The genetics of speciation: genes of small effect underlie sexual isolation in the Hawaiian cricket Laupala

Neurogenomic divergence during speciation by reinforcement of mating behaviors in chorus frogs (Pseudacris)

BACKGROUND

Species interactions can promote mating behavior divergence, particularly when these interactions are costly due to maladaptive hybridization. Selection against hybridization can indirectly cause evolution of reproductive isolation within species, a process termed cascade reinforcement. This process can drive incipient speciation by generating divergent selection pressures among populations that interact with different species assemblages. Theoretical and empirical studies indicate that divergent selection on gene expression networks has the potential to increase reproductive isolation among populations. After identifying candidate synaptic transmission genes derived from neurophysiological studies in anurans, we test for divergence of gene expression in a system undergoing cascade reinforcement, the Upland Chorus Frog (Pseudacris feriarum).

RESULTS

Our analyses identified seven candidate synaptic transmission genes that have diverged between ancestral and reinforced populations of P. feriarum, including five that encode synaptic vesicle proteins. Our gene correlation network analyses revealed four genetic modules that have diverged between these populations, two possessing a significant concentration of neurotransmission enrichment terms: one for synaptic membrane components and the other for metabolism of the neurotransmitter nitric oxide. We also ascertained that a greater number of genes have diverged in expression by geography than by sex. Moreover, we found that more genes have diverged within females as compared to males between populations. Conversely, we observed no difference in the number of differentially-expressed genes within the ancestral compared to the reinforced population between the sexes.

CONCLUSIONS

This work is consistent with the idea that divergent selection on mating behaviors via cascade reinforcement contributed to evolution of gene expression in P. feriarum. Although our study design does not allow us to fully rule out the influence of environment and demography, the fact that more genes diverged in females than males points to a role for cascade reinforcement. Our discoveries of divergent candidate genes and gene networks related to neurotransmission support the idea that neural mechanisms of acoustic mating behaviors have diverged between populations, and agree with previous neurophysiological studies in frogs. Increasing support for this hypothesis, however, will require additional experiments under common garden conditions. Our work points to the importance of future replicated and tissue-specific studies to elucidate the relative contribution of gene expression divergence to the evolution of reproductive isolation during incipient speciation.

A framework for studying behavioral evolution by reconstructing ancestral repertoires

Although different animal species often exhibit extensive variation in many behaviors, typically scientists examine one or a small number of behaviors in any single study. Here, we propose a new framework to simultaneously study the evolution of many behaviors. We measured the behavioral repertoire of individuals from six species of fruit flies using unsupervised techniques and identified all stereotyped movements exhibited by each species. We then fit a Generalized Linear Mixed Model to estimate the intra- and inter-species behavioral covariances, and, by using the known phylogenetic relationships among species, we estimated the (unobserved) behaviors exhibited by ancestral species. We found that much of intra-specific behavioral variation has a similar covariance structure to previously described long-time scale variation in an individual's behavior, suggesting that much of the measured variation between individuals of a single species in our assay reflects differences in the status of neural networks, rather than genetic or developmental differences between individuals. We then propose a method to identify groups of behaviors that appear to have evolved in a correlated manner, illustrating how sets of behaviors, rather than individual behaviors, likely evolved. Our approach provides a new framework for identifying co-evolving behaviors and may provide new opportunities to study the mechanistic basis of behavioral evolution.

Selection, drift, and constraint in cypridinid luciferases and the diversification of bioluminescent signals in sea fireflies

Understanding the genetic causes of evolutionary diversification is challenging because differences across species are complex, often involving many genes. However, cases where single or few genetic loci affect a trait that varies dramatically across a radiation of species provide tractable opportunities to understand the genetics of diversification. Here, we begin to explore how diversification of bioluminescent signals across species of cypridinid ostracods ("sea fireflies") was influenced by evolution of a single gene, cypridinid-luciferase. In addition to emission spectra ("colour") of bioluminescence from 21 cypridinid species, we report 13 new c-luciferase genes from de novo transcriptomes, including in vitro assays to confirm function of four of those genes. Our comparative analyses suggest some amino acid sites in c-luciferase evolved under episodic diversifying selection and may be associated with changes in both enzyme kinetics and colour, two enzymatic functions that directly impact the phenotype of bioluminescent signals. The analyses also suggest multiple other amino acid positions in c-luciferase evolved neutrally or under purifying selection, and may have impacted the variation of colour of bioluminescent signals across genera. Previous mutagenesis studies at candidate sites show epistatic interactions, which could constrain the evolution of c-luciferase function. This work provides important steps toward understanding the genetic basis of diversification of behavioural signals across multiple species, suggesting different evolutionary processes act at different times during a radiation of species. These results set the stage for additional mutagenesis studies that could explicitly link selection, drift, and constraint to the evolution of phenotypic diversification.

