Biology subjects

Jiggins, C. D.

Publications and source records attributed to Jiggins, C. D..

54 records · Page 3Linked to original sources

bioRxiv2019

Adaptive introgression across semipermeable species boundaries between local Helicoverpa zea and invasive Helicoverpa armigera moths

Hybridization between invasive and native species has raised global concern given the dramatic increase in species range shifts and pest outbreaks due to climate change, development of suitable agroecosystems, and anthropogenic dispersal. Nevertheless, secondary contact between sister lineages of local and invasive species provides a natural laboratory to understand the factors that determine introgression and the maintenance or loss of species barriers. Here, we characterize the early evolutionary outcomes following secondary contact between invasive Helicoverpa armigera and H. zea in Brazil. We carried out whole-genome resequencing of Helicoverpa moths from Brazil in two temporal samples: during the outbreak of H. armigera in 2013, and more recent populations from 2017. There is evidence for a burst of hybridization and widespread introgression from local H. zea into invasive H. armigera coinciding with H. armigera expansion in 2013. However, in H. armigera, admixture proportions were reduced between 2013 and 2017, indicating a decline in hybridization rates. Recent populations also showed shorter introgressed tracks suggesting selection against admixture. In contrast to the genome-wide pattern, there was striking evidence for introgression of a single region including an insecticide-resistance allele from the invasive H. armigera into local H. zea, which increased in frequency over time but was localized within the genome. In summary, despite extensive gene-flow after secondary contact, the species boundaries are largely maintained except for the single introgressed region containing the insecticide-resistant locus. We document the worst-case scenario for an invasive species, in which there are now two pest species instead of one, and the native species has acquired resistance to pyrethroid insecticides through introgression and hybridization, with significant implications for pest management in future population expansions and introductions of novel resistance genes from new invasive H. armigera populations. Author summarySecondary contact occurs when related species with non-overlapping ranges are geographically reunited. Scenarios of secondary contact have increased due to anthropogenic movement of species outside of their native range, often resulting in invasive species that successfully spread and stabilised in the new environment. This is the case for Helicoverpa armigera, a major agricultural pest in the Old World that has recently invaded the Americas, where it reunited with its closest relative, H. zea. While some authors reported hybridisation, and hypothesised about the potential emergence of novel ecotypes and the exchange of pesticide-resistant genes, these outcomes have not been tested yet. We examine these outcomes by sequencing individuals from both species in Brazil, collected in 2013 after outbreaks of H. armigera were reported, and individuals collected during 2017. We discovered that despite hybridisation, these moths have not collapsed into a single species nor formed new ecotypes, and that the species distinctiveness is maintained through selection against most of the foreign genotypes that cross species boundaries. However, we found that hybridisation mediated the rapid acquisition of a H. armigera gene conferring resistance to pyrethroids by H. zea. The overall decline in populations of both species during the interval covered by this study means that our results are likely to reflect the consequences of hybridization events early after invasion, despite the likely ongoing introduction of H. armigera genetic diversity through trade across the South American continent. Our results provide a rare example of adaptive transferral of variation right after invasion and elucidate the dynamics of insecticide resistance evolution in H. zea.

evolutionary biology↗

bioRxiv2019

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

Colour pattern has been long recognised as the 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 geographic region and despite being co-mimics they display strong reproductive isolation. In order to test which cues differ between species, and therefore potentially contribute to reproductive isolation, we quantified differences in wing phenotype and male chemical profile. As expected, wing colour pattern was indistinguishable between the two species while the chemical profile of their male sex pheromones 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 pheromones and not wing colour pattern drive the preference of females by conspecific males. In addition, experiments with hybrid males and females suggested an important genetic component for both pheromone production and preference. Altogether, these results suggest that pheromones are the major reproductive barrier opposing gene flow between these two sister and co-mimic species.

evolutionary biology↗

bioRxiv2019

A novel terpene synthase produces an anti-aphrodisiac pheromone in the butterfly Heliconius melpomene

