Can the spectrophotometric response of the elytra explain environmental preferences? A study in seven Onthophagus species (Coleoptera, Scarabaeidae)

Beetles are the most successful and diversified animal taxa characterized by the possession of an external pair of sclerotized wings (elytra). Managing electromagnetic radiations could be one of the functions of the exoskeleton. We studied the spectrophotometric response to ultraviolet, visible, and near-infrared radiations of the elytra of seven closely related and sympatric Onthophagus species to examine if the environmental preferences of these species could be associated with the spectrophotometric behaviour of their elytra. Our results indicated that sibling species can drastically differ in their environmental preferences but not in their spectrophotometric responses. However, our results corroborated that there are interspecific differences in the spectrophotometric characteristics of the elytra, which are mainly explained by morphological features. Among the examined morphological variables, darkness seems to be especially relevant as it facilitates the absorbance and obstructs the transmittance of visible and near-infrared radiations.

Incipient hybrid inferiority between recently introduced, diverging dung beetle populations

Understanding why and how certain clades emerge as speciation hotspots is a fundamental objective of evolutionary biology. Here we investigate divergences between exotic Onthophagus taurus, a dung beetle introduced into the USA and Australia in the 1970s, as a potential model for the widespread recent speciation events characterizing the genus Onthophagus. To do so, we hybridized O. taurus derived from Eastern US (EUS) and Western Australian (WA) populations, and assessed fitness-relevant trait expression in first- and second-generation hybrids. We found that dams invest more in offspring provisioning when paired with a sire from the same population, and that WA dams crossed with EUS sires produce smaller and lighter F1 hybrids, with an unexpectedly male-biased sex ratio. Furthermore, fewer F2 hybrids with vertically inherited WA cytoplasm and microbiome emerged compared with WA backcrosses with WA cytoplasm/microbiome, suggesting that combinations of nuclear genome, cytoplasm and/or microbiome may contribute to hybrid viability. Lastly, we document a dominance of WA genotypes over body size at the point of inflection between minor and major male morphs, a trait of significance in mate competition, which has diverged remarkably between these populations. We discuss our results in light of the evolutionary ecology of onthophagine beetles and the role of developmental evolution in clade diversification.

Coevolution between male and female genitalia in Belostoma angustum Lauck, 1964 (Insecta, Heteroptera, Belostomatidae): disentangling size and shape

Sexual and natural selection mechanisms might drive variation in the genitalia of male animals. All aforementioned mechanisms are known to predict the coevolution of male and female genital morphology. Belostoma angustum is known to have subtle variation in the male and female genitalia of its members. In this species, phallosoma with dorsal arms and ventral diverticulum are assumed to be intromittent male genital traits that interact with the female genital chamber. We thus evaluated the existence of variation after disentangling the size from the shape of male genitalia in B. angustum. Body and genitalia dimensions and photographs of phallosoma with dorsal arms, ventral diverticulum and lateral views of the right paramere (the non-intromittent part) were obtained. Semi-landmarks and landmarks were used to capture phenotypic variation, by eliminating all non-shape variation with a Procrustes superimposition. Male and female specimens collected from the same location or immediate vicinity were grouped, and 12 groups originating from 12 locations were used to conduct two block-Partial Least Squares analyses (PLS). Group structures were also taken into account by adopting a multilevel approach. The male and female genital traits had similarly shallow static allometry slopes, as well as the dispersion values around the mean (i.e. coefficient of variation) and the standard error of the estimate. The correlation between the pooled within-locality covariance matrix of the symmetric component of phallosoma with dorsal arms and the female genital chamber was significant (r-PLS=0.37), as well as that with male body dimensions (r-PLS=0.36), even after controlling for allometry. Specimens with lower PLS shape scores had narrower phallosoma with dorsal arms, with poorly curved outer margins of the dorsal arms, whereas specimens with higher PLS shape scores had slightly shorter dorsal arms, with strongly curved outer margins. Lower shape scores were associated with narrower and especially shorter and narrower female genital chambers. Similar shallow allometric curves among sexes and the correlation between intromittent male parts and the female genital chamber, as well as male dimensions, suggest the coevolution of these contact structures in size and in shape.

