Hydrocarbon divergence and reproductive isolation in Timema stick insects

Divergent dynamics of sexual and habitat isolation at the transition between stick insect populations and species

Speciation is often viewed as a continuum along which populations diverge until they become reproductively-isolated species. However, such divergence may be heterogeneous, proceeding in fits and bursts, rather than being uniform and gradual. We show in Timema stick insects that one component of reproductive isolation evolves non-uniformly across this continuum, whereas another does not. Specifically, we use thousands of host-preference and mating trials to study habitat and sexual isolation among 42 pairs of taxa spanning a range of genomic differentiation and divergence time. We find that habitat isolation is uncoupled from genomic differentiation within species, but accumulates linearly with it between species. In contrast, sexual isolation accumulates linearly across the speciation continuum, and thus exhibits similar dynamics to morphological traits not implicated in reproductive isolation. The results show different evolutionary dynamics for different components of reproductive isolation and highlight a special relevance for species status in the process of speciation.

The role of sexual isolation during rapid ecological divergence: Evidence for a new dimension of isolation in Rhagoletis pomonella

The pace of divergence and likelihood of speciation often depends on how and when different types of reproductive barriers evolve. Questions remain about how reproductive isolation evolves after initial divergence. We tested for the presence of sexual isolation (reduced mating between populations due to divergent mating preferences and traits) in Rhagoletis pomonella flies, a model system for incipient ecological speciation. We measured the strength of sexual isolation between two very recently diverged (~170 generations) sympatric populations, adapted to different host fruits (hawthorn and apple). We found that flies from both populations were more likely to mate within than between populations. Thus, sexual isolation may play an important role in reducing gene flow allowed by early-acting ecological barriers. We also tested how warmer temperatures predicted under climate change could alter sexual isolation and found that sexual isolation was markedly asymmetric under warmer temperatures - apple males and hawthorn females mated randomly while apple females and hawthorn males mated more within populations than between. Our findings provide a window into the early speciation process and the role of sexual isolation after initial ecological divergence, in addition to examining how environmental conditions could shape the likelihood of further divergence.

Disruptive selection and the evolution of discrete color morphs in Timema stick insects

A major unresolved issue in biology is why phenotypic and genetic variation is sometimes continuous, yet other times packaged into discrete units of diversity, such as morphs, ecotypes, and species. In theory, ecological discontinuities can impose strong disruptive selection that promotes the evolution of discrete forms, but direct tests of this hypothesis are lacking. Here, we show that Timema stick insects exhibit genetically determined color morphs that range from weakly to strongly discontinuous. Color data from nature and a manipulative field experiment demonstrate that greater morph differentiation is associated with shifts from host plants exhibiting more continuous color variation to those exhibiting greater coloration distance between green leaves and brown stems, the latter of which generates strong disruptive selection. Our results show how ecological factors can promote discrete variation, and we further present results on how this can have variable effects on the genetic differentiation that promotes speciation.

Species recognition limits mating between hybridizing ant species

Identifying mechanisms limiting hybridization is a central goal of speciation research. Here, we studied premating and postmating barriers to hybridization between two ant species, Formica selysi and Formica cinerea. These species hybridize in the Rhône valley in Switzerland, where they form a mosaic hybrid zone, with limited introgression from F. selysi into F. cinerea. There was no sign of temporal isolation between the two species in the production of queens and males. With choice experiments, we showed that queens and males strongly prefer to mate with conspecifics. Yet, we did not detect postmating barriers caused by genetic incompatibilities. Specifically, hybrids of all sexes and castes were found in the field and F1 hybrid workers did not show reduced viability compared to nonhybrid workers. To gain insights into the cues involved in species recognition, we analyzed the cuticular hydrocarbons (CHCs) of queens, males, and workers and staged dyadic encounters between workers. CHC profiles differed markedly between species, but were similar in F. cinerea and hybrids. Accordingly, workers also discriminated species, but they did not discriminate F. cinerea and hybrids. We discuss how the CHC-based recognition system of ants may facilitate the establishment of premating barriers to hybridization, independent of hybridization costs.