Lighting the way: Forces driving the diversification of bioluminescent signalling in sea fireflies

Understanding the drivers of diversification and processes that maintain biodiversity remains a central theme of evolutionary biology. However, these efforts are often impeded due to disparities across species and environments and the genetic complexity underlying many traits. The factors driving biodiversity can be more readily understood by focusing on the genetics of diversification, of one or few genes shared across species, with large influence over an organism's phenotype (Templeton, 1981; Wright, 1984). In this pursuit, previous studies often focus on the selective pressures that impact phenotypic diversity (Brawand et al., 2014; Yokoyama et al., 2015), often overlooking the contribution of neutral processes (i.e., genetic drift). In this issue of Molecular Ecology, Hensley et al. (2020) use an integrative approach, including RNA sequencing, in vitro protein expression and spectral measurements, to explore the drivers behind the diversification of bioluminescent signalling in cypridinid ostracods (Figure 1). Typical bioluminescent reactions primarily include an enzyme (luciferase) and substrate (luciferin). By focusing on a single gene, this study traces the molecular evolution of (c)luciferase in sea fireflies, elucidating diverse signatures of selection, drift and constraint to decipher the link between genotype and phenotype of their bioluminescent emissions.

Genomic landscape of reproductive isolation in Lucania killifish: The role of sex loci and salinity

Adaptation to different environments can directly and indirectly generate reproductive isolation between species. Bluefin killifish (Lucania goodei) and rainwater killifish (L. parva) are sister species that have diverged across a salinity gradient and are reproductively isolated by habitat, behavioural, extrinsic and intrinsic post-zygotic isolation. We asked if salinity adaptation contributes indirectly to other forms of reproductive isolation via linked selection and hypothesized that low recombination regions, such as sex chromosomes or chromosomal rearrangements, might facilitate this process. We conducted QTL mapping in backcrosses between L. parva and L. goodei to explore the genetic architecture of salinity tolerance, behavioural isolation and intrinsic isolation. We mapped traits relative to a chromosome that has undergone a centric fusion in L. parva (relative to L. goodei). We found that the sex locus appears to be male determining (XX-XY), was located on the fused chromosome and was implicated in intrinsic isolation. QTL associated with salinity tolerance were spread across the genome and did not overly co-localize with regions associated with behavioural or intrinsic isolation. This preliminary analysis of the genetic architecture of reproductive isolation between Lucania species does not support the hypothesis that divergent natural selection for salinity tolerance led to behavioural and intrinsic isolation as a by-product. Combined with previous studies in this system, our work suggests that adaptation as a function of salinity contributes to habitat isolation and that reinforcement may have contributed to the evolution of behavioural isolation instead, possibly facilitated by linkage between behavioural isolation and intrinsic isolation loci on the fused chromosome.

Genomic differentiation across the speciation continuum in three hummingbird species pairs

BACKGROUND

The study of speciation has expanded with the increasing availability and affordability of high-resolution genomic data. How the genome evolves throughout the process of divergence and which regions of the genome are responsible for causing and maintaining that divergence have been central questions in recent work. Here, we use three pairs of species from the recently diverged bee hummingbird clade to investigate differences in the genome at different stages of speciation, using divergence times as a proxy for the speciation continuum.

RESULTS

Population measures of relative differentiation between hybridizing species reveal that different chromosome types diverge at different stages of speciation. Using FST as our relative measure of differentiation we found that the sex chromosome shows signs of divergence early in speciation. Next, small autosomes (microchromosomes) accumulate highly diverged genomic regions, while the large autosomes (macrochromosomes) accumulate genomic regions of divergence at a later stage of speciation.

CONCLUSIONS

Our finding that genomic windows of elevated FST accumulate on small autosomes earlier in speciation than on larger autosomes is counter to the prediction that FST increases with size of chromosome (i.e. with decreased recombination rate), and is not represented when weighted average FST per chromosome is compared with chromosome size. The results of this study suggest that multiple chromosome characteristics such as recombination rate and gene density combine to influence the genomic locations of signatures of divergence.