The evolution of relative trait size and shape: insights from the genitalia of dung beetles

Insects show relatively little genital variation within species compared to extraordinary and often rapid diversification among species. It has been suggested that selection for reproductive isolation through differences in genital shape might explain this phenomenon. This hypothesis predicts that populations diverge faster in genital shape than in genital size. We tested this prediction in males from 10 dung beetle species with known phylogenetic relationships from the genus Onthophagus (Coleoptera: Scarabaeidae), including four species for which we were able to sample multiple populations. Specifically, we compared intra- and interspecific differentiation in shape and relative sizes of genitalia and calculated their respective evolutionary rates. We compared these rates to two similarly sized non-genital traits, the head and the fore-tibia. We found significant intraspecific variation in genital shape in all four species for which multiple populations were sampled, but for three of them we also identified significant relative size variation. We also found that genital shape evolved at higher rates than relative genital size. Genital shape evolved faster than head shape, but not fore-tibia shape. However, shapes of all measured structures evolved faster than their relative size. We discuss the functional constraints that may bias the developmental evolution of relative size and shape of genitalia and other morphological traits.

Rapid Divergence of Nesting Depth and Digging Appendages among Tunneling Dung Beetle Populations and Species

Many dung beetle communities are characterized by species that share very similar morphological, ecological, and behavioral traits and requirements yet appear to be stably maintained. Here, we document that the morphologically nearly indistinguishable, sympatric, and syntopic tunneling sister species Onthophagus taurus and Onthophagus illyricus may be avoiding competitive exclusion by nesting at remarkably different soil depths. Intriguingly, we also find rapid divergence in preferred nesting depth across native and recently established O. taurus populations. Furthermore, geometric morphometric analyses reveal that both inter- and intraspecific divergences in nesting depth are paralleled by similar changes in the shape of the primary digging appendages, the fore tibiae. Collectively, our results identify preferred nesting depth and tibial shape as surprisingly evolutionarily labile and with the potential to ease interspecific competition and/or to facilitate adaptation to local climatic conditions.

Differentiation of ovarian development and the evolution of fecundity in rapidly diverging exotic beetle populations

Fecundity is a fundamental determinant of fitness, yet the proximate developmental and physiological mechanisms that enable its often rapid evolution in natural populations are poorly understood. Here, we investigated two populations of the dung beetle Onthophagus taurus that were established in exotic ranges in the early 1970s. These populations are subject to drastically different levels of resource competition in the field, and have diverged dramatically in female fecundity. Specifically, Western Australian O. taurus experience high levels of resource competition, and exhibit greatly elevated reproductive output compared to beetles from the Eastern US, where resource competition is minimal and female fecundity is low. We compared patterns of ovarian maturation, relative investment into and timing of egg production, and potential trade-offs between ovarian investment and the duration of larval development and adult body size between populations representative of both exotic ranges. We found that the rapid divergence in fecundity between exotic populations is associated with striking differences in several aspects of ovarian development: (1) Western Australian females exhibit accelerated ovarian development, (2) produce more eggs, (3) bigger eggs, and (4) start laying eggs earlier compared to their Eastern US counterparts. At the same time, divergence in ovarian maturation patterns occurred alongside changes in (5) larval developmental time, and (6) adult body size, and (7) mass. Western Australian females take longer to complete larval development and, surprisingly, emerge into smaller yet heavier adults than size-matched Eastern US females. We discuss our results in the context of the evolutionary developmental biology of fecundity in exotic populations.

Are Horn Morphological Patterns Able to Differentiate the Two Closely Related Species Copris klugi Harold and Copris sierrensis Matthews?

