Ecological selection as the cause and sexual differentiation as the consequence of species divergence?

Testing drivers of acoustic divergence in cicadas (Cicadidae: Tettigettalna)

Divergence in acoustic signals may have a crucial role in the speciation process of animals that rely on sound for intra-specific recognition and mate attraction. The acoustic adaptation hypothesis (AAH) postulates that signals should diverge according to the physical properties of the signalling environment. To be efficient, signals should maximize transmission and decrease degradation. To test which drivers of divergence exert the most influence in a speciose group of insects, we used a phylogenetic approach to the evolution of acoustic signals in the cicada genus Tettigettalna, investigating the relationship between acoustic traits (and their mode of evolution) and body size, climate and micro-/macro-habitat usage. Different traits showed different evolutionary paths. While acoustic divergence was generally independent of phylogenetic history, some temporal variables' divergence was associated with genetic drift. We found support for ecological adaptation at the temporal but not the spectral level. Temporal patterns are correlated with micro- and macro-habitat usage and temperature stochasticity in ways that run against the AAH predictions, degrading signals more easily. These traits are likely to have evolved as an anti-predator strategy in conspicuous environments and low-density populations. Our results support a role of ecological selection, not excluding a likely role of sexual selection in the evolution of Tettigettalna calling songs, which should be further investigated in an integrative approach.

Minimal prezygotic isolation between ecologically divergent sibling species

Divergence in mating signals typically accompanies speciation. We examine two ecologically divergent sibling species of crickets to assess the degree and timing of the evolution of prezygotic reproductive isolation. Gryllus saxatilis occurs in rocky habitats throughout western North America with long-winged individuals capable of long-distance dispersal; Gryllus navajo is endemic to red-rock sandstone areas of south-eastern Utah and north-eastern Arizona and has short-winged individuals only capable of limited dispersal. Previous genetic work suggested some degree of introgression and/or incomplete lineage sorting is likely. Here we: (1) use restriction site associated DNA sequencing (RAD-seq) genetic data to describe the degree of genetic divergence among species and populations; (2) examine the strength of prezygotic isolation by (i) quantifying differences among male mating songs, and (ii) testing whether females prefer G. saxatilis or G. navajo calling songs. Our results show that genetically distinct "pure" species populations and genetically intermediate populations exist. Male mating songs are statistically distinguishable, but the absolute differences are small. In playback experiments, females from pure populations had no preference based on song; however, females from a genetically intermediate population preferred G. navajo song. Together these results suggest that prezygotic isolation is minimal, and mediated by female behaviour in admixed populations.

Multivariate stabilizing sexual selection and the evolution of male and female genital morphology in the red flour beetle

Male genitals are highly divergent in animals with internal fertilization. Most studies attempting to explain this diversity have focused on testing the major hypotheses of genital evolution (the lock‐and‐key, pleiotropy, and sexual selection hypotheses), and quantifying the form of selection targeting male genitals has played an important role in this endeavor. However, we currently know far less about selection targeting female genitals or how male and female genitals interact during mating. Here, we use formal selection analysis to show that genital size and shape is subject to strong multivariate stabilizing sexual selection in both sexes of the red flour beetle, Tribolium castaneum. Moreover, we show significant sexual selection on the covariance between the sexes for specific aspects of genital shape suggesting that male and female genitalia also interact to determine the successful transfer of a spermatophore during mating. Our work therefore highlights the important role that both male and female genital morphologies play in determining mating success and that these effects can occur independently, as well as through their interaction. Moreover, it cautions against the overly simplistic view that the sexual selection targeting genital morphology will always be directional in form and restricted primarily to males.

Geographic distributions, phenotypes, and phylogenetic relationships of Phalloceros (Cyprinodontiformes: Poeciliidae): Insights about diversification among sympatric species pools

With 22 described species, Phalloceros is the most species-rich genus of Poeciliidae in South America. Phalloceros diversity is characterized by high degrees of endemism and sympatry in coastal and inland drainages in southeastern South America. The taxa are also characterized by pronounced differentiation in sexual characters (i.e., female urogenital papilla and male gonopodium), which might have contributed to their diversification. Here we estimate phylogenetic relationships based on more than 18,000 loci in 93 individuals representing 19 described species and two putative undescribed species. Morphologically defined species correspond to monophyletic species lineages, with individuals within a species clustering together in phylogenetic estimates, with the main exception being P. harpagos, supporting undiscovered diversity in this morphospecies. Shifts in the female and male sexual traits (i.e., urogenital papilla and gonopodium) occurred in concert multiple times along the phylogeny highlighting the role of sexual selection in driving divergence in this genus. Out of 22 valid species, 14 species are found in sympatry with at least one other species of this genus. However, most co-occurrences are observed among non-sister species suggesting that diversification among closely related species involved mostly allopatric speciation, with only two instances of sympatric sister-species observed. A strong mismatch in sexual traits among sympatric taxa suggests that co-existence may be linked to divergent sexual traits that maintain species genetic distinctiveness through mechanical disruptions of interbreeding.

