Low fertility of wild hybrid male flycatchers despite recent divergence

Regulatory and evolutionary impact of DNA methylation in two songbird species and their naturally occurring F1 hybrids

BACKGROUND

Regulation of transcription by DNA methylation in 5'-CpG-3' context is a widespread mechanism allowing differential expression of genetically identical cells to persist throughout development. Consequently, differences in DNA methylation can reinforce variation in gene expression among cells, tissues, populations, and species. Despite a surge in studies on DNA methylation, we know little about the importance of DNA methylation in population differentiation and speciation. Here we investigate the regulatory and evolutionary impact of DNA methylation in five tissues of two Ficedula flycatcher species and their naturally occurring F1 hybrids.

RESULTS

We show that the density of CpG in the promoters of genes determines the strength of the association between DNA methylation and gene expression. The impact of DNA methylation on gene expression varies among tissues with the brain showing unique patterns. Differentially expressed genes between parental species are predicted by genetic and methylation differentiation in CpG-rich promoters. However, both these factors fail to predict hybrid misexpression suggesting that promoter mismethylation is not a main determinant of hybrid misexpression in Ficedula flycatchers. Using allele-specific methylation estimates in hybrids, we also determine the genome-wide contribution of cis- and trans effects in DNA methylation differentiation. These distinct mechanisms are roughly balanced in all tissues except the brain, where trans differences predominate.

CONCLUSIONS

Overall, this study provides insight on the regulatory and evolutionary impact of DNA methylation in songbirds.

Sperm in the Mammalian Female Reproductive Tract: Surfing Through the Tract to Try to Beat the Odds

Mammalian sperm are deposited in the vagina or the cervix/uterus at coitus or at artificial insemination, and the fertilizing sperm move through the female reproductive tract to the ampulla of the oviduct, the site of fertilization. But the destination of most sperm is not the oviduct. Most sperm are carried by retrograde fluid flow to the vagina, are phagocytosed, and/or do not pass barriers on the pathway to the oviduct. The sperm that reach the site of fertilization are the exceptions and winners of one of the most stringent selection processes in nature. This review discusses the challenges sperm encounter and how the few sperm that reach the site of fertilization overcome them. The sperm that reach the goal must navigate viscoelastic fluid, swim vigorously and cooperatively along the walls of the female tract, avoid the innate immune system, and respond to potential cues to direct their movement.

Evidence that genetic drift not adaptation drives fast-Z and large-Z effects in Ficedula flycatchers

The sex chromosomes have been hypothesized to play a key role in driving adaptation and speciation across many taxa. The reason for this is thought to be the hemizygosity of the heteromorphic part of sex chromosomes in the heterogametic sex, which exposes recessive mutations to natural and sexual selection. The exposure of recessive beneficial mutations increases their rate of fixation on the sex chromosomes, which results in a faster rate of evolution. In addition, genetic incompatibilities between sex-linked loci are exposed faster in the genomic background of hybrids of divergent lineages, which makes sex chromosomes contribute disproportionately to reproductive isolation. However, in birds, which show a Z/W sex determination system, the role of adaptation versus genetic drift as the driving force of the faster differentiation of the Z chromosome (fast-Z effect) and the disproportionate role of the Z chromosome in reproductive isolation (large-Z effect) are still debated. Here, we address this debate in the bird genus Ficedula flycatchers based on population-level whole-genome sequencing data of six species. Our analysis provides evidence for both faster lineage sorting and reduced gene flow on the Z chromosome than the autosomes. However, these patterns appear to be driven primarily by the increased role of genetic drift on the Z chromosome, rather than an increased rate of adaptive evolution. Genomic scans of selective sweeps and fixed differences in fact suggest a reduced action of positive selection on the Z chromosome.

Ecology and age, but not genetic ancestry, predict fetal loss in a wild baboon hybrid zone

OBJECTIVES

Pregnancy failure represents a major fitness cost for any mammal, particularly those with slow life histories such as primates. Here, we quantified the risk of fetal loss in wild hybrid baboons, including genetic, ecological, and demographic sources of variance. We were particularly interested in testing the hypothesis that hybridization increases fetal loss rates. Such an effect would help explain how baboons may maintain genetic and phenotypic integrity despite interspecific gene flow.

MATERIALS AND METHODS

We analyzed outcomes for 1020 pregnancies observed over 46 years in a natural yellow baboon-anubis baboon hybrid zone. Fetal losses and live births were scored based on records of female reproductive state and the appearance of live neonates. We modeled the probability of fetal loss as a function of a female's genetic ancestry (the proportion of her genome estimated to be descended from anubis [vs. yellow] ancestors), age, number of previous fetal losses, dominance rank, group size, climate, and habitat quality using binomial mixed effects models.

