Jaco Greeff
University of Pretoria, Genetics, Faculty Member
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Fig wasps (Hymenoptera, Agaonidae) have been widely utilized in studies of sex ratio evolution, especially local mate competition (LMC), and good fits have been obtained between empirical data and model predictions incorporating LMC and... more
Fig wasps (Hymenoptera, Agaonidae) have been widely utilized in studies of sex ratio evolution, especially local mate competition (LMC), and good fits have been obtained between empirical data and model predictions incorporating LMC and inbreeding effects. These models assume that foundress females within a patch (a fig) oviposit synchronously, producing roughly equal numbers of offspring with the same progeny sex ratios. Working with the fig wasp Eupristina belagaumensis, which pollinates the fig tree Ficus drupacea in India, we investigated whether these assumptions are valid, and then produced an alternative model which incorporates revised biological assumptions. Egg loads at adult emergence were compared with those remaining in females after they had completed their egg-laying and had died. As foundress numbers increased, so did variation in the numbers of eggs which the females managed to lay. In multi-foundress figs certain females (most likely the first ones to enter) often contributed almost complete egg loads while others (which may have entered when most of the oviposition sites had been utilised) contributed relatively few eggs. Assumptions of previous models were therefore violated. Our model assumes totally sequential oviposition by foundresses and differential contributions to broods. The predicted overall sex ratios of the fig wasp progeny are qualitatively similar to models based on LMC and inbreeding effects, despite being based on different biological assumptions, suggesting that previous models may have given the right answers for the wrong reasons. Explicit tests between the models are possible, as the sequential oviposition model predicts that the progeny sex ratios of individual foundresses should vary depending on where in the sequence they enter a fig.
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It is shown that when females can adjust their offspring sex ratios conditionally to the identity of their mates, i.e. sib or non‐sib, split sex ratios are expected. These split sex ratios result from variation in relatedness between... more
It is shown that when females can adjust their offspring sex ratios conditionally to the identity of their mates, i.e. sib or non‐sib, split sex ratios are expected. These split sex ratios result from variation in relatedness between females and their daughters. Haplodiploid females' relatedness to their daughters increases as their relatedness to their mates increases. Therefore, sibmated females' optimal progeny sex ratio is more female biased than that of outbred females. Inbreeding depression that can result from complementary sex determination (CSD) is also considered. The genetic load caused by CSD can be so costly to sibmated females that they switch to the production of males only. The evolutionarily stable sex ratios for a sibmating model is found to be of a weak type. These weak equilibria and split sex ratios can lead to high variation about the mean and are an incentive for further studies on sex ratio variation in conjunction with mating behaviour. The occurrence of split sex ratios in haplodiploid taxa is important because it favours the evolution of eusociality. Partial local mating and alternative mating strategies can thus eventually lead to the evolution of eusociality.
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Research Interests: Botany, Zoology, Morphometrics, Taxonomy, Nematology, and 7 moreBiology, Morphology, Molecular, Phylogeny, Ficus, Monophyly, and Subgenus
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A mere 16 years ago it was suggested that the Cape honeybee (Apis mellifera capensis) should be protected from invasion by the African 'killer bees' (A. m. scutellata). During the last five years, quite the opposite happened when... more
A mere 16 years ago it was suggested that the Cape honeybee (Apis mellifera capensis) should be protected from invasion by the African 'killer bees' (A. m. scutellata). During the last five years, quite the opposite happened when A. m. capensis caused a catastrophe in the A. m. scutellata bee industry. In 1993 an estimated 50 000 A. m. scutellata colonies died) and in 1996 many farmers had to replace 100% or more of their colonies per annum. The disaster is not restricted to the loss of colonies and bee produces. but the pollination service that beekeepers is also seriously impeded.
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Research Interests: Biology and Population
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Sperm Digestion and Reciprocal Sperm Transfer Can Drive Hermaphrodite Sex Allocation to Equality. JM Greeff, NK Michiels American Naturalist 153:44, 421-430, 4/1999. The intensity of sperm competition determines how much reproductive... more
Sperm Digestion and Reciprocal Sperm Transfer Can Drive Hermaphrodite Sex Allocation to Equality. JM Greeff, NK Michiels American Naturalist 153:44, 421-430, 4/1999. The intensity of sperm competition determines how much reproductive effort should be invested in sperm. ...
