Evaluation of Cotton Leaf Curl Virus Resistance in BC1,
BC2, and BC3 Progenies from an Interspecific Cross
between Gossypium arboreum and Gossypium hirsutum
Wajad Nazeer1,2, Abdul Latif Tipu2, Saghir Ahmad2, Khalid Mahmood2, Abid Mahmood3,
Baoliang Zhou1*
1 State Key Laboratory of Crop Genetics and Germpalsm Enhancement, MOE Hybrid Cotton R&D Engineering Research Center, Nanjing Agricultural University, Nanjing,
Jiangsu Province, China, 2 Cotton Research Station, Multan, Ayub Agricultural Research Institute, Faisalabad, Punjab, Pakistan, 3 Cotton Research Institute, Ayub
Agricultural Research Institute, Faisalabad, Punjab, Pakistan
Abstract
Cotton leaf curl virus disease (CLCuD) is an important constraint to cotton production. The resistance of G. arboreum to this
devastating disease is well documented. In the present investigation, we explored the possibility of transferring genes for
resistance to CLCuD from G. arboreum (2n = 26) cv 15-Mollisoni into G. hirsutum (2n = 52) cv CRSM-38 through conventional
breeding. We investigated the cytology of the BC1 to BC3 progenies of direct and reciprocal crosses of G. arboreum and G.
hirsutum and evaluated their resistance to CLCuD. The F1 progenies were completely resistant to this disease, while a
decrease in resistance was observed in all backcross generations. As backcrossing progressed, the disease incidence
increased in BC1 (1.7–2.0%), BC2 (1.8–4.0%), and BC3 (4.2–7.0%). However, the disease incidence was much lower than that
of the check variety CIM-496, with a CLCuD incidence of 96%. Additionally, the disease incidence percentage was lower in
the direct cross 2(G. arboreum)6G. hirsutum than in that of G. hirsutum6G. arboreum. Phenotypic resemblance of BC1 ,BC3
progenies to G. arboreum confirmed the success of cross between the two species. Cytological studies of CLCuD-resistant
plants revealed that the frequency of univalents and multivalents was high in BC1, with sterile or partially fertile plants, but
low in BC2 (in both combinations), with shy bearing plants. In BC3, most of the plants exhibited normal bearing ability due
to the high frequency of chromosome associations (bivalents). The assessment of CLCuD through grafting showed that the
BC1 to BC3 progenies were highly resistant to this disease. Thus, this study successfully demonstrates the possibility of
introgressing CLCuD resistance genes from G. arboreum to G. hirsutum.
Citation: Nazeer W, Tipu AL, Ahmad S, Mahmood K, Mahmood A, et al. (2014) Evaluation of Cotton Leaf Curl Virus Resistance in BC1, BC2, and BC3 Progenies from
an Interspecific Cross between Gossypium arboreum and Gossypium hirsutum. PLoS ONE 9(11): e111861. doi:10.1371/journal.pone.0111861
Editor: David D. Fang, USDA-ARS-SRRC, United States of America
Received July 26, 2014; Accepted October 8, 2014; Published November 5, 2014
Copyright: ß 2014 Nazeer et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Data Availability: The authors confirm that all data underlying the findings are fully available without restriction. All relevant data are within the paper.
Funding: This work was supported by the following. Baoliang Zhou: Research was supported partially by National Key Technology Support Program of China
during the twelfth Five-year Plan Period (2013BAD01B03-04), the Independent Innovation Funds for Agricultural Technology of Jiangsu Province, China [CX (14)
2065] and the Priority Academic Program Development of Jiangsu Higher Education Institutions. Saghir Ahmad: Punjab Agriculture Research Board (PARB) for
providing financial support for these studies under PARB Project no. 27. The funders had no role in study design, data collection and analysis, decision to publish,
or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* Email: baoliangzhou@njau.edu.cn
In Pakistan, an outbreak of CLCuD occurred in the early 1990s.
This disease devastated the Pakistani cotton industry, where it
caused an estimated yield reduction of 30–35%. Between 1992
and 1997, the economic losses due to CLCuD in Pakistan
amounted to approximately 5 billion dollars (US) [14]. Similarly,
in the Indian state of Punjab, this disease reduced cotton
production by almost 70% in 1998 [15]. Singh et al. [16]
observed a reduction of 52.7% in the number of bolls and a
reduction of 54.2% in boll weight due to CLCuD, whereas the
differences in yield loss between resistant and susceptible cultivars
were almost 50% and 85–90%, respectively.
In the late 1990s, several resistant cotton varieties were
gradually introduced into the Indo-Pak region, and losses due to
the disease diminished [17–18]. However, resistance subsequently
broke in 2001–2002 [3,12] due to new strains of CLCuD emerged,
and all of the cotton varieties that were previously known resistant
to CLCuD, such as LRA-5166, CP-15/2, and Cedex, have
Introduction
Cotton production is biotically constrained by various diseases,
which lead to yield instability and reduced seed quality. Cotton
leaf curl disease (CLCuD) is a debilitating disease of cotton in
Africa, Pakistan, and Northwestern India [1–3]. CLCuD is caused
by a pathogen complex of a virus and a DNA beta satellite (DNAb) molecule [4]. There are seven such virus species, all belonging
to the Begomovirus genus, and DNA-b satellites are associated
with CLCuD in these regions [5–8].
CLCuD was first recorded in 1967 in the Multan district,
Pakistan, on scattered Gossypium hirsutum plants [9–11], and it
has spread rapidly to all cotton growing areas of Pakistan and
throughout the Indian subcontinent. Two epidemics of this disease
have been observed during the past three decades due to a loss of
host-plant resistance in existing cotton varieties [12–13].
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Leaf Curl Virus Resistance in Interspecific Cotton
utilized to directly backcross with G. hirsutum. The number of F1
progenies was increased by cuttings. Thus total number of F1 plant
progenies for direct and reciprocal cross was 10 and 15,
respectively, to generate BC1 to BC2 generations. CIM-496, a
cotton variety highly susceptible to CLCuD, was employed as a
standard/control in order to obtain a natural virus inoculum.
become susceptible to CLCuD [6–7,19–23]. Symptoms of this
disease were also reported in China [24], which is located far from
the hot spots of India and Pakistan, and there is great concern that
CLCuD could spread from its origin to other cotton growing areas
of the world where the disease is not currently present. Plant
biologists have attempted to understand the molecular biology of
this disease complex to control CLCuD [25], but the tricky nature
of the pathogen and the rapid evolution/recombination of these
genes have hindered the progress of this research [26–28].