Genome-scale data reveal that endemic Poecilia populations from small sulphidic springs display no evidence of inbreeding

Populations with limited ranges can be highly vulnerable to changes in their environment and are, thus, of high conservation concern. Populations that experience human-induced range reductions are often highly inbred and lack genetic diversity, but it is unknown whether this is also the case for populations with naturally small ranges. The fishes Poecilia sulphuraria (listed as critically endangered) and Poecilia thermalis, which are endemic to small hydrogen sulphide-rich springs in southern Mexico, are examples of such populations with inherently small habitats. We used geometric morphometrics and population genetics to quantify phenotypic and genetic variation within and among two populations of P. sulphuraria and one population of P. thermalis. Principal component analyses revealed phenotypic and genetic differences among the populations. Evidence for inbreeding was low compared to populations that have undergone habitat reduction. The genetic data were also used to infer the demographic history of these populations to obtain estimates for effective population sizes and migration rates. Effective population sizes were large given the small habitats of these populations. Our results imply that these three endemic extremophile populations should each be considered separately for conservation purposes. Additionally, this study suggests that populations in naturally small habitats may have lower rates of inbreeding and higher genetic diversity than expected, and therefore may be better equipped to handle environmental perturbations than anticipated. We caution, however, that the inferred lack of inbreeding and the large effective population sizes could potentially be a result of colonization by genetically diverse ancestors.

Community assembly in Lake Tanganyika cichlid fish: quantifying the contributions of both niche-based and neutral processes

The cichlid family features some of the most spectacular examples of adaptive radiation. Evolutionary studies have highlighted the importance of both trophic adaptation and sexual selection in cichlid speciation. However, it is poorly understood what processes drive the composition and diversity of local cichlid species assemblages on relatively short, ecological timescales. Here, we investigate the relative importance of niche-based and neutral processes in determining the composition and diversity of cichlid communities inhabiting various environmental conditions in the littoral zone of Lake Tanganyika, Zambia. We collected data on cichlid abundance, morphometrics, and local environments. We analyzed relationships between mean trait values, community composition, and environmental variation, and used a recently developed modeling technique (STEPCAM) to estimate the contributions of niche-based and neutral processes to community assembly. Contrary to our expectations, our results show that stochastic processes, and not niche-based processes, were responsible for the majority of cichlid community assembly. We also found that the relative importance of niche-based and neutral processes was constant across environments. However, we found significant relationships between environmental variation, community trait means, and community composition. These relationships were caused by niche-based processes, as they disappeared in simulated, purely neutrally assembled communities. Importantly, these results can potentially reconcile seemingly contrasting findings in the literature about the importance of either niche-based or neutral-based processes in community assembly, as we show that significant trait relationships can already be found in nearly (but not completely) neutrally assembled communities; that is, even a small deviation from neutrality can have major effects on community patterns.

Effects of Temperature, Photoperiod, and Rainfall on Morphometric Variation of Diaphorina citri (Hemiptera: Liviidae)

Phenotypic plasticity provides a mechanism by which an organism can adapt to new or changing environments. Earlier studies have demonstrated the variability of Diaphorina citri Kuwayama (Asian citrus psyllid) population dynamics, but no analysis of morphological changes induced by seasonal or artificial laboratory-induced conditions has yet been documented. Such morphometric variation has been found to correspond in dispersal capabilities in several insect taxa. In this study, the effects of temperature and photoperiod on morphometric variation of D. citri were examined through laboratory rearing of psyllids under controlled temperatures (20 °C, 28 °C, and 30 °C) and under a short photoperiod of 10.5:13.5 (L:D) h and a long photoperiod of 16:8 (L:D) h. Diaphorina citri were field-collected monthly from three citrus groves in Fort Pierce, Gainesville, and Immokalee, FL, to evaluate potential field-associated environmental effects. Both traditional and geometric morphometric data were used to analyze the correlation between environmental and morphometric variation. A strong correlation was found between temperature and shape change, with larger and broader wings at colder temperatures in the laboratory. Short day length resulted in shorter and narrower wings as well. From the field, temperature, rainfall, and photoperiod were moderately associated with shape parameters. Adult D. citri with blue/green abdomens collected in the laboratory and field studies were larger in size and shape than those with brown/gray abdomens.

Differentiation in putative male sex pheromone components across and within populations of the African butterfly Bicyclus anynana as a potential driver of reproductive isolation

Sexual traits are often the most divergent characters among closely related species, suggesting an important role of sexual traits in speciation. However, to prove this, we need to show that sexual trait differences accumulate before or during the speciation process, rather than being a consequence of it. Here, we contrast patterns of divergence among putative male sex pheromone (pMSP) composition and the genetic structure inferred from variation in the mitochondrial cytochrome oxidase 1 and nuclear CAD loci in the African butterfly Bicyclus anynana (Butler, 1879) to determine whether the evolution of "pheromonal dialects" occurs before or after the differentiation process. We observed differences in abundance of some shared pMSP components as well as differences in the composition of the pMSP among B. anynana populations. In addition, B. anynana individuals from Kenya displayed differences in the pMSP composition within a single population that appeared not associated with genetic differences. These differences in pMSP composition both between and within B. anynana populations were as large as those found between different Bicyclus species. Our results suggest that "pheromonal dialects" evolved within and among populations of B. anynana and may therefore act as precursors of an ongoing speciation process.