Parallel genomic architecture underlies repeated sexual signal divergence in Hawaiian Laupala crickets

When the same phenotype evolves repeatedly, we can explore the predictability of genetic changes underlying phenotypic evolution. Theory suggests that genetic parallelism is less likely when phenotypic changes are governed by many small-effect loci compared to few of major effect, because different combinations of genetic changes can result in the same quantitative outcome. However, some genetic trajectories might be favoured over others, making a shared genetic basis to repeated polygenic evolution more likely. To examine this, we studied the genetics of parallel male mating song evolution in the Hawaiian cricket Laupala. We compared quantitative trait loci (QTL) underlying song divergence in three species pairs varying in phenotypic distance. We tested whether replicated song divergence between species involves the same QTL and whether the likelihood of QTL sharing is related to QTL effect size. Contrary to theoretical predictions, we find substantial parallelism in polygenic genetic architectures underlying repeated song divergence. QTL overlapped more frequently than expected based on simulated QTL analyses. Interestingly, QTL effect size did not predict QTL sharing, but did correlate with magnitude of phenotypic divergence. We highlight potential mechanisms driving these constraints on cricket song evolution and discuss a scenario that consolidates empirical quantitative genetic observations with micro-mutational theory.

Within-population variability in a moth sex pheromone blend, part 2: selection towards fixation

To understand how variation in sexual communication systems evolves, the genetic architecture underlying sexual signals and responses needs to be identified. Especially in animals where mating signals are important for mate recognition, and signals and responses are governed by independently assorting genes, it is difficult to envision how signals and preferences can (co)evolve. Moths are a prime example of such animals. In the noctuid moth Heliothis virescens, we found within-population variation in the female pheromone. In previous selection experiments followed by quantitative trait locus (QTL) analysis and expression analysis of candidate desaturase genes, we developed a model involving a trans-acting repressor of the delta-11-desaturase. In our current study with new selection lines, we fixed the most extreme phenotype and found a single underlying mutation: a premature stop codon in the first coding exon of delta-11-desaturase, which we could trace back to its origin in the laboratory. Interestingly, we found no pleiotropic effects of this knock-out mutation on the male physiological or behavioural response, or on growth or fertility. This finding is in contrast to Drosophila melanogaster, where a single desaturase gene affects both female pheromone production and male behavioural response, but similar to other Lepidoptera where these traits are under independent genetic control. To our knowledge, this is the first time that a single point mutation has been identified that underlies the phenotypic variation in the pheromone signal of a moth.

The Genetics of Mating Song Evolution Underlying Rapid Speciation: Linking Quantitative Variation to Candidate Genes for Behavioral Isolation

Differences in mating behaviors evolve early during speciation, eventually contributing to reproductive barriers between species. Knowledge of the genetic and genomic basis of these behaviors is therefore integral to a causal understanding of speciation. Acoustic behaviors are often part of the mating ritual in animal species. The temporal rhythms of mating songs are notably species-specific in many vertebrates and arthropods and often underlie assortative mating. Despite discoveries of mutations that disrupt the temporal rhythm of these songs, we know surprisingly little about genes affecting naturally occurring variation in the temporal pattern of singing behavior. In the rapidly speciating Hawaiian cricket genus Laupala, the striking species variation in song rhythms constitutes a behavioral barrier to reproduction between species. Here, we mapped the largest-effect locus underlying interspecific variation in song rhythm between two Laupala species to a narrow genomic region, wherein we find no known candidate genes affecting song temporal rhythm in Drosophila Whole-genome sequencing, gene prediction, and functional annotation of this region reveal an exciting and promising candidate gene, the putative cyclic nucleotide-gated ion channel-like gene, for natural variation in mating behavior. Identification and molecular characterization of the candidate gene reveals a nonsynonymous mutation in a conserved binding domain, suggesting that ion channels are important targets of selection on rhythmic signaling during establishment of behavioral isolation and rapid speciation.