RESULTS

Female genetic ancestry did not predict fetal loss. Instead, the risk of fetal loss is elevated for very young and very old females. Fetal loss is most robustly predicted by ecological factors, including poor habitat quality prior to a home range shift and extreme heat during pregnancy.

DISCUSSION

Our results suggest that gene flow between yellow and anubis baboons is not impeded by an increased risk of fetal loss for hybrid females. Instead, ecological conditions and female age are key determinants of this component of female reproductive success.

Species-specific song responses emerge as a by-product of tuning to the local dialect

Oscine birds preferentially respond to certain sounds over others from an early age, which focuses subsequent learning onto sexually relevant songs.^1,2,3 Songs vary both across species and, due to cultural evolution, among populations of the same species. As a result, early song responses are expected to be shaped by selection both to avoid the fitness costs of cross-species learning⁴ and to promote learning of population-typical songs.⁵ These sources of selection are not mutually exclusive but can result in distinct geographic patterns of song responses in juvenile birds: if the risks of interspecific mating are the main driver of early song discrimination, then discrimination should be strongest where closely related species co-occur.⁴ In contrast, if early discrimination primarily facilitates learning local songs, then it should be tuned to songs typical of the local dialect.^5,6,7 Here, we experimentally assess the drivers of song discrimination in nestling pied flycatchers (Ficedula hypoleuca). We first demonstrate that early discrimination against the songs of the closely related collared flycatcher (F. albicollis) is not strongly affected by co-occurrence. Second, across six European populations, we show that nestlings' early song responses are tuned to their local song dialect and that responses to the songs of collared flycatchers are similarly weak as to those of other conspecific dialects. Taken together, these findings provide clear experimental support for the hypothesis that cultural evolution, in conjunction with associated learning predispositions, drives the emergence of pre-mating reproductive barriers.

Sexually dichromatic hybrids between two monochromatic duck species, the Chiloé wigeon and the Philippine duck

Captive bird hybrids can provide important data on certain traits, such as hybrid viability and fertility. In this paper, we describe four hybrids between the Chiloé wigeon (Anas sibilatrix) and the Philippine duck (Anas luzonica). These two species diverged about 13 million years ago and are found on different continents, making the occurrence of wild hybrids extremely unlikely. Hence, these captive hybrids provide a unique opportunity to learn more about the outcome of hybridization between these highly divergent species. One pair of hybrids mated and produced six unfertilized eggs, suggesting that hybrids between these species are infertile. Morphologically, the hybrids were slightly larger than the parental species, but had intermediate bill lengths. With regard to plumage patterns, the hybrids displayed characteristics of both parental species: Males developed the iridescent green head pattern of the Chiloé wigeon, whereas the females showed the dark crown and eye stripe of the Philippine duck. Interestingly, Chiloé wigeon and Philippine duck are both sexually monochromatic whereas the hybrids showed clear sexual dimorphism. These hybrids can thus lead to novel insights into the genetic and developmental basis of sexual mono- and dichromatism in ducks.

Should females prefer old males?

Whether females should prefer to mate with old males is controversial. Old males may sire offspring of low quality because of an aging germline, but their proven ability to reach an old age can also be an excellent indicator of superior genetic quality, especially in natural populations. These genetic effects are, however, hard to study in nature, because they are often confounded with direct benefits offered by old males to the female, such as experience and high territory quality. We, therefore, used naturally occurring extra‐pair young to disentangle different aspects of male age on female fitness in a natural population of collared flycatchers because any difference between within‐ and extra‐pair young within a nest should be caused by paternal genetic effects only. Based on 18 years of long‐term data, we found that females paired with older males as social partners experienced an overall reproductive advantage. However, offspring sired by old males were of lower quality as compared to their extra‐pair half‐siblings, whereas the opposite was found in nests attended by young males. These results imply a negative genetic effect of old paternal age, given that extra‐pair males are competitive middle‐age males. Thus, offspring may benefit from being sired by young males but raised by old males, to maximize both genetic and direct effects. Our results show that direct and genetic benefits from pairing with old males may act in opposing directions and that the quality of the germline may deteriorate before other signs of senescence become obvious.