Cuticular Hydrocarbon Plasticity in Three Rice Planthopper Species

Insect cuticular hydrocarbons (CHCs) are organic compounds of the surface lipid layer, which function as a barrier against water loss and xenobiotic penetration, while also serving as chemical signals. Plasticity of CHC profiles can vary depending upon numerous biological and environmental factors. Here, we investigated potential sources of variation in CHC profiles of Nilaparvata lugens, Laodelphax striatellus and Sogatella furcifera, which are considered to be the most important rice pests in Asia. CHC profiles were quantified by GC/MS, and factors associated with variations were explored by conducting principal component analysis (PCA). Transcriptomes were further compared under different environmental conditions. The results demonstrated that CHC profiles differ among three species and change with different developmental stages, sexes, temperature, humidity and host plants. Genes involved in cuticular lipid biosynthesis pathways are modulated, which might explain why CHC profiles vary among species under different environments. Our study illustrates some biological and ecological variations in modifying CHC profiles, and the underlying molecular regulation mechanisms of the planthoppers in coping with changes of environmental conditions, which is of great importance for identifying potential vulnerabilities relating to pest ecology and developing novel pest management strategies.

Candidate genes involved in cuticular hydrocarbon differentiation between cryptic, parabiotic ant species

Insect cuticular hydrocarbons (CHCs) are highly diverse and have multiple functions, including communication and waterproofing. CHC profiles form species-specific, complex blends of up to 150 compounds. Especially in ants, even closely related species can have largely different profiles, raising the question how CHC differences are mirrored in the regulation of biosynthetic pathways. The neotropical ants Crematogaster levior and Camponotus femoratus both consist of two cryptic species each that are morphologically similar, but express strongly different CHC profiles. This is ideal to study the molecular basis of CHC differences. We thus investigated gene expression differences in fat-body transcriptomes of these ants. Despite common garden conditions, we found several thousand differentially expressed transcripts within each cryptic species pair. Many of these were related to metabolic processes, probably accounting for physiological differences. Moreover, we identified candidate genes from five gene families involved in CHC biosynthesis. By assigning candidate transcripts to orthologs in Drosophila, we inferred which CHCs might be influenced by differential gene expression. Expression of these candidate genes was often mirrored in the CHC profiles. For example, Cr. levior A, which has longer CHCs than its cryptic sister species, had a higher expression of elongases and a lower expression of fatty acyl- CoA reductases. This study is one of the first to identify CHC candidate genes in ants and will provide a basis for further research on the genetic basis of CHC biosynthesis.

Cuticular hydrocarbons for identifying Sarcophagidae (Diptera)

The composition and quantity of insect cuticular hydrocarbons (CHCs) can be species-specific as well as sexually dimorphic within species. CHC analysis has been previously used for identification and ageing purposes for several insect orders including true flies (Diptera). Here, we analysed the CHC chemical profiles of adult males and females of eleven species of flesh flies belonging to the genus Sarcophaga Meigen (Sarcophagidae), namely Sarcophaga africa (Wiedemann), S. agnata Rondani, S. argyrostoma Robineau-Desvoidy, S. carnaria (Linnaeus), S. crassipalpis Macquart, S. melanura Meigen, S. pumila Meigen, S. teretirostris Pandellé, S. subvicina Rohdendorf, S. vagans Meigen and S. variegata (Scopoli). Cuticular hydrocarbons extracted from pinned specimens from the collections of the Natural History Museum, London using a customised extraction technique were analysed using Gas Chromatography-Mass Spectrometry. Time of preservation prior to extraction ranged between a few weeks to over one hundred years. CHC profiles (1) allowed reliable identification of a large majority of specimens, (2) differed between males and females of the same species, (3) reliably associated males and females of the same species, provided sufficient replicates (up to 10) of each sex were analysed, and (4) identified specimens preserved for up to over one hundred years prior to extraction.