Several thousand species of beetles evolved impressive, exaggerated horns or horn-like structures. The horn phenotypic patterns and the developmental mechanisms are well documented especially in the Scarabaeidae, the family most predominated by species with horns. The regulation of horn expression appears to be extremely evolutionary labile to the extent that horn allometric patterns have been seen to rapidly diverge between closely related species. For this reason, it has been suggested that horn morphological pattern may be able to differentiate closely related and sibling species even when other traits fail. In this study, we used horn morphological pattern (shape and allometric variation) as a "tool" to evaluate the differentiation of two closely related scarab species, Copris klugi Harold and Copris sierrensis Matthews whose full species status has long been debated due to their high similarity. Combining traditional and geometric morphometric methods, we evidenced that male head horn phenotypic pattern is able to clearly differentiate C. klugi from C. sierrensis, supporting the hypothesis that they are two true species.

Shape--but not size--codivergence between male and female copulatory structures in Onthophagus beetles

Genitalia are among the fastest evolving morphological traits in arthropods. Among the many hypotheses aimed at explaining this observation, some explicitly or implicitly predict concomitant male and female changes of genital traits that interact during copulation (i.e., lock and key, sexual conflict, cryptic female choice and pleiotropy). Testing these hypotheses requires insights into whether male and female copulatory structures that physically interact during mating also affect each other's evolution and patterns of diversification. Here we compare and contrast size and shape evolution of male and female structures that are known to interact tightly during copulation using two model systems: (a) the sister species O. taurus (1 native, 3 recently established populations) and O. illyricus, and (b) the species-complex O. fracticornis-similis-opacicollis. Partial Least Squares analyses indicated very little to no correlation between size and shape of copulatory structures, both in males and females. Accordingly, comparing shape and size diversification patterns of genitalia within each sex showed that the two components diversify readily - though largely independently of each other - within and between species. Similarly, comparing patterns of divergence across sexes showed that relative sizes of male and female copulatory organs diversify largely independent of each other. However, performing this analysis for genital shape revealed a signature of parallel divergence. Our results therefore suggest that male and female copulatory structures that are linked mechanically during copulation may diverge in concert with respect to their shapes. Furthermore, our results suggest that genital divergence in general, and co-divergence of male and female genital shape in particular, can evolve over an extraordinarily short time frame. Results are discussed in the framework of the hypotheses that assume or predict concomitant evolutionary changes in male and female copulatory organs.

Combination of geometric morphometric and genetic approaches applied to a debated taxonomical issue: the status of Onthophagus massai (Coleoptera, Scarabaeidae) as an endemic species vicarious to Onthophagus fracticornis in Sicily

The present study deals with the phenomenon of insular speciation and discusses, as a case study, the debated taxonomical issue of the status of Onthophagus massai (Coleoptera, Sarabaeidae) as an endemic species vicarious to Onthophagus fracticornis in Sicily. The authors investigated the differentiation patterns between an insular population belonging to the supposed species O. massai (collected in its locus typicus, Piano Battaglia) and three Italian O. fracticornis populations (collected along a N-S latitudinal gradient). These patterns are described and analysed using multiple approaches: the qualitative inspection of the microsculpture of elytral surfaces, considered a diagnostic character for O. massai identification; the comparison of horn static allometries, known to be a good indicator of divergence processes between closely related species or isolated populations of the same species; the comparison of the patterns of shape and size difference of the head, epipharynx and genitalia attained with a combination of traditional and geometric (landmark and semilandmark) morphometric methods; and, finally, the estimation of the genetic relationships between Sicilian and continental populations obtained by analysing cytochrome oxidase subunit 1 mitochondrial gene sequences. The integration of the results of these approaches indicates that there is not sufficient evidence to vindicate the species status for O. massai, which should more likely be considered a small-sized version of O. fracticornis (a possible case of insular dwarfism). However, the complex pattern of shape, size and genetic variation observed between the populations analysed hinted at the possibility that a diversification process is ongoing, but not only between insular and continental populations; each population showed a tendency to evolve as an evolutionarily independent unit.