Dipartimento di Scienze della Vita e biologia dei Sistemi

This paper focuses on morphological (both shape and size) differences that quite similar polyphenic sister species evolve during divergence processes. Traits were analysed using a geometrical morphometric approach, which has the ability to evidence also very subtle differences in shape. As a case study, we considered males of the dung beetle sister species pair Onthophagus taurus and Onthophagus illyricus (Coleoptera, Scarabaeidae); these species represent a typical example of polyphenic trait expression concerning the facultative development of horns and considerable body size differences. External shape morphology failed to discriminate O. taurus from O. illyricus, whereas the reproductive system shape showed significant interspecific discrimination power. However, the head of O. taurus was significantly larger than that of O. illyricus and the reverse was true for the elytra. The two species also showed different allometric values of the head with respect to body size. This complex pattern of interspecific morphological divergence is discussed in the light of the differential trait divergence rate hypothesis. In both species, differences between major and minor forms concern the overall shape of head and pronotum: we suggest that such different forms, which likely reflect morphological readjustment to accommodate horns of considerable bulk and disproportionate length, may be nevertheless advantageously used by the two male morphs in their alternative reproductive tactics. Male genitalia sizes were virtually constant with respect to body size; however, the ratio between phallotheca and body size was significantly higher in minor males, in keeping with the hypothesis of a higher investment in genitalia borne by this morph.Cet article porte sur les différences morphologiques (au niveau de la forme et de la taille) que deux espèces voisines polyphéniques ont évolué au cours des processus de divergence. L’étude utilise les méthodes de la morphométrie géométrique, ayant la capacité de mettre en évidence des différences subtiles entre les formes. Comme modèle d’étude nous avons considèré les males de deux espèces voisines de coléoptères, Onthophagus taurus et Onthophagus illyricus (Coleoptera, Scarabaeidae), qui représentent un exemple typique d'expression polyphénique des traits morphologiques concernant des différences importantes de la taille du corps et le développement facultatif des cornes. L'analyse a mis en évidence que la forme des traits de la morphologie externe ne permette pas de distinguer O. taurus par rapport àO. illyricus, alors que la forme des structures génitales masculines montre un signifiant pouvoir de discrimination interspécifique. Cependant, la tête de O. taurus a une taille plus grande par rapport àO. illyricus, et l'inverse est vrai pour la taille des élytres. Les deux espèces montrent aussi une différence au niveau des valeurs allometriques de la taille de la tête respect à la taille du corps. Ce complexe pattern de différences morphologiques interspécifiques est discuté sous l'hypothèse d'un taux de divergence et d’évolution différentiele entre traits morphologiques différentes. Dans chaque espèce, les différences entre les males minor et major concernent la forme générale de la tête et du pronotum: on suggère que ces différences de forme, qui reflétent probablement des changements morphologiques nécessaires à accommoder des cornes d'un poids considérable et d'une longueur disproportionnée, sont, peut-être, cependant avantageuses dans les différentes stratégies reproductives utilisées par le deux morphes. La taille de la phallotheca est résultée virtuellement constant respect à la taille du corps; cependant, les résultats de la standardisation entre la taille de la phallotheca et la taille du corps sont significativement plus hauts dans les mâles minor, en accord avec l'hypothèse d'un plus important investissement dans le système reproductif soutenu par cette morphe.

The present study investigated the morphological and genetic differentiation pattern between two sympatric dung beetle sister species, Onthophagus taurus and Onthophagus illyricus. The geometric morphometric approach coupled with the use of molecular markers allowed examination of the nature of interspecific relationships and an outline of the evolutionary and geographical processes that might have led to interspecific differentiation and the present-day partial sympatric and syntopic pattern of distribution. Geometric morphometrics failed to discrimininate the two species on the ground of external morphological traits, but revealed interspecific differences when the shape of the primary sexual traits was taken into account. The use of two different molecular markers (cytochrome oxidase subunit I gene and amplified fragment-length polymorphism) demonstrates that the two species are genetically well differentiated, forming two distinguishable lineages probably diverged during the Pliocene by allopatric speciation. No evidence of past or recent introgression and no support for hybridization were found, suggesting that sympatry was established subsequently, when speciation was accomplished. Both molecular markers and genital shape indicate that phenotypically intermediate individuals, despite their ambiguous appearance, are members of O. illyricus species rather than hybrids. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 89, 197–211.