Mate attraction, chemical defense, and competition avoidance in the parasitoid wasp Leptopilina pacifica

A major hypothesis for the evolution of chemical signals is that pheromones arise from non-communicative precursor compounds. However, data supporting this hypothesis are rare, primarily because the original functions of the antecedent compounds often have been lost. A notable exception, however, is the parasitoid wasp species Leptopilina heterotoma, whose compound (−)-iridomyrmecin is used as a defensive secretion, a cue for females to avoid competition with con- and hetero-specific females, and as the primary component of the females’ sex pheromone. To better understand the evolution of sex pheromones from defensive compounds, we examined the chemical ecology of L. pacifica, the sister species of L. heterotoma. Here, we show that L. pacifica also produces a defensive secretion containing a species-specific mixture of mostly iridoid compounds. However, the composition of the secretion is more complex than in L. heterotoma, and iridomyrmecin is only a minor component. Moreover, in contrast to L. heterotoma, conspecific female competitors were not avoided by female subjects, and a role of the iridoids in the female sex pheromone of L. pacifica can be excluded, as only the females’ cuticular hydrocarbons (CHCs) resulted in the elicitation of courtship by males. Although closely related, the two sister species show substantial differences in the use of the defensive secretion for communicative purposes. Variation in pheromone usage in this genus still presents a conundrum, highlighting the need for additional studies to understand the selective forces shaping the evolution of pheromone composition.

The evolution of sexually dimorphic cuticular hydrocarbons in blowflies (Diptera: Calliphoridae)

Cuticular hydrocarbons (CHCs) are organic compounds found on the cuticles of all insects which can act as close-contact pheromones, while also providing a hydrophobic barrier to water loss. Given their widespread importance in sexual behaviour and survival, CHCs have likely contributed heavily to the adaptation and speciation of insects. Despite this, the patterns and mechanisms of their diversification have been studied in very few taxa. Here, we perform the first study of CHC diversification in blowflies, focussing on wild populations of the ecologically diverse genus Chrysomya. We convert CHC profiles into qualitative and quantitative traits and assess their inter- and intra-specific variation across 10 species. We also construct a global phylogeny of Chrysomya, onto which CHCs were mapped to explore the patterns of their diversification. For the first time, we demonstrate that blowflies express an exceptional diversity of CHCs, which have diversified in a nonphylogenetic and punctuated manner, are species-specific and sexually dimorphic. It is likely that both ecological and sexual selection have shaped these patterns of CHC diversification, and our study now provides a comprehensive framework for testing such hypotheses.

Chemical signals act as the main reproductive barrier between sister and mimetic Heliconius butterflies

Colour pattern is the main trait that drives mate recognition between Heliconius species that are phylogenetically close. However, when this cue is compromised such as in cases of mimetic, sympatric and closely related species, alternative mating signals must evolve to ensure reproductive isolation and species integrity. The closely related species Heliconius melpomene malleti and H. timareta florencia occur in the same geographical region, and despite being co-mimics, they display strong reproductive isolation. In order to test which cues differ between species, and potentially contribute to reproductive isolation, we quantified differences in the wing phenotype and the male chemical profile. As expected, the wing colour pattern was indistinguishable between the two species, while the chemical profile of the androconial and genital males' extracts showed marked differences. We then conducted behavioural experiments to study the importance of these signals in mate recognition by females. In agreement with our previous results, we found that chemical blends and not wing colour pattern drive the preference of females for conspecific males. Also, experiments with hybrid males and females suggested an important genetic component for both chemical production and preference. Altogether, these results suggest that chemicals are the major reproductive barrier opposing gene flow between these two sister and co-mimic species.