In plant breeding, wild relatives have long been studied due to
the presence of novel genes [29–31], and these wild species have
been exploited most often as sources for biotic and abiotic stress
resistance [32]. Among the wild species of cotton, especially, desi
cotton (G. arboreum L.) has built in desirable resistant genes for all
kind of Begomoviruses associated with CLCuD [33]. Additionally,
G. arboreum is known to combat various stresses like drought [34–
35], heat [36], root rot, cotton leaf curl virus [37] and insect pests
(bollworms and aphids) [12]. Interspecific hybridization of cotton
has been performed with varying degrees of success [21,38–40].
For example, Sacks and Robinson [41] transferred nematode
(Rotylenchulus reniformis) resistance into tetraploid G. hirsutum.
Chen et al. [42] and Nazeer et al. [2] employed Gossypium
australe and Gossypium stocksii to introgress some novel genes for
drought and CLCuD resistance into G. hirsutum, respectively.
The interspecific hybridization is quite difficult, especially,
between G. arboreum and G. hirsutum, and some scientists
explored bridge lines for introgression of genes form wild species
[43].
At present, no single variety of G. hirsutum is resistant to
CLCuD; however, G. arboreum is documented to have resistance
against CLCuD [31]. Due to the importance of this disease and
significant features of this species, we initiated a project to explore
the possibility of successful transferring CLCuD resistance genes
from Desi cotton (G. arboreum, 2n = 26) into cultivated upland
cotton (G. hirsutum, 2n = 52) genotypes through conventional
hybridization and backcrossing without developing bridging line.
In this way maximum desirable donor genes of G. arboreum can
be transferred into G. hirsutum to improve the resistance to
CLCuD of the cultivated G. hirsutum.
Development of backcross progenies
The F1 CLCuD-resistant progenies consisting of two cross
combinations, 2(G. arboreum)6G. hirsutum and G. hirsutum6G.
arboreum, were backcrossed with G. hirsutum to produce the BC1
progenies in 2011. The BC1 progenies were planted in the field at
Cotton Research Station in Multan, Pakistan in May 2012 and
backcrossed with G. hirsutum to generate the BC2 progenies.
These BC2 progenies were again backcrossed with G. hirsutum cv
CRSM-38 to produce the BC3 progenies in 2013. One thing
should be noticed here that only those normal morphological plant
progenies that produce more fruits but no symptoms of CLCuD
were selected for backcrossing. The plant progenies that showed
even minor spots of CLCuD were rejected to utilize for
backcrossing. The scheme for the development of the backcross
progenies are shown in Figure 2. Emasculation was carried out in
the evening, and emasculated flowers were manually pollinated
the next morning.
Use of plant growth hormones for hybridization
Normally, embryos fail to develop in hybridizations between G.
arboreum and G. hirsutum. This obstacle was overcome by the
application of plant hormones such as gibberellic acid (GA3) and
naphthalene acetic acid. Specifically, 50 mgL21 GA3 and 100 mg
L21 naphthalene acetic acid were applied to the bases of pedicles
24 hours after pollination for 3 consecutive days to reduce embryo
and boll shedding. The number of cross boll sets was counted, and
the bolls were picked at harvest time.
Cross fertility studies
Fertility studies for BC1 to BC3 progenies of 2(G. arboreum)6G.
hirsutum and G. hirsutum6G. arboreum were measured in term of
cross boll setting and their germination percentage by given
formula:
Materials and Methods
Plant materials
Boll setting(%)~
The plant materials used in this study include G. hirsutum cv
CRSM-38 (2n = 4x = AADD = 52), G. arboreum cv 15-Mollisoni
(2n = 2x = AA = 26), and an artificial autotetraploid of G. arboreum cv 15-Mollisoni (2n = 4x = 52; Figure 1). The F1 CLCuDresistant progeny involving these parents, which was developed by
Ahmad et al. [44], comprising direct cross [2(G. arboreum)6G.
hirsutum] and its reciprocal cross (G. hirsutum6G. arboreum), was
Total number of cross boll picked
|100;
Total number of pollinations
Seed germination(%)~
Number of seed geminated
|100
Total number of seed tested
Figure 1. Parents of interspecific hybridization. A. G. arboreum cv 15-Mollisoni (2n = 2x = 26); B. G. hirsutum cv CRSM-38 (2n = 4x = 52); C. 2(G.
arboreum) (2n = 4x = 52); D. [2(G. arboreum)6G. hirsutum] F1 (2n = 4x = 52); E. (G. hirsutum6G. arboreum) F1 (2n = 3x = 39).
doi:10.1371/journal.pone.0111861.g001
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Leaf Curl Virus Resistance in Interspecific Cotton
Figure 2. Scheme for the development of the BC1 to BC3 progenies for interspecific cross 2(G. arboreum)6G. hirsutum and G.
hirsutum6G. arboretum.
doi:10.1371/journal.pone.0111861.g002
Morphological characteristics
Maintenance of virus inoculum and screening for CLCuD
Observations of growth habit, stem color, leaf texture, leaf
shape, leaf hairiness, bracteole size, corolla color, petal spots, the
position of the staminal column, anther color, and dehiscence in
the parents, as well as in BC1 to BC3, were recorded. The
phenotypic resemblance of BC1 to BC3 progenies to G. arboreum
having desirable traits with good resistance to CLCuD will be
helpful for selection of introgression progenies.
Artificial inoculation techniques is not available for CLCuD,
therefore, the only way to study the response of cotton germplasm
is to expose the introgression progenies to high inoculum pressure
by planting in natural hot spots [45], so sick plot technique was
used to arrange spreader plants among BC introgression lines. In
this sick plot technique, we planted susceptible variety CIM-496
after each two rows of CLCuD resistant lines to encourage
uniform spread of the disease. Planting of BC1,BC3 progenies
was done after 3rd week of May for the three seasons i.e. 2011–
2013. Sowing was done manually and row to row (75 cm) and
plant to plant (30 cm) distance was maintained. Row length for
each genotype was 450cm and plot size was variable depending
upon the seed availability.
Cytological studies
Morphological normal plants producing more fruits were
selected from BC1 to BC3 progenies for cytological studies. Young
buds of BC1 to BC3 plants, along with those of the parents, were
collected and fixed in Carnoy’s solution at 8 to 9 am and preserved
in 70% ethanol after 24 h. Three to four anthers were squashed on
a slide with a drop of 2.5% acetocarmine solution to examine the
pollen mother cells (PMCs). Chromosomal configurations such as
univalent (I’s), bivalents (II’s), trivalents (III’s), quadrivalents (IV’s),
and division stage were examined under a Labomed microscope,
and photographs were also taken using a camera mounted on a
Labomed microscope.
Phenotypic assessment of BC1 to BC3 progenies against
CLCuD
The resistance of the BC1 to BC3 progenies against CLCuD was
assessed under natural field conditions using an inoculum of CIM496 at Cotton Research Station in Multan, Pakistan which is hot
spot of CLCuD. Data for CLCuD were recorded following the
rating system described in Table 1 to calculate the severity index
Table 1. Disease rating (symptom rating) scale for evaluation of cotton leaf curl virus disease.