SUMMARY Onthophagus taurus is a polyphenic beetle in which males express alternative major (horned) and minor (hornless) morphologies largely dependent on larval nutrition. O. taurus was originally limited to a Turanic–European–Mediterranean distribution, but became introduced to several exotic regions in the late 1960s. Using geometric morphometrics, we investigate the present-day morphological shape differentiation patterns among native (Italian) and introduced (Western Australian and Eastern US) populations. We then contrast these divergences to those observed between native O. taurus and its sympatric sister species O. illyricus. Our analysis failed to find significant divergences between O. taurus populations in external morphological traits (head, pronotum) when analyses were conducted separately for each sex. However, when sexes and male morphs were analyzed together, three important differences among populations emerged. First, relative warp analyses showed that native and introduced populations diverged in certain shape components that normally distinguish major and minor male morphs. Second, comparison of covariation of body regions (head vs. pronotum) in the three populations showed that populations diverged in the nature of this covariation, suggesting that different body regions are not totally constrained to evolve in concert. Lastly, and most importantly, the analysis of genitalic shape revealed little to no divergence of female genitalia, but unexpected substantial differentiation of male genitalia among the three O. taurus populations. This suggests that genitalic shape divergence can occur extremely rapidly even in the absence of sympatry and possible reinforcement, and that the genitalia of males and females may diverge independent of one another, at least during the early stage of interpopulational divergence. Interpopulation divergences in O. taurus mirrored aspects of interspecific divergences between O. taurus and O. illyricus in some cases but not others.

This paper focuses on morphological (both shape and size) differences that quite similar polyphenic sister species evolve during divergence processes. Traits were analysed using a geometrical morphometric approach, which has the ability to evidence also very subtle differences in shape. As a case study, we considered males of the dung beetle sister species pair Onthophagus taurus and Onthophagus illyricus (Coleoptera, Scarabaeidae); these species represent a typical example of polyphenic trait expression concerning the facultative development of horns and considerable body size differences. External shape morphology failed to discriminate O. taurus from O. illyricus, whereas the reproductive system shape showed significant interspecific discrimination power. However, the head of O. taurus was significantly larger than that of O. illyricus and the reverse was true for the elytra. The two species also showed different allometric values of the head with respect to body size. This complex pattern of interspecific morphological divergence is discussed in the light of the differential trait divergence rate hypothesis. In both species, differences between major and minor forms concern the overall shape of head and pronotum: we suggest that such different forms, which likely reflect morphological readjustment to accommodate horns of considerable bulk and disproportionate length, may be nevertheless advantageously used by the two male morphs in their alternative reproductive tactics. Male genitalia sizes were virtually constant with respect to body size; however, the ratio between phallotheca and body size was significantly higher in minor males, in keeping with the hypothesis of a higher investment in genitalia borne by this morph.Cet article porte sur les différences morphologiques (au niveau de la forme et de la taille) que deux espèces voisines polyphéniques ont évolué au cours des processus de divergence. L’étude utilise les méthodes de la morphométrie géométrique, ayant la capacité de mettre en évidence des différences subtiles entre les formes. Comme modèle d’étude nous avons considèré les males de deux espèces voisines de coléoptères, Onthophagus taurus et Onthophagus illyricus (Coleoptera, Scarabaeidae), qui représentent un exemple typique d'expression polyphénique des traits morphologiques concernant des différences importantes de la taille du corps et le développement facultatif des cornes. L'analyse a mis en évidence que la forme des traits de la morphologie externe ne permette pas de distinguer O. taurus par rapport àO. illyricus, alors que la forme des structures génitales masculines montre un signifiant pouvoir de discrimination interspécifique. Cependant, la tête de O. taurus a une taille plus grande par rapport àO. illyricus, et l'inverse est vrai pour la taille des élytres. Les deux espèces montrent aussi une différence au niveau des valeurs allometriques de la taille de la tête respect à la taille du corps. Ce complexe pattern de différences morphologiques interspécifiques est discuté sous l'hypothèse d'un taux de divergence et d’évolution différentiele entre traits morphologiques différentes. Dans chaque espèce, les différences entre les males minor et major concernent la forme générale de la tête et du pronotum: on suggère que ces différences de forme, qui reflétent probablement des changements morphologiques nécessaires à accommoder des cornes d'un poids considérable et d'une longueur disproportionnée, sont, peut-être, cependant avantageuses dans les différentes stratégies reproductives utilisées par le deux morphes. La taille de la phallotheca est résultée virtuellement constant respect à la taille du corps; cependant, les résultats de la standardisation entre la taille de la phallotheca et la taille du corps sont significativement plus hauts dans les mâles minor, en accord avec l'hypothèse d'un plus important investissement dans le système reproductif soutenu par cette morphe.