Terpenes, a group of structurally diverse compounds, are the biggest class of secondary metabolites. While the biosynthesis of terpenes by enzymes known as terpene synthases (TPSs) has been described in plants and microorganisms, few TPSs have been identified in insects, despite the presence of terpenes in multiple insect species. Indeed, in many insect species, it remains unclear whether terpenes are sequestered from plants or biosynthesised de novo. No homologs of plant TPSs have been found in insect genomes, though insect TPSs with an independent evolutionary origin have been found in Hemiptera and Coleoptera. In the butterfly Heliconius melpomene, the monoterpene (E)-{beta}-ocimene acts as an anti-aphrodisiac pheromone, where it is transferred during mating from males to females to avoid re-mating by deterring males. To date only one insect monoterpene synthase has been described, in Ips pini (Coleoptera), and is a multifunctional TPS and isoprenyl diphosphate synthase (IDS). Here, we combine linkage mapping and expression studies to identify candidate genes involved in the biosynthesis of (E)-{beta}-ocimene. We confirm that H. melpomene has two enzymes that exhibit TPS activity, and one of these, HMEL037106g1 is able to synthesise (E)-{beta}-ocimene in vitro. Unlike the enzyme in Ips pini, these enzymes only exhibit residual IDS activity, suggesting they are more specialised TPSs, akin to those found in plants. Phylogenetic analysis shows that these enzymes are unrelated to previously described plant and insect TPSs. The distinct evolutionary origin of TPSs in Lepidoptera suggests that they have evolved multiple times in insects.\n\nSignificance statementTerpenes are a diverse class of natural compounds, used by both plants and animals for a variety of functions, including chemical communication. In insects it is often unclear whether they are synthesised de novo or sequestered from plants. Some plants and insects have converged to use the same compounds. For instance, (E)-{beta}-ocimene is a common component of floral scent and is also used by the butterfly Heliconius melpomene as an anti-aphrodisiac pheromone. We describe two novel terpene synthases, one of which synthesises (E)-{beta}-ocimene in H. melpomene, unrelated not only to plant enzymes but also other recently identified insect terpene synthases. This provides the first evidence that the ability to synthesise terpenes has arisen multiple times independently within the insects.

biochemistry↗

bioRxiv2019

A major locus controls a biologically active pheromone component in Heliconius melpomene

Understanding the production, response, and genetics of signals used in mate choice can inform our understanding of the evolution of both intraspecific mate choice and reproductive isolation. Sex pheromones are important for courtship and mate choice in many insects, but we know relatively little of their role in butterflies. The butterfly Heliconius melpomene uses a complex blend of wing androconial compounds during courtship. Electroantennography in H. melpomene and its close relative H. cydno showed that responses to androconial extracts were not species-specific. Females of both species responded equally strongly to extracts of both species, suggesting conservation of peripheral nervous system elements across the two species. Individual blend components provoked little to no response, with the exception of octadecanal, a major component of the H. melpomene blend. Supplementing octadecanal on the wings of octadecanal-rich H. melpomene males led to an increase in the time until mating, demonstrating the bioactivity of octadecanal in Heliconius. Using quantitative trait locus (QTL) mapping, we identified a single locus on chromosome 20 responsible for 41% of the parental species difference in octadecanal production. This QTL does not overlap with any of the major wing color or mate choice loci, nor does it overlap with known regions of elevated or reduced FST. A set of 16 candidate fatty acid biosynthesis genes lies underneath the QTL. Pheromones in Heliconius carry information relevant for mate choice and are under simple genetic control, suggesting they could be important during speciation.

evolutionary biology↗

bioRxiv2019

Whole-chromosome hitchhiking driven by a male-killing endosymbiont

Neo-sex chromosomes are found in many taxa, but the forces driving their emergence and spread are poorly understood. The female-specific neo-W chromosome of the African monarch (or queen) butterfly Danaus chrysippus presents an intriguing case study because it is restricted to a single contact zone population, involves a putative colour patterning supergene, and co-occurs with infection by the the male-killing endosymbiont Spiroplasma. We investigated the origin and evolution of this system using whole genome sequencing. We first identify the BC supergene, a broad region of suppressed recombination across nearly half a chromosome, which links two colour patterning loci. Association analysis suggests that the genes yellow and arrow in this region control the forewing colour pattern differences between D. chrysippus subspecies. We then show that the same chromosome has recently formed a neo-W that has spread through the contact zone within [~]2200 years. We also assembled the genome of the male-killing Spiroplasma, and find that it shows perfect genealogical congruence with the neo-W, suggesting that the neo-W has hitchhiked to high frequency as the male killer has spread through the population. The complete absence of female crossing-over in the Lepidoptera causes whole-chromosome hitchhiking of a single neo-W haplotype, carrying a single allele of the BC supergene, and dragging multiple non-synonymous mutations to high frequency. This has created a population of infected females that all carry the same recessive colour patterning allele, making the phenotypes of each successive generation highly dependent on uninfected male immigrants. Our findings show how hitchhiking can occur between the unlinked genomes of host and endosymbiont, with dramatic consequences.