Disease index (%)
Severity grade
Symptoms
Remarks
0
0
No Symptoms
Resistant
1–20
1
Thickening of only secondary and tertiary veins.
Highly tolerant
21–30
2
Thickening of secondary and primary (mid rib) veins.
Tolerant
31–50
3
Vein thickening (V.T), leaf curling (L.C) or enation (E) or both.
Susceptible
.50
4
Stunting along with vein thickening leaf curling/enation.
Highly susceptible
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67.0
46.1
Sum of all disease ratings
100
|
Total plants
Maximum grade
The percent disease tolerance (PDT) was calculated by selecting
a minimum of 100 plants on a diagonal from one corner to the
other, and diseased plants were counted to determine the PDT
using the formula:
276
24
65.5
45.5
11
340
52.1
35.1
20
25
~
412
3123
52
Data regarding the latent period, number of virus-infected
plants, disease incidence percentage, disease severity index,
infection type, and disease reaction were recorded.
Inoculation of CLCuD through grafting
42.9
1.5
15
980
7263
519
22
1.3
39.1
19
117
1495
299
57
2.6
3.3
338
265
12890
48
Total plants - diseased plants
Percent disease tolerance~
|100
Total plants
8144
No. of seed
obtained
Boll setting
(%)
No. of bolls
picked
No. of
pollinations
Percent disease index
A petiole and rootstock from CIM-496 were used to transfer
virus inoculum into healthy plants. Two grafting techniques, i.e.,
approach grafting and petiole grafting, were employed to confirm
the resistance against CLCuD in BC1 to BC3 plants. For approach
grafting, the resistant plants of the BC1, BC2, and BC3 progenies
were used as scions, whereas virus-susceptible G. hirsutum plants
were used as stock. For petiole grafting, young petioles from
CLCuD-infected plants were selected and inserted into the test
plants. Two infected petioles were also grafted onto the same plant
to introduce additional virus inoculum. The following data were
recorded: grafting success, infectivity, latent period, infection type,
disease severity index, and disease incidence percentage at 40 and
70 days after grafting (DAG).
Cross fertility studies
Examination of the cross ability of BC1 to BC3 of the
combination 2(G. arboreum)6G. hirsutum and G. hirsutum6G.
arboreum revealed that the maximum percentage of boll set
(42.9%) and germination (67.0%) were observed in the BC3 (G.
hirsutum6G. arboreum) progenies (Table 2). Minimum boll setting
(1.3%) was recorded in the cross BC2 [2(G. arboreum)6G.
hirsutum]. A minimum percentage of viable seeds (35.1%) were
obtained in BC1 [2(G. arboreum)6G. hirsutum]. The boll setting
and germination (%) gradually increased from BC1 to BC3
(Figure 3).
*Number of plants used for pollination and recording data.
doi:10.1371/journal.pone.0111861.t002
12
225
2013
BC3 (G. hirsutum6G. arboreum)
2013
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BC3 [2(G. arboreum)6G. hirsutum]
14
155
2012
2012
BC2 (G. hirsutum6G. arboreum)
BC2 [2(G. arboreum)6G. hirsutum]
15
12
2009–11
2009–11
BC1 (G. hirsutum6G. arboreum)
BC1 [2(G. arboreum)6G. hirsutum]
No. of
plants*
Year
Results
Parentage
Table 2. Fertility studies of interspecific hybrid between G. arboreum and G. hirsutum to produce the BC1 to BC3 progenies.
No. of seed
germinated
Germination
(%)
(SI), percent disease index (%, DI), and disease reaction. Individual
plant ratings for each genotype were added and means were
calculated to generate the corresponding SI. The DI was
calculated using the following formula:
Morphological studies
Examination of the morphological characteristics of the parents
and BC1 to BC3 of 2(G. arboreum)6G. hirsutum revealed that in
BC1 to BC3, leaf hairiness, flower size, corolla color, petal spots,
and pollen color were segregated for the male and female parents.
Stem color, leaf lobation, flower size, corolla color, petal number,
petal size, anther dehiscence, and pollen color of BC1 to BC3 were
similar to those of the female parents. Stem hairiness, gossypol
glands, leaf size, leaf hairiness, leaf texture, petiole length, and
bracteole number and size were dominant characters of the male
parents. Bracteole dentation, petiole size, petal spots, and position
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Figure 3. Advancement of boll setting and germination (%) across different generations (BC1 to BC3). D = Direct cross [2(G.
arboreum)6G. hirsutum]; R = Reciprocal cross [G. hirsutum6G. arboreum].
doi:10.1371/journal.pone.0111861.g003
The meiotic behavior in the artificial autotetraploid of G.
arboreum parent showed two I’s, 23 II’s, and one IV (Figure 6C).
Meiosis in BC1. [2(G. arboreum)6G. hirsutum]. The
progenies of this combination comprised 15 plants; only seven
normal morphological plants with better boll setting were studied
cytologically. Cytological studies at Metaphase-I revealed that
there were six I’s, 21 II’s, and one IV’s (Figure 6D). The number
of I’s, II’s, III’s, and IV’s for 76 PMCs ranged from 5–12, 18–22,
0–1, and 0–1, respectively, for a total of 52 chromosomes
(Table 5), while the average number of I’s, II’s, III’s, and IV’s
was 8.2, 20.4, 0.2, and 0.7, respectively. A few lagging
chromosomes were also observed at Anaphase-I. The high
frequency of univalents (5–12) and multivalents (0–1) caused
meiotic disturbance; the plants were partially fertile/sterile.
G. hirsutum6G. arboretum. The plant progenies of this
combination comprised 12 plants; cytological studies were
conducted on six normal morphological plants with better boll
setting. The cytological configuration at Metaphase-I of the BC1
plants revealed two I’s, 23 II’s, and one IV’s (Figure 6E). In the 53
PMCs of these hybrid plants, there were 1–5 I’s, 20–25 II’s, and 0–
1 III’s and IV’s, for a total of 52 chromosomes (Table 5), while the
average number of I’s, II’s, III’s, and IV’s for 53 PMCs was 2.2,
23.2, 0.4, and 0.6, respectively. Although multivalent association
was observed, the high frequency of bivalents (20–25) caused these
plants to be fertile or partially fertile.
of the staminal column of BC1 to BC3 were intermediate between
those of both parents (Table 3). The hybrid plant progenies of BC1
to BC3 of 2(G. arboreum)6G. hirsutum are shown in Figure 4.