The present study investigated the morphological and genetic differentiation pattern between two sympatric dung beetle sister species, Onthophagus taurus and Onthophagus illyricus. The geometric morphometric approach coupled with the use of molecular markers allowed examination of the nature of interspecific relationships and an outline of the evolutionary and geographical processes that might have led to interspecific differentiation and the present-day partial sympatric and syntopic pattern of distribution. Geometric morphometrics failed to discrimininate the two species on the ground of external morphological traits, but revealed interspecific differences when the shape of the primary sexual traits was taken into account. The use of two different molecular markers (cytochrome oxidase subunit I gene and amplified fragment-length polymorphism) demonstrates that the two species are genetically well differentiated, forming two distinguishable lineages probably diverged during the Pliocene by allopatric speciation. No evidence of past or recent introgression and no support for hybridization were found, suggesting that sympatry was established subsequently, when speciation was accomplished. Both molecular markers and genital shape indicate that phenotypically intermediate individuals, despite their ambiguous appearance, are members of O. illyricus species rather than hybrids. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 89, 197–211.

SUMMARY Onthophagus taurus is a polyphenic beetle in which males express alternative major (horned) and minor (hornless) morphologies largely dependent on larval nutrition. O. taurus was originally limited to a Turanic–European–Mediterranean distribution, but became introduced to several exotic regions in the late 1960s. Using geometric morphometrics, we investigate the present-day morphological shape differentiation patterns among native (Italian) and introduced (Western Australian and Eastern US) populations. We then contrast these divergences to those observed between native O. taurus and its sympatric sister species O. illyricus. Our analysis failed to find significant divergences between O. taurus populations in external morphological traits (head, pronotum) when analyses were conducted separately for each sex. However, when sexes and male morphs were analyzed together, three important differences among populations emerged. First, relative warp analyses showed that native and introduced populations diverged in certain shape components that normally distinguish major and minor male morphs. Second, comparison of covariation of body regions (head vs. pronotum) in the three populations showed that populations diverged in the nature of this covariation, suggesting that different body regions are not totally constrained to evolve in concert. Lastly, and most importantly, the analysis of genitalic shape revealed little to no divergence of female genitalia, but unexpected substantial differentiation of male genitalia among the three O. taurus populations. This suggests that genitalic shape divergence can occur extremely rapidly even in the absence of sympatry and possible reinforcement, and that the genitalia of males and females may diverge independent of one another, at least during the early stage of interpopulational divergence. Interpopulation divergences in O. taurus mirrored aspects of interspecific divergences between O. taurus and O. illyricus in some cases but not others.