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The optimal progeny sex ratio produced by a mother depends on the amount of competition between same sexed sibs (Hamilton, 1967; Clark, 1978) and on the mother’s relatedness to each sex (Hamilton, 1972; Herre, 1985). In haplodiploid... more
The optimal progeny sex ratio produced by a mother depends on the amount of competition between same sexed sibs (Hamilton, 1967; Clark, 1978) and on the mother’s relatedness to each sex (Hamilton, 1972; Herre, 1985). In haplodiploid species where the mating population is structured in groups of related individuals, two phenomena of importance occur: (1) brothers compete against each other for mating opportunities, referred to as local mating competition or LMC (Hamilton, 1967) and (2) sib-mating occurs, which results in mothers being more related to daughters than sons (Hamilton, 1972; Herre, 1985). Due to these two factors, optimal sex ratios become more female biased as relatedness in the local mating population increases. Agaonine fig wasps (Hymenoptera: Agaonidae: Agaoninae) have provided a useful empirical system for testing models of sex ratio optimization. Their usefulness is due to various aspects of their natural history, which is characterized by a mutualism with plants of the genus Ficus (Moraceae). Ficus inflorescences are shaped like a hollow ball, lined on the inside with hundreds or thousands of unisexual flowers. When the female flowers are ready to be pollinated, attractive volatiles are released (Ware et af., 1993) and the bracts lining the ostiole (a bract-lined tunnel) loosen to allow the female wasps to squeeze their way through (Galil, 1977). The duration of volatile release and ostiole opening are linked to pollinator entry. Unentered figs can remain attractive for several weeks, whereas after a single pollinator had entered syconia of Ecarica and Eaurea they were found to remain attractive for a maximum of 4 days (Khadari et al., 1995). Once one or more pollen-bearing mated fig wasp females (foundresses) have passed through the ostiole they reach the lumen of the fruit, where they pollinate the fig flowers and lay their eggs inside a proportion of the ovules. Females have a short adult life-span ofjust 1-3 days (Kjellberg etal., 1988; Compton et al.; 1994) and typically die inside the fruit. The length of the period between entry of the first wasp and the fig flowers becoming unsuitable for oviposition is not known. The offspring develop to maturity in the galled ovules, whereupon the male wasps emerge, chew holes in the females’ galls and mate with
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Sirex noctilio is an economically important invasive pest of commercial pine forestry in the Southern Hemisphere. Newly established invasive populations of this woodwasp are characterized by highly male‐biased sex ratios that subsequently... more
Sirex noctilio is an economically important invasive pest of commercial pine forestry in the Southern Hemisphere. Newly established invasive populations of this woodwasp are characterized by highly male‐biased sex ratios that subsequently revert to those seen in the native range. This trend was not observed in the population of S. noctilio from the summer rainfall regions in South Africa, which remained highly male‐biased for almost a decade. The aim of this study was to determine the cause of this persistent male bias. As an explanation for this pattern, we test hypotheses related to mating success, female investment in male versus female offspring, and genetic diversity affecting diploid male production due to complementary sex determination. We found that 61% of females in a newly established S. noctilio population were mated. Microsatellite data analysis showed that populations of S. noctilio from the summer rainfall regions in South Africa are far less genetically diverse than ...
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Calculation of the probability of sibmating, derivation of the indiscriminate model and supplementary tables.
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Book Title: DispersalBook editors: J. Clobert, E. Danchin,A.A. Dhondt & J.D. Nichols.Oxford University Press, Oxford. 2001. Pp. 452. Price £24.95 (paperback). ISBN 0 19 85065 9
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Additional file 4. Figure 10 data. Single Foundress sex ratios, fraction of single foundress figs and clutch size of 39 species.
Local mate competition (LMC) favours female biased clutch sex ratios because it reduces competition between brothers and provides extra mating opportunities for sons. Fig wasps seem to fit LMC model assumptions and lay female-biased sex... more
Local mate competition (LMC) favours female biased clutch sex ratios because it reduces competition between brothers and provides extra mating opportunities for sons. Fig wasps seem to fit LMC model assumptions and lay female-biased sex ratios as predicted. These female biased sex ratios increase fitness greatly. In line with predictions, their sex ratios become less female-biased as the number of mothers laying in the same fig increases. However, this variation results in comparatively small fitness benefits compared to just biased ratios and data suggest substantial mismatches with LMC theory. The mismatches are due to several factors. (1) Multiple foundresses typically lay too many daughters. (2) Single foundress sex ratios are explained by sequential oviposition and ladies-last models. (3) Mortality that typically exceeds 10% may decouple the link between primary sex ratios, the focus of model predictions, and secondary sex ratios of adult wasps that are counted by researchers. ...