An analysis of the morphological characteristics of the parents
and BC1 to BC3 of G. hirsutum6G. arboreum revealed that in BC1
to BC3, gossypol, bracteole number and size, and pistil size was
dominant characters of the female parents. Leaf size, leaf lobation,
corolla color, petal spots, and pollen color were the dominant
characteristics of the male parents. Stem color and hairiness, leaf
texture, bracteole number and size, and position of staminal
column of BC1 to BC3 were intermediate between those of both
parents, while leaf hairiness was segregated (Table 4). The hybrid
plant progenies of BC1 to BC3 of G. hirsutum6G. arboreum are
shown in Figure 5. Morphological characteristics particularly leaf
texture, leaf size, bracteole size, corolla color and petal spots from
G. arboreum into BC1 to BC3 progenies of both crosses helped for
selection of plant progenies that have some resemblance of G.
arboreum and also showed CLCuD resistance.
Cytological studies
Meiosis in parents. The course of meiosis was examined in
the G. hirsutum and G. arboreum parents. In these species, the
reduction division was normal, with regular pairing of chromosomes. The number of bivalents in G. hirsutum and G. arboreum
at Metaphase-I was 26 and 13, respectively (Figures 6A and 6B).
The disjunction of the chromosomes was normal at Anaphase-I.
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Table 3. Morphological characteristics of parents and the BC1 to BC3 progenies from the cross 2(G. arboreum)6G. hirsutum.
Morphological
characteristic
2(G. arboreum) G. hirsutum
BC1
BC2
BC3
Stem color
Greenish
brown
Green
Brown
Brown
Brown
Stem hairiness
Profusely
hairy
Hairy
Hairy
Hairy
Hairy
Black glands
Dense
Sparse
Sparse
Sparse
Sparse
Leaf color
Dark
green
Green
Light/dark green
Light/dark green
Light/dark
green
Leaf size(cm)
Medium
(7.369.9)
Large
(10614)
Small/Large
(7.068.0 cm)/(9.0611.0 cm)
Small/Large
(7.168.3)/(9.2610.8)
Medium/Large
(7.668.9)/(9.6611.4)
Leaf hairiness
Profusely
hairy
Hairy
Hairy/profusely
hairy
Hairy/profusely
hairy
Hairy/profusely
hairy
Leaf lobation
3–5
narrrow,
deep
lobed
3–5 broad,
shallow lobed
3–5 broad
lobed
3–5 broad
lobed
3–5 broad
lobed
Leaf texture
Thick,
Leathery
Herbaceous
Herbaceous
Herbaceous
Herbaceous
Petiole length
(cm)
Medium
(4.4)
Long
(8.8)
Long (7.3)
Long (7.2)
Long (7.5.0)
Bracteole number
and size (cm)
2–3, large
(3.062.6),
united at
base
3 large
(3.361.8)
3 Large (3.062.3)
3 Large
(3.362.2)
3 Large
(3.162.4)
Bracteole dentation
Entire
5–11,
deep narrow
4–9 medium
3–10 medium
3–11 medium
Stem characteristics
Leaf characteristics
Boll characteristics
Flower characteristics
Flower size
Medium
Large
Medium
Medium
Medium
Pedicel size (cm)
Long (1.7)
Long (1.2)
Long (1.3)
medium(1.0)
medium(0.9)
Calyx
5 sepal
forming a
cup with
wavy
margins
5 sepals
forming a
cup with
teeth
5 sepal forming
a cup with
wavy margins
5 sepal forming
a cup with
wavymargins
5 sepal forming
a cup with
wavymargins
Corolla color
Light
yellow
Creamy
Creamy/light
yellow
Creamy/ligt
yellow
Creamy/light
yellow
Petal number
and size (cm)
5, medium,
(3.062.6)
5, large
(4.664.5)
5, medium
(3.564.1)
5, medium
(3.464.2)
5, medium
(3.363.5)
Petal spot
Dark
pink
Absent
Present/absent
Present/absent
Present/absent
Position of
staminalcolum
and size (cm)
Short (0.4)
Long (2.0)
Medium (1.5)
Medium (1.7)
Medium (1.6)
Anther
dehiscence
Partial
Normal
Partial
Normal
Normal
Pollen
color
Light
yellow
Creamy
Yellow
Creamy/light
yellow
Creamy/light
yellow
Pistil
size (cm)
Long (2.5)
Long (2.9)
Long (2.6)
Long (2.8)
Long (2.9)
doi:10.1371/journal.pone.0111861.t003
Meiosis in BC2. [2(G. arboreum)6G. hirsutum].
These plant progenies consisted of 14 plants; only five normal
morphological plants with better boll setting were studied
cytologically. The chromosomal conformation at Metaphase-I
was two I’s+23 II’s+1 IV’s (Figure 6F). A study of 60 PMCs
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revealed 2–4 I’s, 23–25 II’s, and 0–1 IV’s, for a total of 52
chromosomes (Table 5), while the average number of I’s, II’s, and
IV’s for 60 PMCs was 3.0, 24.1, and 0.2, respectively. Trivalents
were not observed in these plants. The low frequency of uni- and
multi-valents, as well as the high frequency of chromosome
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Figure 4. Hybrid Progenies of [2(G. arboreum)6G. hirsutum]. A = BC1; B = BC2; C = BC3.
doi:10.1371/journal.pone.0111861.g004
BC1 hybrid plants of 2(G. arboreum)6G. hirsutum and G.
hirsutum6G. arboreum were resistant to CLCuD, with good plant
growth. The BC2 hybrid plants of 2(G. arboreum)6G. hirsutum
and G. hirsutum6G. arboreum developed disease symptoms at 30–
35 and 28–30 DAG, respectively. The infection type range for
2(G. arboreum)6G. hirsutum and G. hirsutum6G. arboreum was
0–1 and 0–2, respectively, and the same trend for the first
appearance of disease symptoms was observed for BC3 plants from
both crosses. All BC3 progenies from 2(G. arboreum)6G. hirsutum
and G. hirsutum6G. arboreum were highly tolerant to CLCuD,
with good fruit bearing and normal growth compared with
susceptible variety CIM-496. By and large, the hybrid plants of
cross 2(G. arboreum)6G. hirsutum showed better resistance/
tolerance to CLCuD than those of cross G. hirsutum6G.
arboreum.
association (23–25 II’s), caused the plants to be fertile but shy
bearing.
G. hirsutum6G. arboretum. The plant progenies of this
combination comprised 161 plants; only 10 normal morphological
plants with better boll setting were studied cytologically. The
chromosomal constitution at Metaphase-I revealed 3 I’s+21 II’s+1
III’s+1 IV’s (Figure 6G). However, in 110 PMCs, there were 1–4
I’s, 21–25 II’s, and 0–1 III’s and IV’s, for a total of 52
chromosomes (Table 5), and the average number of I’s, II’s, III’s,
and IV’s was 2.4, 23.2, 0.4, and 0.5, respectively. Low frequencies
of univalents (1–4) and multivalents (0–1), as well as high
frequencies of bivalents (21–25), were observed. The plants were
fertile. A few shy bearing plants were also observed.