This paper focuses on morphological (both shape and size) differences that quite similar polyphenic sister species evolve during divergence processes. Traits were analysed using a geometrical morphometric approach, which has the ability to evidence also very subtle differences in shape. As a case study, we considered males of the dung beetle sister species pair Onthophagus taurus and Onthophagus illyricus (Coleoptera, Scarabaeidae); these species represent a typical example of polyphenic trait expression concerning the facultative development of horns and considerable body size differences. External shape morphology failed to discriminate O. taurus from O. illyricus, whereas the reproductive system shape showed significant interspecific discrimination power. However, the head of O. taurus was significantly larger than that of O. illyricus and the reverse was true for the elytra. The two species also showed different allometric values of the head with respect to body size. This complex pattern of interspecific morphological divergence is discussed in the light of the differential trait divergence rate hypothesis. In both species, differences between major and minor forms concern the overall shape of head and pronotum: we suggest that such different forms, which likely reflect morphological readjustment to accommodate horns of considerable bulk and disproportionate length, may be nevertheless advantageously used by the two male morphs in their alternative reproductive tactics. Male genitalia sizes were virtually constant with respect to body size; however, the ratio between phallotheca and body size was significantly higher in minor males, in keeping with the hypothesis of a higher investment in genitalia borne by this morph.Cet article porte sur les différences morphologiques (au niveau de la forme et de la taille) que deux espèces voisines polyphéniques ont évolué au cours des processus de divergence. L’étude utilise les méthodes de la morphométrie géométrique, ayant la capacité de mettre en évidence des différences subtiles entre les formes. Comme modèle d’étude nous avons considèré les males de deux espèces voisines de coléoptères, Onthophagus taurus et Onthophagus illyricus (Coleoptera, Scarabaeidae), qui représentent un exemple typique d'expression polyphénique des traits morphologiques concernant des différences importantes de la taille du corps et le développement facultatif des cornes. L'analyse a mis en évidence que la forme des traits de la morphologie externe ne permette pas de distinguer O. taurus par rapport àO. illyricus, alors que la forme des structures génitales masculines montre un signifiant pouvoir de discrimination interspécifique. Cependant, la tête de O. taurus a une taille plus grande par rapport àO. illyricus, et l'inverse est vrai pour la taille des élytres. Les deux espèces montrent aussi une différence au niveau des valeurs allometriques de la taille de la tête respect à la taille du corps. Ce complexe pattern de différences morphologiques interspécifiques est discuté sous l'hypothèse d'un taux de divergence et d’évolution différentiele entre traits morphologiques différentes. Dans chaque espèce, les différences entre les males minor et major concernent la forme générale de la tête et du pronotum: on suggère que ces différences de forme, qui reflétent probablement des changements morphologiques nécessaires à accommoder des cornes d'un poids considérable et d'une longueur disproportionnée, sont, peut-être, cependant avantageuses dans les différentes stratégies reproductives utilisées par le deux morphes. La taille de la phallotheca est résultée virtuellement constant respect à la taille du corps; cependant, les résultats de la standardisation entre la taille de la phallotheca et la taille du corps sont significativement plus hauts dans les mâles minor, en accord avec l'hypothèse d'un plus important investissement dans le système reproductif soutenu par cette morphe.

Horns of Onthophagus beetles are typical examples of phenotypically plastic traits: they are expressed as a function of environmental (nutritional) stimuli, and their reaction norm (i.e. the full set of horn lengths expressed as a response to different degrees of nutritional states) can be either linear or threshold-dependent. Horned males of Onthophagus (Palaeonthophagus) fracticornis (Preyssler, 1790) bear a single triangular cephalic protrusion of vertex carina, which has received phylogenetic support as the most primitive horn shape in the genus. Inter- and intra-sexual patterns of horn expression were studied in O. fracticornis by means of static allometries while associated variations in head shape were assessed using geometric morphometric techniques. The relation between log-transformed measurements of body size and vertex carina supported an isometric scaling in females. On the contrary, a sigmoidal model described better the horn length-body size allometry in males, with a switch point between alternative morphs at a pronotum width of 3.88 mm. Sigmoidal static allometries of horns in Onthophagus populations arise from a threshold-dependent developmental process of horn growth. This process underlies the expression of both plesiomorphic and apomorphic horn shapes in the genus. Given that the single-horn model has been identified as primitive, we propose that such a developmental process giving rise to it may be evolutionarily ancient as well. Horn expression was accompanied by a deformation of the head which makes minor and major morphs appear even more different. Therefore, in this species both horn and head shape expression contribute to male dimorphism.Le strutture cefaliche degli Scarabeidi sono tipici esempi di tratti soggetti a plasticità fenotipica: vengono espresse in funzione di stimoli ambientali e le norme di reazione sottese al loro sviluppo, ovvero l’intero range di lunghezze delle corna espresso in risposta a differenti condizioni nutrizionali della larva, può avere andamento lineare o essere caratterizzate da un effetto soglia. In Onthophagus (Palaeonthophagus) fracticornis (Preyssler, 1790) i maschi possono esprimere una singola protrusione cefalica triangolare, emergente dalla carena del vertice ed estesa in forma di lamina, un carattere considerato plesiomorfo all’interno del genere, mentre le femmine esibiscono una carena del vertex pronunciata, senza tuttavia sviluppare corna. I pattern inter- e intrasessuali di espressione del corno di O. fracticornis sono stati analizzati in termini di relazioni di allometria statica; le modificazioni della forma complessiva del capo che si evidenziano in concomitanza con lo sviluppo del corno sono state indagate attraverso l’uso di tecniche di morfometria geometrica. Nelle femmine, la regressione tra le misure, logaritmicamente trasformate, della larghezza del pronoto e dell’altezza della carena cefalica indica una relazione isometrica tra queste due variabili; al contrario, nei maschi questa relazione risulta meglio descritta da un modello sigmoidale. La transizione tra maschi minor e major avviene ad una larghezza del pronoto di 3.88 mm. Le allometrie statiche sigmoidali già descritte nelle popolazioni di Onthophagus risultano da specifici processi di sviluppo che coinvolgono soglie di sensibilità alla neotenina nei singoli individui. Si può ritenere che tali processi siano relativamente antichi, in considerazione del fatto che sono alla base dello sviluppo polifenico in specie che esprimono strutture sessuali secondarie plesiomorfe. Nella specie analizzata, l’espressione del corno è accompagnata da una deformazione cefalica che differenzia le morfe anche sulla base della morfologia generale del capo: di conseguenza, sia il grado di espressione del corno sia la forma del capo concorrono a determinare il dimorfismo maschile.