Cuticular hydrocarbons as potential mediators of cryptic species divergence in a mutualistic ant association

Upon advances in sequencing techniques, more and more morphologically identical organisms are identified as cryptic species. Often, mutualistic interactions are proposed as drivers of diversification. Species of the neotropical parabiotic ant association between Crematogaster levior and Camponotus femoratus are known for highly diverse cuticular hydrocarbon (CHC) profiles, which in insects serve as desiccation barrier but also as communication cues. In the present study, we investigated the association of the ants' CHC profiles with genotypes and morphological traits, and discovered cryptic species pairs in both genera. To assess putative niche differentiation between the cryptic species, we conducted an environmental association study that included various climate variables, canopy cover, and mutualistic plant species. Although mostly sympatric, the two Camponotus species seem to prefer different climate niches. However in the two Crematogaster species, we could not detect any differences in niche preference. The strong differentiation in the CHC profiles may thus suggest a possible role during speciation itself either by inducing assortative mating or by reinforcing sexual selection after the speciation event. We did not detect any further niche differences in the environmental parameters tested. Thus, it remains open how the cryptic species avoid competitive exclusion, with scope for further investigations.

Repeated Evolution of Asexuality Involves Convergent Gene Expression Changes

Asexual reproduction has evolved repeatedly from sexual ancestors across a wide range of taxa. Whereas the costs and benefits associated with asexuality have received considerable attention, the molecular changes underpinning the evolution of asexual reproduction remain relatively unexplored. In particular, it is completely unknown whether the repeated evolution of asexual phenotypes involves similar molecular changes, as previous studies have focused on changes occurring in single lineages. Here, we investigate the extent of convergent gene expression changes across five independent transitions to asexuality in stick insects. We compared gene expression of asexual females to females of close sexual relatives in whole-bodies, reproductive tracts, and legs. We identified a striking amount of convergent gene expression change (up to 8% of genes), greatly exceeding that expected by chance. Convergent changes were also tissue-specific, and most likely driven by selection for functional changes. Genes showing convergent changes in the reproductive tract were associated with meiotic spindle formation and centrosome organization. These genes are particularly interesting as they can influence the production of unreduced eggs, a key barrier to asexual reproduction. Changes in legs and whole-bodies were likely involved in female sexual trait decay, with enrichment in terms such as sperm-storage and pigmentation. By identifying changes occurring across multiple independent transitions to asexuality, our results provide a rare insight into the molecular basis of asexual phenotypes and suggest that the evolutionary path to asexuality is highly constrained, requiring repeated changes to the same key genes.

The Role of Transposable Elements in Speciation

Understanding the phenotypic and molecular mechanisms that contribute to genetic diversity between and within species is fundamental in studying the evolution of species. In particular, identifying the interspecific differences that lead to the reduction or even cessation of gene flow between nascent species is one of the main goals of speciation genetic research. Transposable elements (TEs) are DNA sequences with the ability to move within genomes. TEs are ubiquitous throughout eukaryotic genomes and have been shown to alter regulatory networks, gene expression, and to rearrange genomes as a result of their transposition. However, no systematic effort has evaluated the role of TEs in speciation. We compiled the evidence for TEs as potential causes of reproductive isolation across a diversity of taxa. We find that TEs are often associated with hybrid defects that might preclude the fusion between species, but that the involvement of TEs in other barriers to gene flow different from postzygotic isolation is still relatively unknown. Finally, we list a series of guides and research avenues to disentangle the effects of TEs on the origin of new species.

Release from prey preservation behavior via prey switch allowed diversification of cuticular hydrocarbon profiles in digger wasps

The cuticle of insects is covered by a layer of hydrocarbons (CHC), whose original function is the protection from desiccation and pathogens. However, in most insects CHC profiles are species specific. While this variability among species was largely linked to communication and recognition functions, additional selective forces may shape insect CHC profiles. Here, we show that in Philanthinae digger wasps (Crabronidae) the CHC profile coevolved with a peculiar brood-care strategy. In particular, we found that the behavior to embalm prey stored in the nest with hydrocarbons is adaptive to protect larval food from fungi in those species hunting for Hymenoptera. The prey embalming secretion is identical in composition to the alkene-dominated CHC profile in these species, suggesting that their profile is adaptively conserved for this purpose. In contrast, prey embalming is not required in those species that switched to Coleoptera as prey. Released from this chemical brood-care strategy, Coleoptera-hunting species considerably diversified their CHC profiles. Differential needs to successfully protect prey types used as larval food have thus driven the diversification of CHCs profiles of female Philanthinae wasps. To the best of our knowledge, this is the first evidence of a direct link between selection pressure for food preservation and CHC diversity.