Genomic heterozygosity and hybrid breakdown in cotton (Gossypium): different traits, different effects

Background

Hybrid breakdown has been well documented in various species. Relationships between genomic heterozygosity and traits-fitness have been extensively explored especially in the natural populations. But correlations between genomic heterozygosity and vegetative and reproductive traits in cotton interspecific populations have not been studied. In the current study, two reciprocal F₂ populations were developed using Gossypium hirsutum cv. Emian 22 and G. barbadense acc. 3–79 as parents to study hybrid breakdown in cotton. A total of 125 simple sequence repeat (SSR) markers were used to genotype the two F₂ interspecific populations.

Results

To guarantee mutual independence among the genotyped markers, the 125 SSR markers were checked by the linkage disequilibrium analysis. To our knowledge, this is a novel approach to evaluate the individual genomic heterozygosity. After marker checking, 83 common loci were used to assess the extent of genomic heterozygosity. Hybrid breakdown was found extensively in the two interspecific F₂ populations particularly on the reproductive traits because of the infertility and the bare seeds. And then, the relationships between the genomic heterozygosity and the vegetative reproductive traits were investigated. The only relationships between hybrid breakdown and heterozygosity were observed in the (Emian22 × 3–79) F₂ population for seed index (SI) and boll number per plant (BN). The maternal cytoplasmic environment may have a significant effect on genomic heterozygosity and on correlations between heterozygosity and reproductive traits.

Conclusions

A novel approach was used to evaluate genomic heterozygosity in cotton; and hybrid breakdown was observed in reproductive traits in cotton. These findings may offer new insight into hybrid breakdown in allotetraploid cotton interspecific hybrids, and may be useful for the development of interspecific hybrids for cotton genetic improvement.

Electronic supplementary material

The online version of this article (doi:10.1186/s12863-016-0366-5) contains supplementary material, which is available to authorized users.

Biomechanical Diversity of Mating Structures among Harvestmen Species Is Consistent with a Spectrum of Precopulatory Strategies

Diversity in reproductive structures is frequently explained by selection acting at individual to generational timescales, but interspecific differences predicted by such models (e.g., female choice or sexual conflict) are often untestable in a phylogenetic framework. An alternative approach focuses on clade- or function-specific hypotheses that predict evolutionary patterns in terms neutral to specific modes of sexual selection. Here we test a hypothesis that diversity of reproductive structures in leiobunine harvestmen (daddy longlegs) of eastern North America reflects two sexually coevolved but non-overlapping precopulatory strategies, a primitive solicitous strategy (females enticed by penis-associated nuptial gifts), and a multiply derived antagonistic strategy (penis exerts mechanical force against armature of the female pregenital opening). Predictions of sexual coevolution and fidelity to precopulatory categories were tested using 10 continuously varying functional traits from 28 species. Multivariate analyses corroborated sexual coevolution but failed to partition species by precopulatory strategy, with multiple methods placing species along a spectrum of mechanical antagonistic potential. These findings suggest that precopulatory features within species reflect different co-occurring levels of solicitation and antagonism, and that gradualistic evolutionary pathways exist between extreme strategies. The ability to quantify antagonistic potential of precopulatory structures invites comparison with ecological variables that may promote evolutionary shifts in precopulatory strategies.

Human-caused habitat fragmentation can drive rapid divergence of male genitalia

The aim of this study rests on three premises: (i) humans are altering ecosystems worldwide, (ii) environmental variation often influences the strength and nature of sexual selection, and (iii) sexual selection is largely responsible for rapid and divergent evolution of male genitalia. While each of these assertions has strong empirical support, no study has yet investigated their logical conclusion that human impacts on the environment might commonly drive rapid diversification of male genital morphology. We tested whether anthropogenic habitat fragmentation has resulted in rapid changes in the size, allometry, shape, and meristics of male genitalia in three native species of livebearing fishes (genus: Gambusia) inhabiting tidal creeks across six Bahamian islands. We found that genital shape and allometry consistently and repeatedly diverged in fragmented systems across all species and islands. Using a model selection framework, we identified three ecological consequences of fragmentation that apparently underlie observed morphological patterns: decreased predatory fish density, increased conspecific density, and reduced salinity. Our results demonstrate that human modifications to the environment can drive rapid and predictable divergence in male genitalia. Given the ubiquity of anthropogenic impacts on the environment, future research should evaluate the generality of our findings and potential consequences for reproductive isolation.