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Additional file 1: Figure S1. Non-European admixture fractions (of K=6) sorted by ancestry fraction. Figure S2. Principal component analysis for PC1-PC10 and the variation explained by PCs. Figure S3. Results from f3-test. Populations are... more
Additional file 1: Figure S1. Non-European admixture fractions (of K=6) sorted by ancestry fraction. Figure S2. Principal component analysis for PC1-PC10 and the variation explained by PCs. Figure S3. Results from f3-test. Populations are colored according to regional affiliation. Figure S4. Results from f3-test. The CEU (A) and Khomani (B) populations are fixed to show the best African and non-African sources to the Afrikaner population. Figure S5. Fraction of shared private alleles between the Afrikaner population and a comparative population. Figure S6. Shared private alleles between the Afrikaner populations and populations with West-African ancestry. Figure S7. A) Afrikaner individuals (black circles) projected on a PCA based on European genetic variation from the POPRES dataset. B) Population variation on PC 1 and 2 summarized as averages and standard deviations. Figure S8. Admixture analyses of the Asian extended dataset. Figure S9. Manhattan plot of Locus specific branch len...
Sex allocation theory has long generated insights into the nature of natural selection. Classical models have elucidated causal phenomena such as local mate competition and inbreeding on the degree of female bias exhibited by various... more
Sex allocation theory has long generated insights into the nature of natural selection. Classical models have elucidated causal phenomena such as local mate competition and inbreeding on the degree of female bias exhibited by various invertebrates. Typically, these models assume mothers facultatively adjust sex allocation using predictive cues of future offspring mating conditions. Here we relax this assumption by developing a sex allocation model for haplodiploid mothers experiencing local mate competition that lay a fixed number of male eggs first. Female egg number is determined by remaining oviposition sites or remaining eggs of the mother, depending on which is exhausted first. Our model includes parameters for variation in foundress number, patch size, fecundity and offspring mortality that allow us to generate secondary sex ratio predictions based on specific parameterizations for natural populations. Simulations show that: 1) in line with classical models, factors that increase sib-mating result in mothers laying relatively more female eggs; 2) high offspring mortality leads to relatively more males as fertilization insurance; 3) unlike classical model predictions, sub-optimal predictions, such as more males than females are possible. In addition, our model provides the first quantitative predictions for the expected number of males and females in a patch where typically only one mother utilizes a given patch. We parameterized the model with data obtained from seven species of southern African fig wasps to predict expected means and variances for numbers of male and female offspring for typical numbers of mothers utilizing a patch. These predictions were compared to secondary sex ratio data from single foundress patches, the most commonly encountered situation for these species. Our predictions matched both the observed number and variance of male and female offspring with a high degree of accuracy suggesting that facultative adjustment is not required to produce evolutionary stable sex ratios
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While male mate choice in insects is a widely accepted concept, there is still limited evidence showing that lek formation is compatible with the evolution of male mate choice. In the woodwasp Sirex noctilio, males form leks that are used... more
While male mate choice in insects is a widely accepted concept, there is still limited evidence showing that lek formation is compatible with the evolution of male mate choice. In the woodwasp Sirex noctilio, males form leks that are used by females to select a mate. However, males have been observed to ignore certain females, suggesting the presence of male mate choice despite the presence of a lek mating system. In this study we demonstrate that males only attempt to mate with certain females. To understand the criteria used by males and females to select a mate, we also tested the effect of age, size, and male to female size ratio on the number of mating attempts made by males and on female receptivity. We demonstrate that size and age play a role in both male and female mate choice. Our results suggest that males must reach sexual maturity after emergence and are neither receptive nor attractive to females during the first few days of their lives. We also show that older females...
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Nematodes of figs and fig wasps have received limited attention in Africa since their discovery in 1973. Sixteen of the 25 species of native South African figs were sampled for nematode associates using molecular barcoding with three loci... more
Nematodes of figs and fig wasps have received limited attention in Africa since their discovery in 1973. Sixteen of the 25 species of native South African figs were sampled for nematode associates using molecular barcoding with three loci (SSU, LSU D2-D3 and mtCOI) and fourteen (93%) were positive for at least one nematode species. Thirty-three putative species of nematodes were identified and classified according to the loci that were amplified and successfully sequenced. Fourteen putative nematode species were classified as Aphelenchoididae, of which nine were identified as Ficophagus from four species of Ficus from the section Galoglychia (i.e., five ex F. burkei including one shared with F. natalensis, one ex F. glumosa, one ex F. lutea, and one ex F. stuhlmannii) and one species ex F. sur from the section Sycomorus. In addition, there were four nematode species classified as Schistonchus s.s. from section Galoglychia figs (i.e., one ex F. burkei, two ex F. trichopoda, and one e...