Geographic contact drives increased reproductive isolation in two cryptic Empidonax flycatchers

Geographic contact between sister lineages often occurs near the final stages of speciation, but its role in speciation's completion remains debated. Reproductive isolation may be essentially complete prior to secondary contact. Alternatively, costly interactions between partially reproductively isolated species - such as maladaptive hybridization or competition for resources - may select for divergence, increasing reproductive isolation and driving speciation toward completion. Here, we use coalescent demographic modelling and whole-genome data sets to show that a period of contact and elevated hybridization between sympatric eastern North American populations of two cryptic bird species preceded a major increase in reproductive isolation between these populations within the last 10,000 years. In contrast, substantial introgression continues to the present in a western contact zone where geographic overlap is much narrower and probably of more recent origin. In the sympatric eastern region where reproductive isolation has increased, it is not accompanied by character displacement in key morphometric traits, plumage coloration, or ecological traits. While the precise trait and underlying mechanism driving increased reproductive isolation remains unknown, we discuss several possibilities and outline avenues for future research. Overall, our results highlight how demographic models can reveal the geographic context in which reproductive isolation was completed, and demonstrate how contact can accelerate the final stages of speciation.

Sperm length divergence as a potential prezygotic barrier in a passerine hybrid zone

The saltmarsh sparrow Ammospiza caudacuta and Nelson's sparrow A. nelsoni differ in ecological niche, mating behavior, and plumage, but they hybridize where their breeding distributions overlap. In this advanced hybrid zone, past interbreeding and current backcrossing result in substantial genomic introgression in both directions, although few hybrids are currently produced in most locations. However, because both species are nonterritorial and have only brief male-female interactions, it is difficult to determine to what extent assortative mating explains the low frequency of hybrid offspring. Since females often copulate with multiple males, a role of sperm as a postcopulatory prezygotic barrier appears plausible. Here, we show that sperm length differs between the two species in the hybrid zone, with low among-male variation consistent with strong postcopulatory sexual selection on sperm cells. We hypothesize that divergence in sperm length may constitute a reproductive barrier between species, as sperm length co-evolves with the size of specialized female sperm storage tubules. Sperm does not appear to act as a postzygotic barrier, as sperm from hybrids was unexceptional.

Adaptive coloration in pied flycatchers (Ficedula hypoleuca)-The devil is in the detail

Understanding the origin and persistence of phenotypic variation within and among populations is a major goal in evolutionary biology. However, the eagerness to find unadulterated explanatory models in combination with difficulties in publishing replicated studies may lead to severe underestimations of the complexity of selection patterns acting in nature. One striking example is variation in plumage coloration in birds, where the default adaptive explanation often is that brightly colored individuals signal superior quality across environmental conditions and therefore always should be favored by directional mate choice. Here, we review studies on the proximate determination and adaptive function of coloration traits in male pied flycatchers (Ficedula hypoleuca). From numerous studies, we can conclude that the dark male color phenotype is adapted to a typical northern climate and functions as a dominance signal in male-male competition over nesting sites, and that the browner phenotypes are favored by relaxed intraspecific competition with more dominant male collared flycatchers (Ficedula albicollis) in areas where the two species co-occur. However, the role of avoidance of hybridization in driving character displacement in plumage between these two species may not be as important as initially thought. The direction of female choice on male coloration in pied flycatchers is not simply as opposite in direction in sympatry and allopatry as traditionally expected, but varies also in relation to additional contexts such as climate variation. While some of the heterogeneity in the observed relationships between coloration and fitness probably indicate type 1 errors, we strongly argue that environmental heterogeneity and context-dependent selection play important roles in explaining plumage color variation in this species, which probably also is the case in many other species studied in less detail.

Tissue-specific patterns of regulatory changes underlying gene expression differences among Ficedula flycatchers and their naturally occurring F1 hybrids

Changes in interacting cis- and trans-regulatory elements are important candidates for Dobzhansky-Muller hybrid incompatibilities and may contribute to hybrid dysfunction by giving rise to misexpression in hybrids. To gain insight into the molecular mechanisms and determinants of gene expression evolution in natural populations, we analyzed the transcriptome from multiple tissues of two recently diverged Ficedula flycatcher species and their naturally occurring F₁ hybrids. Differential gene expression analysis revealed that the extent of differentiation between species and the set of differentially expressed genes varied across tissues. Common to all tissues, a higher proportion of Z-linked genes than autosomal genes showed differential expression, providing evidence for a fast-Z effect. We further found clear signatures of hybrid misexpression in brain, heart, kidney, and liver. However, while testis showed the highest divergence of gene expression among tissues, it showed no clear signature of misexpression in F₁ hybrids, even though these hybrids were found to be sterile. It is therefore unlikely that incompatibilities between cis-trans regulatory changes explain the observed sterility. Instead, we found evidence that cis-regulatory changes play a significant role in the evolution of gene expression in testis, which illustrates the tissue-specific nature of cis-regulatory evolution bypassing constraints associated with pleiotropic effects of genes.