Satellite DNAs are conserved and differentially transcribed among Gryllus cricket species

Satellite DNA (satDNA) is an abundant class of non-coding repetitive DNA that is preferentially found as tandemly repeated arrays in gene-poor heterochromatin but is also present in gene-rich euchromatin. Here, we used DNA- and RNA-seq from Gryllus assimilis to address the content and transcriptional patterns of satDNAs. We also mapped RNA-seq libraries for other Gryllus species against the satDNAs found in G. assimilis and G. bimaculatus genomes to investigate their evolutionary conservation and transcriptional profiles in Gryllus. Through DNA-seq read clustering analysis using RepeatExplorer, dotplots analysis and fluorescence in situ hybridization mapping, we found that ∼4% of the G. assimilis genome is represented by 11 well-defined A + T-rich satDNA families. These are mainly located in heterochromatic areas, with some repeats able to form high-order repeat structures. By in silico transcriptional analysis we identified satDNAs that are conserved in Gryllus but differentially transcribed. The data regarding satDNA presence in G. assimilis genome were discussed in an evolutionary context, with transcriptional data enabling comparisons between sexes and across tissues when possible. We discuss hypotheses for the conservation and transcription of satDNAs in Gryllus, which might result from their role in sexual differentiation at the chromatin level, heterochromatin formation and centromeric function.

Geographic variation in hybridization across a reinforcement contact zone of chorus frogs (Pseudacris)

Reinforcement contact zones, which are secondary contact zones where species are diverging in reproductive behaviors due to selection against hybridization, represent natural laboratories for studying speciation‐in‐action. Here, we examined replicate localities across the entire reinforcement contact zone between North American chorus frogs Pseudacris feriarum and P. nigrita to investigate geographic variation in hybridization frequencies and to assess whether reinforcement may have contributed to increased genetic divergence within species. Previous work indicated these species have undergone reproductive character displacement (RCD) in male acoustic signals and female preferences due to reinforcement. We also examined acoustic signal variation across the contact zone to assess whether signal characteristics reliably predict hybrid index and to elucidate whether the degree of RCD predicts hybridization rate. Using microsatellites, mitochondrial sequences, and acoustic signal information from >1,000 individuals across >50 localities and ten sympatric focal regions, we demonstrate: (1) hybridization occurs and (2) varies substantially across the geographic range of the contact zone, (3) hybridization is asymmetric and in the direction predicted from observed patterns of asymmetric RCD, (4) in one species, genetic distance is higher between conspecific localities where one or both have been reinforced than between nonreinforced localities, after controlling for geographic distance, (5) acoustic signal characters strongly predict hybrid index, and (6) the degree of RCD does not strongly predict admixture levels. By showing that hybridization occurs in all sympatric localities, this study provides the fifth and final line of evidence that reproductive character displacement is due to reinforcement in the chorus frog contact zone. Furthermore, this work suggests that the dual action of cascade reinforcement and partial geographic isolation is promoting genetic diversification within one of the reinforced species.

Quantitative Trait Locus Analysis of Mating Behavior and Male Sex Pheromones in Nasonia Wasps

A major focus in speciation genetics is to identify the chromosomal regions and genes that reduce hybridization and gene flow. We investigated the genetic architecture of mating behavior in the parasitoid wasp species pair Nasonia giraulti and Nasonia oneida that exhibit strong prezygotic isolation. Behavioral analysis showed that N. oneida females had consistently higher latency times, and broke off the mating sequence more often in the mounting stage when confronted with N. giraulti males compared with males of their own species. N. oneida males produce a lower quantity of the long-range male sex pheromone (4R,5S)-5-hydroxy-4-decanolide (RS-HDL). Crosses between the two species yielded hybrid males with various pheromone quantities, and these males were used in mating trials with females of either species to measure female mate discrimination rates. A quantitative trait locus (QTL) analysis involving 475 recombinant hybrid males (F₂), 2148 reciprocally backcrossed females (F₃), and a linkage map of 52 equally spaced neutral single nucleotide polymorphism (SNP) markers plus SNPs in 40 candidate mating behavior genes revealed four QTL for male pheromone amount, depending on partner species. Our results demonstrate that the RS-HDL pheromone plays a role in the mating system of N. giraulti and N. oneida, but also that additional communication cues are involved in mate choice. No QTL were found for female mate discrimination, which points at a polygenic architecture of female choice with strong environmental influences.