evolutionary biology↗

bioRxiv2019

Classic and introgressed selective sweeps shape mimicry loci across a butterfly adaptive radiation

Natural selection leaves distinct signatures in the genome that can reveal the targets and history of adaptive evolution. By analysing high-coverage genome sequence data from four major colour pattern loci sampled from nearly 600 individuals in 53 populations, we show pervasive selection on wing patterns across the Heliconius adaptive radiation. The strongest signatures correspond to loci with the greatest phenotypic effects, consistent with visual selection by predators, and are found in colour patterns with geographically restricted distributions. These recent sweeps are similar between co-mimics and indicate colour pattern turn-over events despite strong stabilizing selection. Using simulations we compare sweep signatures expected under classic hard sweeps with those resulting from adaptive introgression, an important aspect of mimicry evolution in Heliconius. Simulated recipient populations show a distinct volcano pattern with peaks of increased genetic diversity around the selected target, consistent with patterns found in some populations. Our genomic data provide unprecedented insights into the recent history of selection across the Heliconius adaptive radiation.

evolutionary biology↗

bioRxiv2019

The conspicuousness of the toxic Heliconius butterflies across time and habitat

Forests are a mosaic of light spectra, and colour signal efficiency might change in different light environments. Local adaptation in Heliconius butterflies is linked to microhabitat use and the colourful wing colour patterns may be adapted for signalling in different light environments. These toxic butterflies exhibit conspicuous colours as a warning to predators that they should be avoided, but also find and choose potential mates based on colour signals. The two selection pressures of predation and mate preference are therefore acting together. Colour conspicuousness should show habitat-specific contrast for the butterflies, which would facilitate detection and species identification. On the other hand, selection for signal stability would be stronger in the avian visual system. In this study we analysed the contrast of two Heliconius mimicry rings in their natural habitats under varying degrees of forest fragmentation and light conditions. We used digital image analyses and mapped the bird and butterfly vision colour space in order to examine whether warning colours have greater contrast and if they transmit a consistent signal across time of the day and habitat in a tropical forest. We tested conspicuousness using opponent colour channels against a natural green background. For avian vision, colours are generally very stable through time and habitat. For butterfly vision, there is some evidence that species are more contrasting in their own habitats, where conspicuousness is higher for red and yellow bands in the border and for white in the forest. Light environment affects Heliconius butterflies warning signal transmission to a higher degree through their own vision, but to a lesser degree through avian predator vision. This work provides insight into the use of colour signals in sexual and natural selection in the light of ecological adaptation.

ecology↗

bioRxiv2019

Species specificity and intraspecific variation in the chemical profiles of Heliconius butterflies across a large geographic range

In many animals, mate choice is important for the maintenance of reproductive isolation between species. Traits important for mate choice and behavioural isolation are predicted to be under strong stabilising selection within species, however such traits can also exhibit variation at the population level driven by neutral and adaptive evolutionary processes. Here, we describe patterns of divergence among androconial and genital chemical profiles at inter- and intra-specific levels in mimetic Heliconius butterflies. Most variation in chemical bouquets was found between species, but there were also quantitative differences at the population level. We found a strong correlation between interspecific chemical and genetic divergence, however, this correlation varied in intraspecific comparisons. We identified indicator compounds characteristic of particular species that included known biologically active compounds, suggesting an approach for identification of candidate compounds for future behavioural studies in novel systems. Overall, the strong signal of species identity suggests a role for these compounds in species recognition, but with additional potentially neutral variation at the population level.

evolutionary biology↗

bioRxiv2019

Conservation and flexibility in the gene regulatory landscape of Heliconiine butterfly wings

BackgroundMany traits evolve by cis-regulatory modification, by which changes to non-coding sequences affect the binding affinity for available transcription factors and thus modify the expression profile of genes. Multiple examples of cis-regulatory evolution have been described at pattern switch genes responsible for butterfly wing pattern polymorphism, including in the diverse neotropical genus Heliconius, but the identities of the factors that can regulate these switch genes have not been identified. ResultsWe investigated the spatial transcriptomic landscape across the wings of three closely related butterfly species, two of which have a convergently-evolved, co-mimetic pattern, the other having a divergent pattern. We identified candidate factors for regulating the expression of wing patterning genes, including transcription factors with a conserved expression profile in all three species, and others, including both transcription factors and Wnt pathway genes, with markedly different profiles in each of the three species. We verified the conserved expression profile of the transcription factor homothorax by immunofluorescence, and showed that its expression profile strongly correlates with that of the selector gene optix in butterflies with the Amazonian forewing pattern element dennis. ConclusionsHere we show that, in addition to factors with conserved expression profiles like homothorax, there are also a variety of transcription factors and signaling pathway components that appear to vary in their expression profiles between closely related butterfly species, highlighting the importance of genome-wide regulatory evolution between species.