The Scent Chemistry of Heliconius Wing Androconia

Neotropical Heliconius butterflies are members of various mimicry rings characterized by diverse colour patterns. In the present study we investigated whether a similar diversity is observed in the chemistry of volatile compounds present in male wing androconia. Recent research has shown that these androconia are used during courting of females. Three to five wild-caught male Heliconius individuals of 17 species and subspecies were analyzed by GC/MS. Most of the identified compounds originate from common fatty acids precursors, including aldehydes, alcohols, acetates or esters preferentially with a C₁₈ and C₂₀ chain, together with some alkanes. The compounds occurred in species-specific mixtures or signatures. For example, octadecanal is characteristic for H. melpomene, but variation in composition between the individuals was observed. Cluster analysis of compound occurrence in individual bouquets and analyses based on biosynthetic motifs such as functional group, chain length, or basic carbon-backbone modification were used to reveal structural patterns. Mimetic pairs contain different scent bouquets, but also some compounds in common, whereas sympatric species, both mimetic and non-mimetic, have more distinct compound compositions. The compounds identified here may play a role in mate choice thus helping maintain species integrity in a butterfly genus characterized by pervasive interspecific gene flow.

Transitions between phases of genomic differentiation during stick-insect speciation

Speciation can involve a transition from a few genetic loci that are resistant to gene flow to genome-wide differentiation. However, only limited data exist concerning this transition and the factors promoting it. Here, we study phases of speciation using data from >100 populations of 11 species of Timema stick insects. Consistent with early phases of genic speciation, adaptive colour-pattern loci reside in localized genetic regions of accentuated differentiation between populations experiencing gene flow. Transitions to genome-wide differentiation are also observed with gene flow, in association with differentiation in polygenic chemical traits affecting mate choice. Thus, intermediate phases of speciation are associated with genome-wide differentiation and mate choice, but not growth of a few genomic islands. We also find a gap in genomic differentiation between sympatric taxa that still exchange genes and those that do not, highlighting the association between differentiation and complete reproductive isolation. Our results suggest that substantial progress towards speciation may involve the alignment of multi-faceted aspects of differentiation.

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.

Nematode endoparasites do not codiversify with their stick insect hosts

Host-parasite coevolution stems from reciprocal selection on host resistance and parasite infectivity, and can generate some of the strongest selective pressures known in nature. It is widely seen as a major driver of diversification, the most extreme case being parallel speciation in hosts and their associated parasites. Here, we report on endoparasitic nematodes, most likely members of the mermithid family, infecting different Timema stick insect species throughout California. The nematodes develop in the hemolymph of their insect host and kill it upon emergence, completely impeding host reproduction. Given the direct exposure of the endoparasites to the host's immune system in the hemolymph, and the consequences of infection on host fitness, we predicted that divergence among hosts may drive parallel divergence in the endoparasites. Our phylogenetic analyses suggested the presence of two differentiated endoparasite lineages. However, independently of whether the two lineages were considered separately or jointly, we found a complete lack of codivergence between the endoparasitic nematodes and their hosts in spite of extensive genetic variation among hosts and among parasites. Instead, there was strong isolation by distance among the endoparasitic nematodes, indicating that geography plays a more important role than host-related adaptations in driving parasite diversification in this system. The accumulating evidence for lack of codiversification between parasites and their hosts at macroevolutionary scales contrasts with the overwhelming evidence for coevolution within populations, and calls for studies linking micro- versus macroevolutionary dynamics in host-parasite interactions.