Meiosis in BC3. [2(G. arboreum)6G. hirsutum]. The
plant progenies consisted of 12 plants. A total of 35 PMCs were
sampled from two plants for microscopic studies. Metaphase-I of
these PMCs showed 2 I’s+25 II’s (Figure 6H). The average range
of these PMCs revealed that there were 2 I’s and 25 II’s, for a total
of 52 chromosomes (Table 5); the plants were fertile.
G. hirsutum6G. arboretum. The plant progenies consisted
of 225 plants. The chromosome pairing was normal (26 II’s) in
most of the PMCs (Figure 6I). The average number of chromosomes among 40 PMCs exhibited normal disjunction (Table 5);
the plants were fertile.
Testing of BC1 to BC3 progenies against CLCuD under
natural field conditions
The BC1 to BC3 hybrid plants of 2(G. arboreum)6G. hirsutum
and G. hirsutum6G. arboreum were tested under natural field
conditions. Nineteen plants of BC1 of the combination [2(G.
arboreum)6G. hirsutum] and 15 plants of reciprocal cross G.
hirsutum6G. arboreum revealed disease indices of 1.3% and 1.6%,
respectively, whereas the average severity index was 0.05 and 0.06
at 40 DAS, respectively (Table 7). However, the disease index and
severity index were zero after 70 DAS because the minor spots of
vein thickening that were observed on a single plant of each cross
disappeared after 70 DAS. CIM 496, the control variety used in
this trial, had a disease index of 94.3%, and enation was also
observed at 70 DAS. Fourteen plants of BC2 of the combination
2(G. arboreum)6G. hirsutum and 161 plants of the combination G.
hirsutum6G. arboreum raised through backcrossing of BC1 with
G. hirsutum had disease indices of 1.8% and 4.0%, respectively, at
40 DAS, and the disease index increased to 3.5% and 6.8%,
respectively, at 70 DAS. The grade of disease severity in 2(G.
arboreum)6G. hirsutum was 0.07 (40 DAS) to 0.1 (70 DAS),
whereas it was 0.17 (40 DAS) to 0.2 (70 DAS) for G. hirsutum6G.
arboreum. The susceptible cotton variety CIM 496 in this trial had
a disease index of 97.7% with a disease severity grade of 3.9.
Twelve plants of BC3 of the combination 2(G. arboreum)6G.
hirsutum and 225 plants of the combination G. hirsutum6G.
arboreum raised through backcrossing with G. hirsutum had a
4.2% and 7.0% disease index, respectively, at 40 DAS. And the
Testing of BC1 to BC3 progenies against CLCuD through
grafting
The resistance/susceptibility of the plants was confirmed
through petiole and approach grafting, as indicated in Figure 7,
and only resistant plants were used for backcrossing to produce the
next generation. Grafting for BC1 to BC3 hybrid plants of 2(G.
arboreum)6G. hirsutum and G. hirsutum6G. arboreum was
carried out under greenhouse conditions as well as in the natural
field. All plants from both crosses [2(G. arboreum)6G. hirsutum
and G. hirsutum6G. arboreum] showed 100% infectivity and
grafting success (Table 6). Plants of susceptible variety CIM-496
showed symptoms of CLCuD at 11–14 days after germination.
Grafts of BC1 from cross 2(G. arboreum)6G. hirsutum remained
asymptomatic to this disease throughout their lifecycles, whereas
only two grafts from BC1 of G. hirsutum6G. arboreum showed
minor spots (3–5) of vein thickening at 40 DAG, which appeared
on a few leaves. These minor spots become quite small at 70 DAG
and were only detected after careful observation. Therefore, the
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Leaf Curl Virus Resistance in Interspecific Cotton
Table 4. Morphological characteristics of parents and the BC1 to BC3 progenies from the cross G. hirsutum6G. arboretum.
Morphological characteristics
G. hirsutum
G. arboreum
BC1
BC2
BC3
Green
Stem characteristcs
Stem color
Green
Green
Green
Green
Stem hairiness
Hairy
Hairy
Hairy
Hairy
Hairy
Black glands
Sparse
Sparse/dense
Sparse
Sparse
Sparse
Leaf color
Green
Green
Green
Green/dark green
Green/dark green
Leaf size(cm)
large
(10.0614.0)
Small/medium
(6.068.3)
Medium (7.167.9)
medium (7.068.4)
medium (7.368.3)
Leaf lobation
3–5 broad,
shallow lobed
3–5 narrrow,
deep lobed
3–5 medium lobed
3–5 broad lobed
3–5 broad lobed
Leaf texture
Herbaceous
Herbaceous
Herbaceous
Herbaceous
Herbaceous
Leaf hairiness
Hairy
Hairy/profusely
hairy
Hairy
Hairy/profusely
hairy
Hairy/profusely
hairy
Petiole
length (cm)
Long (8.8)
Medium (4.4)
Long (7.2)
Long (7.5)
Long (7.4)
Bracteole number and size (cm)
3 large (3.361.8)
3, small
(2.762.1),
united at base
3, large (3.062.6)
3, large (3.062.0)
3, large (3.262.2)
Bracteole dentation
3–7, superficial
5–11, deep
narrow
4–9, superficial
3–11,
superficial
3–9, superficial
Leaf characteristcs
Boll characteristcs
Flower characteristcs
Flower size
Large
Small
Medium
Medium
Medium/large
Pedicel size (cm)
Long (1.2)
Long (1.2)
Long (1.1)
Long (1.3)
Long (1.2)
Calyx
5 sepal
forming a
cup with
teeth
5 sepals
forming
a cup with
wavy margins
5 sepals
forming a cup
with wavy
margins
5 sepals forming
a cup with wavy
margins
5 sepals forming a
cup with wavy
margins
Corolla color
Creamy
Yellow
Creamy/Light
Yellow
Creamy/light
yellow
Creamy/light
yellow
Petal number
and size (cm)
5, large,
(4.664.5)
5, small
(2.662.5)
5, large,
(4.564.4)
5, large,
(4.464.6)
5, large,
(4.664.4)
Petal
spot
Absent
Light
pink
Present/absent
Present/absent
Present/absent
Position of
staminalcolum
and size (cm)
long (2.0)
small (1.0)
Medium (1.5)
Medium (1.5)
Medium (1.6)
Anther
dehiscence
Normal
Normal
Partial/normal
Normal
Normal
Pollen
color
Creamy
Yellow
Creamy/light
Yellow
Creamy/light
Yellow
Creamy/light
Yellow
Pistil
size (cm)
Long (2.9)
Small (2.1)
Long (3.1)
Long (2.9)
Long (3.2)
doi:10.1371/journal.pone.0111861.t004
disease index increased to 8.3% and 12.0%, respectively, at 70
DAS. The grade of disease severity in 2(G. arboreum)6G. hirsutum
was 0.17 (40 DAS) to 0.3 (70 DAS), whereas it was 0.28 (40 DAS)
to 0.5 (70 DAS) for G. hirsutum6G. arboreum. CIM 496 had a
disease index of 95.0% with a disease severity grade of 3.8
(Table 7).