Insect speciation rules: unifying concepts in speciation research

The study of speciation is concerned with understanding the connection between causes of divergent evolution and the origin and maintenance of barriers to gene exchange between incipient species. Although the field has historically focused either on examples of recent divergence and its causes or on the genetic basis of reproductive isolation between already divergent species, current efforts seek to unify these two approaches. Here we integrate these perspectives through a discussion of recent progress in several insect speciation model systems. We focus on the evolution of speciation phenotypes in each system (i.e., those phenotypes causally involved in reducing gene flow between incipient species), drawing an explicit connection between cause and effect (process and pattern). We emphasize emerging insights into the genomic architecture of speciation as well as timely areas for future research.

Additive genetic architecture underlying a rapidly evolving sexual signaling phenotype in the Hawaiian cricket genus Laupala

Complex, quantitative traits are often the function of the coordinated action of many physically independent genetic factors. Interactive properties of multilocus genotypes, such as epistasis, are thought to be pervasive components of the genetic architecture of complex phenotypes. Here, we utilize a panel of interspecific backcross introgression lines to evaluate the genetic architecture of song variation, a quantitative sexual signaling phenotype, in the Hawaiian swordtail cricket genus Laupala. Allelic effects across five quantitative trait loci are consistent with a purely additive model of gene action, where alleles at multiple loci are found to have fully independent and discrete effects with respect to the sexual signaling phenotype. Whereas a more complex genetic architecture featuring non-additive dominance and epistasis components may constrain potential evolutionary trajectories and reduce the rate of evolutionary change, the polygenic, additive genetic architecture observed for sexual signaling in Laupala should respond rapidly to directional selection pressures and freely move throughout phenotypic space. This classic type I genetic architecture may facilitate the explosive radiation of song variation observed across the Laupala genus.

Developmental gene discovery in a hemimetabolous insect: de novo assembly and annotation of a transcriptome for the cricket Gryllus bimaculatus

Most genomic resources available for insects represent the Holometabola, which are insects that undergo complete metamorphosis like beetles and flies. In contrast, the Hemimetabola (direct developing insects), representing the basal branches of the insect tree, have very few genomic resources. We have therefore created a large and publicly available transcriptome for the hemimetabolous insect Gryllus bimaculatus (cricket), a well-developed laboratory model organism whose potential for functional genetic experiments is currently limited by the absence of genomic resources. cDNA was prepared using mRNA obtained from adult ovaries containing all stages of oogenesis, and from embryo samples on each day of embryogenesis. Using 454 Titanium pyrosequencing, we sequenced over four million raw reads, and assembled them into 21,512 isotigs (predicted transcripts) and 120,805 singletons with an average coverage per base pair of 51.3. We annotated the transcriptome manually for over 400 conserved genes involved in embryonic patterning, gametogenesis, and signaling pathways. BLAST comparison of the transcriptome against the NCBI non-redundant protein database (nr) identified significant similarity to nr sequences for 55.5% of transcriptome sequences, and suggested that the transcriptome may contain 19,874 unique transcripts. For predicted transcripts without significant similarity to known sequences, we assessed their similarity to other orthopteran sequences, and determined that these transcripts contain recognizable protein domains, largely of unknown function. We created a searchable, web-based database to allow public access to all raw, assembled and annotated data. This database is to our knowledge the largest de novo assembled and annotated transcriptome resource available for any hemimetabolous insect. We therefore anticipate that these data will contribute significantly to more effective and higher-throughput deployment of molecular analysis tools in Gryllus.

Identification of genetically linked female preference and male trait

Genetic variation in male traits and the female preferences for those traits allows for the evolution of sexual behavior. Trait-preference combinations are thought to improve the effectiveness of runaway sexual selection within a species, and are considered necessary for the induction of divergence between species. Novel traits, or variants of existing traits, and their associated preferences in the opposite sex are more likely to be maintained if they are genetically linked in proximity on a chromosome (the genetic coupling hypothesis), yet there is little empirical evidence that this genetic linkage occurs. Here we show for the first time that natural genetic variation at a single-linked region can induce both species-specific female choosiness and the male trait they are discriminating against. We found this effect in two separate regions of the genome, demonstrating that this linkage may be common. In contrast, female choosiness and male unattractiveness could not be alleviated by a single region. The close linkage of these loci and the strength of their effect provide an evolutionary means by which this preference-trait combination could arise and be maintained, thus enabling a more rapid route for runaway sexual selection, and providing empirical evidence supporting the genetic coupling hypothesis.