Multiple sexual selection pressures drive the rapid evolution of complex morphology in a male secondary genital structure

The genitalia of internally fertilizing taxa represent a striking example of rapid morphological evolution. Although sexual selection can shape variation in genital morphology, it has been difficult to test whether multiple sexual selection pressures combine to drive the rapid evolution of individual genital structures. Here, we test the hypothesis that both pre‐ and postcopulatory sexual selection can act in concert to shape complex structural variation in secondary genital morphology. We genetically modified the size and shape of the posterior lobes of Drosophila melanogaster males and tested the consequences of morphological variation on several reproductive measures. We found that the posterior lobes are necessary for genital coupling and that they are also the targets of multiple postcopulatory processes that shape quantitative variation in morphology, even though these structures make no direct contact with the external female genitalia or internal reproductive organs during mating. We also found that males with smaller and less structurally complex posterior lobes suffer substantial fitness costs in competitive fertilization experiments. Our results show that sexual selection mechanisms can combine to shape the morphology of a single genital structure and that the posterior lobes of D. melanogaster are the targets of multiple postcopulatory selection pressures.

Selection on a genetic polymorphism counteracts ecological speciation in a stick insect

The interplay between selection and aspects of the genetic architecture of traits (such as linkage, dominance, and epistasis) can either drive or constrain speciation [1-3]. Despite accumulating evidence that speciation can progress to "intermediate" stages-with populations evolving only partial reproductive isolation-studies describing selective mechanisms that impose constraints on speciation are more rare than those describing drivers. The stick insect Timema cristinae provides an example of a system in which partial reproductive isolation has evolved between populations adapted to different host plant environments, in part due to divergent selection acting on a pattern polymorphism [4, 5]. Here, we demonstrate how selection on a green/melanistic color polymorphism counteracts speciation in this system. Specifically, divergent selection between hosts does not occur on color phenotypes because melanistic T. cristinae are cryptic on the stems of both host species, are resistant to a fungal pathogen, and have a mating advantage. Using genetic crosses and genome-wide association mapping, we quantify the genetic architecture of both the pattern and color polymorphism, illustrating their simple genetic control. We use these empirical results to develop an individual-based model that shows how the melanistic phenotype acts as a "genetic bridge" that increases gene flow between populations living on different hosts. Our results demonstrate how variation in the nature of selection acting on traits, and aspects of trait genetic architecture, can impose constraints on both local adaptation and speciation.

Premating isolation is determined by larval rearing substrates in cactophilic Drosophila mojavensis. X. Age-specific dynamics of adult epicuticular hydrocarbon expression in response to different host plants

Analysis of sexual selection and sexual isolation in Drosophila mojavensis and its relatives has revealed a pervasive role of rearing substrates on adult courtship behavior when flies were reared on fermenting cactus in preadult stages. Here, we assessed expression of contact pheromones comprised of epicuticular hydrocarbons (CHCs) from eclosion to 28 days of age in adults from two populations reared on fermenting tissues of two host cacti over the entire life cycle. Flies were never exposed to laboratory food and showed significant reductions in average CHC amounts consistent with CHCs of wild-caught flies. Overall, total hydrocarbon amounts increased from eclosion to 14–18 days, well past age at sexual maturity, and then declined in older flies. Most flies did not survive past 4 weeks. Baja California and mainland populations showed significantly different age-specific CHC profiles where Baja adults showed far less age-specific changes in CHC expression. Adults from populations reared on the host cactus typically used in nature expressed more CHCs than on the alternate host. MANCOVA with age as the covariate for the first six CHC principal components showed extensive differences in CHC composition due to age, population, cactus, sex, and age × population, age × sex, and age × cactus interactions. Thus, understanding variation in CHC composition as adult D. mojavensis age requires information about population and host plant differences, with potential influences on patterns of mate choice, sexual selection, and sexual isolation, and ultimately how these pheromones are expressed in natural populations. Studies of drosophilid aging in the wild are badly needed.