As the backcross progressed from BC1 to BC3, the PDT
gradually decreased (Figure 8). However, the PDT was fairly high
in 2(G. arboreum)6G. hirsutum compared to the combination G.
hirsutum6G. arboreum.
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Discussion
G. hirsutum has low genetic diversity and lacks resistance
against CLCuD. In general, wild diploid species of Gossypium
possess resistance against many challenges, such as insects, pests,
diseases, and many abiotic factors [37,46]. Hence, there is a great
need to exploit this resource to develop resistance against CLCuD
in cultivated tetraploid species [2]. Cotton breeders have long tried
to obtain hybrids between diploid and tetraploid species [47].
However, several incompatibility factors hinder the development
8
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Leaf Curl Virus Resistance in Interspecific Cotton
Figure 5. Hybrid Progenies of (G. hirsutum6G. arboreum). A = BC1; B, C = BC2; D = BC3.
doi:10.1371/journal.pone.0111861.g005
Figure 6. Chromosome configurations in PMCs at Metaphase-I of meiosis. A. G. hirsutum, 26 II’s; B. G. arboreum, 13 II’s; C. 2(G. arboreum), 2
I’s+23 II’s+1 IV; D. [2(G. arboreum)6G. hirsutum] BC1, 6 I’s+21 II’s+1 IV; E. [G. hirsutum6G. arboreum] BC1, 2 I’s+23 II’s+1 IV; F. [2(G. arboreum)6G.
hirsutum] BC2, 2 I’s+23 II’s+1 IV; G. [G. hirsutum6G. arboreum] BC2, 3 I’s+21 II’s+1 III+1 IV; H. [2(G. arboreum)6G. hirsutum] BC3, 2 I’s+25 II’s; I. [G.
hirsutum6G. arboreum] BC3, 26 II’s.
doi:10.1371/journal.pone.0111861.g006
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Table 5. Cytological comparison of BC1 to BC3 plants from an interspecific cross between G. arboreum and G. hirsutum.
Cross congifuration
Plant number
PMC
I’s
II’s
III’s
IV’s
Total
Chromosomal configuarion for BC1
2(G. arboreum)6G. hirsutum
P2
12
5
20
1
1
52
//
P3
10
6
21
0
1
52
//
P4
8
6
21
0
1
52
//
P9
10
8
20
0
1
52
//
P11
15
8
22
0
0
52
//
P13
11
12
20
0
0
52
//
P4
10
12
18
0
1
52
5–12
18–22
0–1
0–1
Range
8.2
20.4
0.2
0.7
P1
Average of 76 cells
5
2
25
0
0
52
//
P2
12
1
22
1
1
52
//
P5
8
5
20
1
1
52
//
P9
10
2
23
0
1
52
//
P10
6
2
25
0
0
52
//
P11
12
2
25
0
0
52
Range
1–5
20–25
0–1
0–1
Average of 53 cells
2.2
23.2
0.4
0.6
G. hirsutum6G. arboreum
10
Chromosomal configuarion for BC2
2(G. arboreum)6G. hirsutum
10
2
25
0
0
52
P4
15
4
24
0
0
52
//
P7
8
2
25
0
0
52
//
P10
12
2
23
0
1
52
//
P11-(1)
15
4
24
0
0
52
2–4
23–25
0
0–1
0.2
Range
Average of 60 cells
(G. hirsutum6G. arboreum)
//
3.0
24.1
0
P1(16)
15
1
24
1
0
52
P2
10
3
23
1
0
52
12
2
25
0
0
52
P4 (1)
10
2
23
0
1
52
P5(3)
10
2
25
0
0
52
8
4
22
0
1
52
52
//
//
P5(14)
5
3
21
1
1
//
P7(15)
5
3
23
1
0
52
10
2
23
0
1
52
10
2
23
0
1
52
//
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November 2014 | Volume 9 | Issue 11 | e111861
P3
//
0
Average of 40 cells
26
0
0
52
0
0
26
0
Range
doi:10.1371/journal.pone.0111861.t005
52
0
0
0
0
26
26
0
0
20
of hybrids under in situ conditions [48,49]. Abortion of the
embryo after fertilization and the lack of retention of cross bolls
[50,51] is a common stumble in interspecific crosses. Some species
like G. barbadense can be hybridize easily with G. hirsutum and
produce fertile F1 progeny [52] without hormones application.
These two species i.e. G. hirsutum and G. barbadense have
chromosome homology and the tetraploid genomes, are not
separated by any large scale chromosomal rearrangement [53].
However, crosses between G. hirsutum and G. arboreum L. are
rarely successful without hormone application [44,54]. Plant
hormones are known to control pollen tube growth [55].
Exogenous application of growth hormones has been used to
overcome the crossing barrier and to facilitate interspecific crosses
in many crops, i.e., cotton [56], wheat [57], and tomato [58].
Altman [56] compared exogenous application with in vitro
techniques, i.e., ovule and embryo culture, and found that
exogenous hormone application in conjunction with standard
hybridization methods is superior to in vitro methods. Interspecific
hybridization of cotton is enhanced by the application of
exogenous hormones after pollination. Exogenous hormone
application alone may be used to overcome certain crossing
barriers within Gossypium [59–60]. The extract of garlic acid has
been used as a growth regulator to obtain interspecific hybrids
between tetraploid G. hirsutum and diploid G. arboreum species of
cotton [56,61]. The in situ development of BC1 to BC3 plants
using exogenous hormones in the current study was superior to
that using in vitro methods, which is in agreement with an earlier
report [55]. The average number of seeds per boll varied from
immature seeds to 1.5 seeds per boll. In the absence of exogenous
hormones, pollinated flowers produce 0.1% seed development
[56].
The boll setting and seed germiantion is very low in interspecific
crosses and fertility of interspecific crosses can be measured in
terms of boll setting percentage [52,62]. The cross fertility of BC1
to BC3 between [2(G. arboreum)6G. hirsutum] and G. hirsutum6G. arboreum showed that the boll set was maximum (42.9%)
in cross BC3, G. hirsutum6G. arboreum, but minimum (1.3%) in
cross BC2, 2(G. arboreum)6G. hirsutum (Table 2). Viable seeds
were obtained in both combinations. From BC1 to BC3, an
increasing trend of boll setting and germination (%) was observed.