evolutionary biology↗

bioRxiv2018

Genomic architecture and introgression shape a butterfly radiation

We here pioneer a low-cost assembly strategy for 20 Heliconiini genomes to characterize the evolutionary history of the rapidly radiating genus Heliconius. A bifurcating tree provides a poor fit to the data, and we therefore explore a reticulate phylogeny for Heliconius. We probe the genomic architecture of gene flow, and develop a new method to distinguish incomplete lineage sorting from introgression. We find that most loci with non-canonical histories arose through introgression, and are strongly underrepresented in regions of low recombination and high gene density. This is expected if introgressed alleles are more likely to be purged in such regions due to tighter linkage with incompatibility loci. Finally, we identify a hitherto unrecognized inversion, and show it is a convergent structural rearrangement that captures a known color pattern switch locus within the genus. Our multi-genome assembly approach enables an improved understanding of adaptive radiation.

evolutionary biology↗

bioRxiv2018

Genome-wide admixture is common across the Heliconius radiation

How frequent is gene flow between species? The pattern of evolution is typically portrayed as a phylogenetic tree, implying that speciation is a series of splits between lineages. Yet gene flow between good species is increasingly recognized as an important mechanism in the diversification of radiations, often spreading adaptive traits and leading to a complex pattern of phylogenetic incongruence. This process has thus far been studied in cases involving few species, or geographically restricted to spaces like islands, but not on the scale of a continental radiation. Previous studies have documented gene flow, adaptive introgression and hybrid speciation in a small subsection of the charismatic Neotropical butterflies Heliconius. Using genome-wide resequencing of 40 out of 45 species in the genus we demonstrate for the first time that admixture has played a role throughout the evolution of Heliconius and the sister genus Eueides. Modelling of phylogenetic networks based on 6848 orthologous autosomal genes (Maximum Pseudo-Likelihood Networks) or 5,483,419 high quality SNPs (Ancestral Recombination Graph) uncovers nine new cases of interspecific gene flow at up to half of the genome. However, f4 statistics of admixture show that the extent of the process has varied between subgenera. Evidence for introgression is found at all five loci controlling the colour and shape of the mimetic wing patterns, including in the putative hybrid species H. hecalesia, characterised by an unusual hindwing. Due to hybridization and incomplete coalescence during rapid speciation, individual gene trees show rampant discordance. Although reduced gene flow and faster coalescence are expected at the Z chromosome, we discover high levels of conflict between the 416 sex-linked loci. Despite this discordant pattern, both concatenation and multispecies coalescent approaches yield surprisingly consistent and fully supported genome-wide phylogenies. We conclude that the imposition of the bifurcating tree model without testing for interspecific gene flow may distort our perception of adaptive radiations and thus the ability to study trait evolution in a comparative framework.

evolutionary biology↗

bioRxiv2018

Generalisation behaviour of predators toward warning signals displayed by harmful prey: answers from a video-game played by humans.

The persistence of several warning signals in sympatry is a puzzling evolutionary question because selection favours convergence of colour patterns among toxic species. Such convergence is shaped by predators reaction to similar but not identical stimulus, i.e. generalisation behaviour. However, studying generalisation behaviour in complex natural communities of predators is challenging, and is thus generally limited to simple variations of prey colour patterns. Here, we used humans as surrogate predators to investigate generalisation behaviours on two prey communities with different level of warning signals complexity. Humans generalisation capacities were estimated using a computer game simulating a simple (4 morphs) and a complex (10 morphs) community of defended (associated with a penalty) and palatable butterflies. Colour patterns used in the game are actually observed in natural populations of the defended butterflies H. numata, and generalisation behaviour of natural predators communities on these colour patterns have previously been investigated in the wild, allowing direct comparison with human behaviour. We investigated human predation behaviour by recording attack rates on the different defended and palatable colour patterns, as well as player survival time (i.e. score). Phenotypic similarity among the different colour patterns was precisely quantified using a custom algorithm accounting for both colour and pattern variations (CPM method). By analysing attack behaviours of 491 game players, we found that learning was more efficient in the simple prey community. Additionally, profitable prey gained protection from sharing key visual features with unprofitable prey in both communities while learning, in accordance with natural predator behaviours. Moreover, other behaviours observed in natural predators, such as colour neophobia, were detected in humans and shaped morph vulnerability during the game. Similarities between our results in humans and the reaction of natural predator communities to the same colour patterns validate our video-game as a useful proxy to study predator behaviour. This experimental set-up can thus be compared to natural systems, enabling further investigations of generalisation on mimicry evolution.