One quantitative trait locus for intra- and interspecific variation in a sex pheromone

Even though premating isolation is hypothesized to be a major driving force in speciation, its genetic basis is poorly known. In the noctuid moth Heliothis subflexa, one group of sex pheromone components, the acetates, emitted by the female, plays a crucial isolating role in preventing interspecific matings to males of the closely related Heliothis virescens, in which females do not produce acetates and males are repelled by them. We previously found intraspecific variation in acetates in H. subflexa: females in eastern North America contain significantly more acetates than females in Western Mexico. Here we describe the persistence of this intraspecific variation in laboratory-reared strains and the identification of one major quantitative trait locus (QTL), explaining 40% of the variance in acetate amounts. We homologized this intraspecific QTL to our previously identified interspecific QTL using restriction-associated DNA (RAD) tags. We found that a major intraspecific QTL overlaps with one of the two major interspecific QTL. To identify candidate genes underlying the acetate variation, we investigated a number of gene families with known or suspected acetyl- or acyltransferase activity. The most likely candidate genes did not map to our QTL, so that we currently hypothesize that a transcription factor underlies this QTL. Finding a single, large QTL that impacts variation in pheromone blends between and within species is, to our knowledge, the first such example for traits that have been demonstrated to affect premating isolation.

Cellular basis for singing motor pattern generation in the field cricket (Gryllus bimaculatus DeGeer)

The singing behavior of male crickets allows analyzing a central pattern generator (CPG) that was shaped by sexual selection for reliable production of species-specific communication signals. After localizing the essential ganglia for singing in Gryllus bimaculatus, we now studied the calling song CPG at the cellular level. Fictive singing was initiated by pharmacological brain stimulation. The motor pattern underlying syllables and chirps was recorded as alternating spike bursts of wing-opener and wing-closer motoneurons in a truncated wing nerve; it precisely reflected the natural calling song. During fictive singing, we intracellularly recorded and stained interneurons in thoracic and abdominal ganglia and tested their impact on the song pattern by intracellular current injections. We identified three interneurons of the metathoracic and first unfused abdominal ganglion that rhythmically de- and hyperpolarized in phase with the syllable pattern and spiked strictly before the wing-opener motoneurons. Depolarizing current injection in two of these opener interneurons caused additional rhythmic singing activity, which reliably reset the ongoing chirp rhythm. The closely intermeshing arborizations of the singing interneurons revealed the dorsal midline neuropiles of the metathoracic and three most anterior abdominal neuromeres as the anatomical location of singing pattern generation. In the same neuropiles, we also recorded several closer interneurons that rhythmically hyper- and depolarized in the syllable rhythm and spiked strictly before the wing-closer motoneurons. Some of them received pronounced inhibition at the beginning of each chirp. Hyperpolarizing current injection in the dendrite revealed postinhibitory rebound depolarization as one functional mechanism of central pattern generation in singing crickets.

Recombinant inbred systems can advance research in behavioral ecology

Recombinant inbred (RI) systems such as the BXD mouse family represent a population with defined genetic architecture and variation that approximates those of natural populations. With the development of novel RI lines and sophisticated methods that conjointly analyze phenotype, gene sequence, and expression data, RI systems such as BXD are a timely and powerful tool to advance the field of behavioral ecology. The latter traditionally focused on functional questions such as the adaptive value of behavior but largely ignored underlying genetics and mechanisms. In this perspective, we argue that using RI systems to address questions in behavioral ecology and evolutionary biology has great potential to advance research in these fields. We outline key questions and how they can be tackled using RI systems and BXD in particular. The unique opportunity to analyze genetic and phenotypic data from studies conducted in different laboratories and at different times is a key benefit of RI systems and may lead the way to a better understanding of how adaptive phenotypes arise from genetic and environmental factors.

Russian comparative embryology takes form: a conceptual metamorphosis toward "evo-devo"

This essay recapitulates major paths followed by the Russian tradition of what we refer to today as evolutionary developmental biology ("evo-devo"). The article addresses several questions regarding the conceptual history of evolutionary embryological thought in its particularly Russian perspective: (1) the assertion by the St. Petersburg academician Wolff regarding the possible connections between environmental modifications during morphogenesis and the "transformation" of species, (2) the discovery of shared "principles" underlying animal development by von Baer, (3) the experimental expression of Baer's principles by Kowalevsky and Mechnikoff, (4) Severtsov's theory of phylembryogenesis, (5) Filatov's approach to the study of evolution using comparative "developmental mechanics", and (6) Shmalgausen's concept of "stabilizing" selection as an attempt to elucidate the evolution of developmental mechanisms. The focus on comparative evolutionary embryology, which was established by Kowalevsky and Mechnikoff, still continues to be popular in present-day "evo-devo" research in Russia.