Seed setting improvement was also recorded in Brassica by
backcrossing with the recurrent parent [63]. The factor responsible for the semi-sterile condition are transmitted rarely through
the pollen but readily through the egg cell. Boll setting and
germination (%) was higher in reciprocal cross (G. hirsutum6
G.arboreum) as compared to direct cross 2(G. arboreum)6G.
hirsutum [64].
In general, the BC1 to BC3 hybrid plants of both cross
combinations [2(G. arboreum)6G. hirsutum and G. hirsutum6G.
arboreum] were intermediate in several traits between the two
parents. The prevalence of yellow pollen in both crosses (direct
and reciprocal) in most of the plants validated the inheritance of
this character from G. arboreum, because this color is more
common in G. arboreum species [65,66], and it revealed the
dominance in inheritance [67]. By contrast, in BC1 to BC3, leaf
hairiness, flower size, corolla color, petal spots, pollen color, and so
on were segregated in both parents. Moreover, morphological
characteristics particularly leaf texture, leaf size, bracteole size,
corolla color and petal spots from G. arboreum into BC1 to BC3
progenies [68] of both crosses were helpful for selection of plant
progenies that have resemblance to G. arboreum and also showed
CLCuD resistance. The frequency of plant progenies that showed
20
P14
P1
G. hirsutum6G. arboreum
//
0
0
0
0
25
Average of 35 cells
25
2
2
Range
52
52
0
0
0
0
25
25
2
15
20
P1
2(G. arboreum)6G. hirsutum
P2
2
0.5
0.4
23.2
2.4
Average of 110 cells
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Chromosomal configuarion for BC3
52
52
0–1
0–1
21–25
1–4
Range
1
1
0
1
21
22
4
3
7
8
P9(16)
P13(17)
//
PMC
Plant number
Cross congifuration
Table 5. Cont.
I’s
II’s
III’s
IV’s
Total
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Leaf Curl Virus Resistance in Interspecific Cotton
Figure 7. Testing of CLCuD through grafting. A and B. Cleft grafts; C = Single petiole grafts; D = Double petiole grafts.
doi:10.1371/journal.pone.0111861.g007
good plant architecture were higher in reciprocal crosses as
compared to direct cross.
When developing interspecific hybrids for resistance, a thorough
knowledge of the chromosomal behavior in hybrids and backcross
progenies is essential. In the present study, in hybrid 2(G.
arboreum)6G. hirsutum, the ‘AD’ genome was introgressed into
the ‘AA’ genome of G. arboreum, producing an ‘AAAD’ genomic
constitution. In hybrid G. hirsutum6G. arboreum, the A-genome
of G. arboreum was introgressed into the ‘AD’ genome of G.
hirsutum, producing the genomic constitution ‘AAD’. In G.
arboreum and G. hirsutum, normal orientation, association, and
disjunction of chromosomes were observed, while in F1 hybrids of
the above genomic constitution, quadrivalents and a low
frequency of chromosome association (bivalents) were observed.
The univalents observed in this study can be attributed to
asynapsis due to the lack of homology between the different sets of
chromosomes. The presence of laggards demonstrates the
occurrence of meiotic disturbances, leading to an imbalance in
the daughter cells. In BC1 hybrid plants of both combinations, the
frequency of univalents and multivalents was high, and the plants
were sterile/partially fertile. In BC2 hybrids of both combinations,
the frequency of univalents and multivalents was low, and the
plants were shy bearing. In BC3 hybrids of both combinations, the
frequency of chromosome association (bivalents) was 25–26;
hence, the plants were fertile. The average of univalent (I’s)
chromosomes was higher in 2(G. arboreum)6G. hirsutum in
comparison with G. hirsutum6G.arboreum. However, the average
of bivalents (II’s) chromosomes was higher in G. hirsutum6G.arboreum. Thus G. hirsutum6G.arboreum was more fertile and more
adaptive to the environment than 2(G.arboreum)6G. hirsutum.
Studies of resistance/susceptibility are rather difficult and
laborious due to the involvement of vectors, the efficiency of
transmission, and the persistent nature of the virus/CLCuD.
Grafting may successfully lead to the transmission of the virus
when other methods fail, as it involves the union of cambial layers
of the root sock and scion [69–70]. Thus, to screen CLCuDresistant germplasm, transmission by grafting is the best alternative
to natural transmission by vector, as most viruses of a persistent
nature, such as CLCuD, cannot be transmitted through mechanical inoculation [71]. Ahmad et al. [72] used sick plot techniques
to screen the exotic and local germplasm against CLCuD.
The BC1 to BC3 of 2(G. arboreum)6G. hirsutum and G.
hirsutum6G. arboreum were tested through grafting under natural
field/greenhouse conditions. These hybrids remained resistant to
CLCuD [39]. The results of evaluation of the BC1 to BC3
progenies revealed a high degree of variability for CLCuD in the
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field and through grafting. All plants from both crosses [2(G.
arboreum)6G. hirsutum and G. hirsutum6G. arboreum] showed
100% infectivity and grafting success. However, latent period and
infection type range for BC1–BC2 was better in 2(G.arboreum)6G.
hirsutum cross than G. hirsutum6G.arboreum. The grafts of BC1
from cross 2(G. arboreum)6G. hirsutum remained asymptomatic
to this disease. However, BC1 of G. hirsutum6G. arboreum
showed minor vein thickening, but the vein thickening was highly
reduced after 70 days of grafting [46], whereas CIM-496 showed
symptoms of CLCuD within 11–14 days after germination.
Although minor symptoms of CLCuD appeared in BC1 of G.
hirsutum6G. arboreum, this disease did not affect the growth of
the plants. Therefore, we can conclude that these plants were also
resistant to CLCuD. The BC2 and BC3 hybrid plants of both cross
combinations 2(G. arboreum)6G. hirsutum and G. hirsutum6G.
arboreum developed disease symptoms after 28–35 DAG, and the
average disease severity was grade 1.0 (70 DAG). Additionally,
these plants showed good tolerance to CLCuD, with no symptoms
of stunted growth. Therefore, these BC2 and BC3 plants were
highly tolerant to CLCuD compared with susceptible variety
CIM-496, which showed CLCuD symptoms after 11 DAS with no
boll setting. Ullah et al. [46] also observed mild symptoms of
CLCuD on the introgressed material following grafting, but the
amount of viral DNA was significantly lower than the levels found
in G. hirsutum. The same trend/response for latent period to
acquire CLCuD was observed in the field for BC1 to BC3 hybrid
plants. The average severity index for 2(G. arboreum)6G.
hirsutum and G. hirsutum6G. arboreum was 0.05 and 0.06 (40
DAS), respectively. However, the disease index and severity index
were zero after 70 DAS. Thus, the resistant hybrid plants of both
crosses 2(G. arboreum)6G. hirsutum and G. hirsutum6G.
arboreum showed better tolerance to CLCuD, with not deleterious
effects on yield or growth. However, 2(G. arboreum)6G. hirsutum
plants were more tolerant regarding number of virus infected
plants, disease index (%), severity index and infection type range
than those of cross G. hirsutum6G. arboreum. Collectively, plants
from these crosses had better tolerance to CLCuD than CIM-496.