evolutionary biology↗

bioRxiv2018

Sexually dimorphic gene expression and transcriptome evolution provides mixed evidence for a fast-Z effect in Heliconius

Sex chromosomes have different evolutionary properties as compared to the autosomes due to their hemizygous nature. In particular, recessive mutations are more readily exposed to selection, which can lead to faster rates of molecular evolution. Here, we report patterns of gene expression and molecular evolution in the sex chromosomes of a group of tropical butterflies. We first improved the completeness of the Heliconius melpomene reference annotation, a neotropical butterfly with a ZW sex determination system. Then we sequenced RNA from male and female whole abdomens and female ovary and gut tissue to identify sex and tissue specific gene expression profiles in H. melpomene. Using these expression profiles we compare sequence divergence and polymorphism, the strength of positive and negative selection and rates of adaptive evolution for Z and autosomal genes between two species of Heliconius butterflies, H. melpomene and H. erato.\n\nWe show that the rate of adaptive substitutions is higher for Z as compared to autosomal genes, but contrary to expectation it is also higher for male as compared to female biased genes. There is therefore mixed evidence that hemizygosity influences the rate of adaptive substitutions. Additionally, we find no significant increase in the rate of adaptive evolution or purifying selection on genes expressed in ovary tissue, a heterogametic specific tissue. Together our results provide limited support for fast-Z evolution. This contributes to a growing body of literature from other ZW systems that also provide mixed evidence for a fast-Z effect.

evolutionary biology↗

bioRxiv2018

Male pheromone composition depends on larval but not adult diet in Heliconius melpomene

Condition-dependent traits can act as honest signals of mate quality, with fitter individuals able to display preferred phenotypes. Nutrition is known to be an important determinant of individual condition, with diet known to affect many secondary sexual traits. In Heliconius butterflies, male chemical signalling plays an important role in female mate choice. Heliconius pheromones are sexually dimorphic, found only in mature males, but it is unclear what information they convey to the female. Here, we manipulate both the larval and adult diet of male Heliconius melpomene rosina to test for environmental effects on wing and genital pheromone production. We find no evidence that adult pollen consumption affects pheromone production in the first ten days after eclosion. We also find strong overlap in the chemical profiles of individuals reared on different larval host plants. However, individual compounds were found in different amounts between host plant treatments. Further electrophysiological and behavioural experiments will be needed to determine the biological significance of these differences.

ecology↗

bioRxiv2017

Patterns of Z chromosome divergence among Heliconius species highlight the importance of historical demography

Sex chromosomes are disproportionately involved in reproductive isolation and adaptation. In support of such a large-X effect, genome scans between recently diverged populations or species pairs often identify distinct patterns of divergence on the sex chromosome compared to autosomes. When measures of divergence between populations are higher on the sex chromosome compared to autosomes, such patterns could be interpreted as evidence for faster divergence on the sex chromosome, i.e. faster-X, or barriers to gene flow on the sex chromosome. However, demographic changes can strongly skew divergence estimates and are not always taken into consideration. We used 224 whole genome sequences representing 36 populations from two Heliconius butterfly clades (H. erato and H. melpomene) to explore patterns of Z chromosome divergence. We show that increased divergence compared to equilibrium expectations can in many cases be explained by demographic change. Among Heliconius erato populations, for instance, population size increase in the ancestral population can explain increased absolute divergence measures on the Z chromosome compared to the autosomes, as a result of increased ancestral Z chromosome genetic diversity. Nonetheless, we do identify increased divergence on the Z chromosome relative to the autosomes in parapatric or sympatric species comparisons that imply post-zygotic reproductive barriers. Using simulations, we show that this is consistent with reduced gene flow on the Z chromosome, perhaps due to greater accumulation of species incompatibilities. Our work demonstrates the importance of constructing an appropriate demographic null model in order to interpret patterns of divergence on the Z chromosome, but nonetheless provides evidence to support the Z chromosome as a strong barrier to gene flow in incipient Heliconius butterfly species.

evolutionary biology↗

bioRxiv2017

What shapes the continuum of reproductive isolation? Lessons from Heliconius butterflies.