Evolution of multiple additive loci caused divergence between Drosophila yakuba and D. santomea in wing rowing during male courtship

In Drosophila, male flies perform innate, stereotyped courtship behavior. This innate behavior evolves rapidly between fly species, and is likely to have contributed to reproductive isolation and species divergence. We currently understand little about the neurobiological and genetic mechanisms that contributed to the evolution of courtship behavior. Here we describe a novel behavioral difference between the two closely related species D. yakuba and D. santomea: the frequency of wing rowing during courtship. During courtship, D. santomea males repeatedly rotate their wing blades to face forward and then back (rowing), while D. yakuba males rarely row their wings. We found little intraspecific variation in the frequency of wing rowing for both species. We exploited multiplexed shotgun genotyping (MSG) to genotype two backcross populations with a single lane of Illumina sequencing. We performed quantitative trait locus (QTL) mapping using the ancestry information estimated by MSG and found that the species difference in wing rowing mapped to four or five genetically separable regions. We found no evidence that these loci display epistasis. The identified loci all act in the same direction and can account for most of the species difference.

The genetic basis of behavioral isolation between Drosophila mauritiana and D. sechellia

Understanding how species form is a fundamental question in evolutionary biology. Identifying the genetic bases of barriers that prevent gene flow between species provides insight into how speciation occurs. Here, I analyze a poorly understood reproductive isolating barrier, prezygotic reproductive isolation. I perform a genetic analysis of prezygotic isolation between two closely related species of Drosophila, D. mauritiana and D. sechellia. I first confirm the existence of strong behavioral isolation between D. mauritiana females and D. sechellia males. Next, I examine the genetic basis of behavioral isolation by (1) scanning an existing set of introgression lines for chromosomal regions that have a large effect on isolation; and (2) mapping quantitative trait loci (QTL) that underlie behavioral isolation via backcross analysis. In particular, I map QTL that determine whether a hybrid backcross female and a D. sechellia male will mate. I identify a single significant QTL, on the X chromosome, suggesting that few major-effect loci contribute to behavioral isolation between these species. In further work, I refine the map position of the QTL to a small region of the X chromosome.

What do we need to know about speciation?

Speciation has been a major focus of evolutionary biology research in recent years, with many important advances. However, some of the traditional organising principles of the subject area no longer provide a satisfactory framework, such as the classification of speciation mechanisms by geographical context into allopatric, parapatric and sympatry classes. Therefore, we have asked where speciation research should be directed in the coming years. Here, we present a distillation of questions about the mechanisms of speciation, the genetic basis of speciation and the relationship between speciation and diversity. Our list of topics is not exhaustive; rather we aim to promote discussion on research priorities and on the common themes that underlie disparate speciation processes.

Widespread genetic linkage of mating signals and preferences in the Hawaiian cricket Laupala

The evolution of novel sexual communication systems is integral to the process of speciation, as it discourages gene flow between incipient species. Physical linkage between genes underlying male-female communication (i.e. sexual signals and preferences for them) facilitates both rapid and coordinated divergence of sexual communication systems between populations and reduces recombination in the face of occasional hybridization between diverging populations. Despite these ramifications of the genetic architecture of sexual communication for sexual selection and speciation, few studies have examined this relationship empirically. Previous studies of the closely related Hawaiian crickets Laupala paranigra and Laupala kohalensis have indirectly suggested that many of the genes underlying the difference in pulse rate of male song are physically linked with genes underlying the difference in female preference for pulse rate. Using marker-assisted introgression, we moved 'slow pulse rate' alleles from L. paranigra at five known quantitative trait loci (QTL) underlying male pulse rate into the 'fast pulse rate' genetic background of L. kohalensis and assessed the effect of these loci on female preference. An astounding four out of five song QTL predicted the preferences of female fourth-generation backcrosses, providing direct evidence for the extensive genetic linkage of song and preference in one of the fastest diversifying genera currently known.