The PDT was higher in 2(G. arboreum)6G. hirsutum than in G.
hirsutum6G. arboreum. The frequency of ideotype plants was
higher in G. hirsutum6G. arboreum compared with 2(G.
arboreum)6G. hirsutum.
Conclusions
The results indicate that the BC1 to BC3 progenies were highly
tolerant to CLCuD, indicating the possibility of transferring
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Table 6. Evaluation of plants from an interspecific cross between G. arboreum and G. hirsutum against cotton leaf curl virus disease through grafting.
BC1
BC2
BC3
Progeny
Year
No. of
plants
tested
[2(G. arboreum)6G. hirsutum]
2011
15
Grafting
success
(%)
Infectivity
(%)
Latent
period
(days)
Infection
type
rangeA
Av
disease
severity
after 70
(DAG)
100
100
Symptomless
0
0
Disease
reaction
Resistant
G. hirsutum6G. arboreum
2011
12
100
100
39–41
0–1
1
Highly tolerant
CIM-496 (Std.)
2011
20
100
100
14
3–4E*
4E
Highly susceptible
[2(G. arboreum)6G. hirsutum]
2012
14
100
100
30–35
0–1
1
Highly tolerant
G. hirsutum6G. arboreum
2012
20
100
100
28–30
0–2
1
Highly tolerant
CIM-496 (Std.)
2012
10
100
100
11
3–4E
4E
Highly susceptible
[2(G. arboreum)6G. hirsutum]
2013
12
100
100
28–30
0–2
1
Highly tolerant
G. hirsutum6G. arboreum
2013
16
100
100
25–30
0–3
1
Highly tolerant
CIM-496 (Std.)
2013
10
100
100
11
3–4E
4E
Highly susceptible
A
Infection type range is based on the 0–4 scale described in Table 1,
*Enation where observed.
doi:10.1371/journal.pone.0111861.t006
13
Table 7. Evaluation of plants from an interspecific cross between G. arboreum and G. hirsutum against cotton leaf curl virus disease under natural field conditions.
BC1
BC2
BC3
Latent
period
(days)
Disease
index
(%)
No. of virus
infected plants
Severity
index
Infection
type
rangeA
40 DAS
70 DAS
40
DAS
70
DAS
40
DAS
70
DAS
40
DAS
Disease
reaction#
70
DAS
[2(G. arboreum)6G. hirsutum]
19
Symptomless
19(180+11)
19(190)
1.3
0
0.05
0
0
0
R
(G. hirsutum6G. arboreum)
15
35–40
15(140+11)
15(150)
1.6
0
0.06
0
0–1
0
R
CIM-496 (Std.)
31
14
31(12+53+254)
31(43+274)
94.35
96.7E*
3.7
3.9
2–4E
3–4E
HS
14(120+21)
1.8
3.5
0.07
0.1
0–1
0–1
HT
0
1
[2(G. arboreum)6G. hirsutum]
14
30–35
14(13 +1 )
G. hirsutum6G. arboreum
161
25–30
161(1460+71+52+33)
2
3
4E
CIM-496 (Std.)
168
13
168(4 +7 +157 )
[2(G. arboreum)6G. hirsutum]
12
25–30
12(110+12)
161(1380+91+82+53+14)
4
6.8
0.17
0.2
0–3
0–4
HT
168(42+113+1534E)
97.7
97.1
3.9
3.9
2–4E
2–4E
HS
12(90+21+12)
HT
4.2
8.3
0.17
0.3
0–2
0–2
G. hirsutum6G. arboreum
225
25–30
225(200 +5 +8 +6 +6 )
225(1760+141+122+113+94)
7
12
0.28
0.5
0–4
0–4
HT
CIM-496 (Std.)
190
15
190(42+63+1804E)
190(62+153+1694E)
95
96.4
3.8
3.8
2–4E
2–4E
HS
0
A
Infection type range is based on the 0–4 scale described in Table 1;
*Enation where observed; R Resistant; HT Highly tolerant; HS Highly susceptible;
#
Disease reaction based on disease index 70 DAS.
doi:10.1371/journal.pone.0111861.t007
1
2
3
4
Leaf Curl Virus Resistance in Interspecific Cotton
November 2014 | Volume 9 | Issue 11 | e111861
Parentage
No. of
plants
tested
Leaf Curl Virus Resistance in Interspecific Cotton
BC1 (1.3–1.6%) to BC2 (1.8–4.0%) to BC3 (4.2–7.0%). However,
the disease incidence was much lower than that of the commercial
cultivar CIM-496, which exhibited a very high incidence of
CLCuD (97.7%). The disease incidence was lower in combination
2(G. arboreum)6G. hirsutum than in G. hirsutum6G. arboreum.
As ‘‘A’’ genome is an invaluable genetic resource for improving
modern tetraploid cotton (G. hirsutum). We observed very wide
genetic variability among BC1 to BC3 progenies, which will
certainly facilitate improvement of cotton resistances to diseases.
And various scientists also utilized G. arboreum L. for introgression
of desirable resistant genes into cultivated tetraploid cotton for
drought [34–35], heat [36], root rot, cotton leaf curl virus [37,[44]
and insect pests (bollworms and aphids) [12]. Therefore, the
introgression lines of G. arboreum developed with or wothout
resistance in this study can be employed to map the resistance
gene(s)/loci, which will be very useful for future diverse (a)biotictolerant cotton breeding.
Acknowledgments
We thank Mr. Abdul Latif Sheikh for providing technical assistance.
Figure 8. Percent disease tolerance across different generations (BC1 to BC3). D. Direct cross [2(G. arboreum)6G. hirsutum]; R.
Reciprocal cross [G. hirsutum6G. arboreum].
doi:10.1371/journal.pone.0111861.g008
Author Contributions
Conceived and designed the experiments: WN SA. Performed the
experiments: ALT KM. Analyzed the data: BZ. Contributed reagents/
materials/analysis tools: BZ SA. Contributed to the writing of the
manuscript: WN BZ SA. Provided the breeding material: AM SA.
CLCuD resistance genes from G. arboreum to G. hirsutum
through conventional hybridization and backcrossing. As the
backcross progressed, the disease incidence also increased, from
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