The process by which species evolve can be illuminated by investigating barriers that limit gene flow between taxa. Recent radiations, such as Heliconius butterflies, offer the opportunity to compare isolation between pairs of taxa at different stages of ecological, geographic and phylogenetic divergence. We carry out a comparative analysis of existing and novel data in order to quantify the strength and direction of isolating barriers within a well-studied clade of Heliconius. Our results highlight that increased divergence is associated with the accumulation of stronger and more numerous barriers to gene flow. Wing pattern is both under natural selection for Mullerian mimicry and involved in mate choice, and therefore underlies several isolating barriers. However, pairs which share a similar wing pattern, also display strong reproductive isolation mediated by traits other than wing pattern. This suggests that, while wing pattern is a key factor for early stages of divergence, it is not essential at a higher level. Additional factors including habitat isolation, hybrid sterility and chemically-mediated mate choice are associated with complete speciation. Therefore, although most previous work has emphasised the role of wing pattern, our comparative results highlight that speciation is a multidimensional process, whose completion is stabilized by many factors.

evolutionary biology↗

bioRxiv2017

North Andean origin and diversification of the largest ithomiine butterfly genus

The Neotropics harbour the most diverse flora and fauna on Earth. The Andes are a major centre of diversification and source of diversity for adjacent areas in plants and vertebrates, but studies on insects remain scarce, even though they constitute the largest fraction of terrestrial biodiversity. Here, we combine molecular and morphological characters to generate a dated phylogeny of the butterfly genus Pteronymia (Nymphalidae: Danainae), which we use to infer spatial, elevational and temporal diversification patterns. We first propose six taxonomic changes that raise the generic species total to 53, making Pteronymia the most diverse genus of the tribe Ithomiini. Our biogeographic reconstruction shows that Pteronymia originated in the Northern Andes, where it diversified extensively. Some lineages colonized lowlands and adjacent montane areas, but diversification here remained scarce. The recent colonization of lowland areas was reflected by an increase in the rate of evolution of species elevational ranges towards present. By contrast, speciation rate decelerated with time, with no extinction. The geological history of the Andes and adjacent regions have likely contributed to Pteronymia diversification by providing compartmentalized habitats and an array of biotic and abiotic conditions, and by limiting dispersal between some areas while promoting interchange across others.

evolutionary biology↗

bioRxiv2016

Recombination Suppression is Unlikely to Contribute to Speciation in Sympatric Heliconius Butterflies

Mechanisms that suppress recombination are known to help maintain species barriers by preventing the breakup of co-adapted gene combinations. The sympatric butterfly species H. melpomene and H. cydno are separated by many strong barriers, but the species still hybridise infrequently in the wild, with around 40% of the genome influenced by introgression. We tested the hypothesis that genetic barriers between the species are reinforced by inversions or other mechanisms to reduce between-species recombination rate. We constructed fine-scale recombination maps for Panamanian populations of both species and hybrids to directly measure recombination rate between these species, and generated long sequence reads to detect inversions. We find no evidence for a systematic reduction in recombination rates in F1 hybrids, and also no evidence for inversions longer than 50 kb that might be involved in generating or maintaining species barriers. This suggests that mechanisms leading to global or local reduction in recombination do not play a significant role in the maintenance of species barriers between H. melpomene and H. cydno.\n\nAuthor SummaryIt is now possible to study the process of species formation by sequencing the genomes of multiple closely related species. Heliconius melpomene and Heliconius cydno are two butterfly species that have diverged over the past 2 million years and have different colour patterns, mate preferences and host plants. However, they still hybridise infrequently in the wild and exchange large parts of their genomes. Typically, when genomes are exchanged, chromosomes are recombined and gene combinations are broken up, preventing species from forming. Theory predicts that gene variants that define species might be linked together because of structural differences in their genomes, such as inverted pieces of chromosomes that will not be broken up when the species hybridise. However, in this paper, we use deep sequencing of large crosses of butterflies to show that there are no long chromosome regions that are not broken up during hybridisation, and no long chromosome inversions anywhere between the two genomes. This suggests that hybridisation is rare enough and mate preference is strong enough that inversions are not necessary to maintain the species barrier.

evolutionary biology↗