The present study investigated the morphological and genetic differentiation pattern between two sympatric dung beetle sister species, Onthophagus taurus and Onthophagus illyricus. The geometric morphometric approach coupled with the use of molecular markers allowed examination of the nature of interspecific relationships and an outline of the evolutionary and geographical processes that might have led to interspecific differentiation and the present-day partial sympatric and syntopic pattern of distribution. Geometric morphometrics failed to discrimininate the two species on the ground of external morphological traits, but revealed interspecific differences when the shape of the primary sexual traits was taken into account. The use of two different molecular markers (cytochrome oxidase subunit I gene and amplified fragment-length polymorphism) demonstrates that the two species are genetically well differentiated, forming two distinguishable lineages probably diverged during the Pliocene by allopatric speciation. No evidence of past or recent introgression and no support for hybridization were found, suggesting that sympatry was established subsequently, when speciation was accomplished. Both molecular markers and genital shape indicate that phenotypically intermediate individuals, despite their ambiguous appearance, are members of O. illyricus species rather than hybrids. © 2006 The Linnean Society of London, Biological Journal of the Linnean Society, 2006, 89, 197–211.

SUMMARY Onthophagus taurus is a polyphenic beetle in which males express alternative major (horned) and minor (hornless) morphologies largely dependent on larval nutrition. O. taurus was originally limited to a Turanic–European–Mediterranean distribution, but became introduced to several exotic regions in the late 1960s. Using geometric morphometrics, we investigate the present-day morphological shape differentiation patterns among native (Italian) and introduced (Western Australian and Eastern US) populations. We then contrast these divergences to those observed between native O. taurus and its sympatric sister species O. illyricus. Our analysis failed to find significant divergences between O. taurus populations in external morphological traits (head, pronotum) when analyses were conducted separately for each sex. However, when sexes and male morphs were analyzed together, three important differences among populations emerged. First, relative warp analyses showed that native and introduced populations diverged in certain shape components that normally distinguish major and minor male morphs. Second, comparison of covariation of body regions (head vs. pronotum) in the three populations showed that populations diverged in the nature of this covariation, suggesting that different body regions are not totally constrained to evolve in concert. Lastly, and most importantly, the analysis of genitalic shape revealed little to no divergence of female genitalia, but unexpected substantial differentiation of male genitalia among the three O. taurus populations. This suggests that genitalic shape divergence can occur extremely rapidly even in the absence of sympatry and possible reinforcement, and that the genitalia of males and females may diverge independent of one another, at least during the early stage of interpopulational divergence. Interpopulation divergences in O. taurus mirrored aspects of interspecific divergences between O. taurus and O. illyricus in some cases but not others.