Mitochondria and the Origin of Species: Bridging Genetic and Ecological Perspectives on Speciation Processes

Mitochondria have been known to be involved in speciation through the generation of Dobzhansky-Muller incompatibilities, where functionally neutral co-evolution between mitochondrial and nuclear genomes can cause dysfunction when alleles are recombined in hybrids. We propose that adaptive mitochondrial divergence between populations can not only produce intrinsic (Dobzhansky-Muller) incompatibilities, but could also contribute to reproductive isolation through natural and sexual selection against migrants, post-mating prezygotic isolation, as well as by causing extrinsic reductions in hybrid fitness. We describe how these reproductive isolating barriers can potentially arise through adaptive divergence of mitochondrial function in the absence of mito-nuclear coevolution, a departure from more established views. While a role for mitochondria in the speciation process appears promising, we also highlight critical gaps of knowledge: (1) many systems with a potential for mitochondrially-mediated reproductive isolation lack crucial evidence directly linking reproductive isolation and mitochondrial function; (2) it often remains to be seen if mitochondrial barriers are a driver or a consequence of reproductive isolation; (3) the presence of substantial gene flow in the presence of mito-nuclear incompatibilities raises questions whether such incompatibilities are strong enough to drive speciation to completion; and (4) it remains to be tested how mitochondrial effects on reproductive isolation compare when multiple mechanisms of reproductive isolation coincide. We hope this perspective and the proposed research plans help to inform future studies of mitochondrial adaptation in a manner that links genotypic changes to phenotypic adaptations, fitness, and reproductive isolation in natural systems, helping to clarify the importance of mitochondria in the formation and maintenance of biological diversity.

Heterosis in cattle crossbreeding schemes in tropical regions: meta-analysis of effects of breed combination, trait type, and climate on level of heterosis

The aim of this study was to investigate the effects of animal trait, breed combination, and climate on the expressed levels of heterosis in crossbreeding schemes using tropical cattle. A meta-analysis of 42 studies was carried out with 518 heterosis estimates. In total, 62.5% of estimates were found to be significantly different from zero, the majority of which (89.8%) were beneficial for the studied trait. Trait and breed combination were shown to have a significant effect on the size of heterosis (P < 0.001 and P = 0.044, respectively). However, climate did not have a significant effect. Health, longevity, and milk production traits showed the highest heterosis (31.84 ± 10.73%, 35.13 ± 14.35%, and 35.15 ± 3.29%, respectively), whereas fertility, growth, and maternal traits showed moderate heterosis (12.02 ± 4.10%, 12.25 ± 2.69%, and 15.69 ± 3.26%, respectively). Crosses between breeds from different types showed moderate to high heterosis ranging from 9.95 ± 4.53% to 19.53 ± 3.62%, whereas crosses between breeds from the same type did not express heterosis that was significantly different from zero. These results show that heterosis has significant and favorable impact on productivity of cattle farming in tropical production systems, particularly in terms of fitness but also milk production traits.

Maladaptive secondary sexual characteristics reduce the reproductive success of hybrids between native and non-native salmonids

Human-mediated hybridization between introduced and native species is one of the most serious threats to native taxa. Although field studies have attempted to quantify the relative fitness or reproductive success of parental species and their hybrids, only a few studies have unraveled the factors determining the fitness of hybrids. Here, we hypothesized that maladaptive secondary sexual characteristics may reduce fitness of hybrids between two fish species. To test this, we evaluated the reproductive success of introduced brook trout (BT: Salvelinus fontinalis), native white-spotted charr (WSC: S. leucomaenis) and their hybrids in a natural stream in Hokkaido, Japan, where the two parental species show remarkably different male secondary sexual characteristics, such as elongated jaws and deeper bodies. We predicted that introgression from WSC is maladaptive for BT males because the BT male has more prominent secondary sexual characteristics. Our results suggest that both sexual selection and outbreeding depression in males and females significantly influence an individual's reproductive success. Our results also suggest that asymmetric introgression may increase the risks to persistence in the recipient species.

Genomic islands of differentiation in two songbird species reveal candidate genes for hybrid female sterility

Hybrid sterility is a common first step in the evolution of postzygotic reproductive isolation. According to Haldane's Rule, it affects predominantly the heterogametic sex. While the genetic basis of hybrid male sterility in organisms with heterogametic males has been studied for decades, the genetic basis of hybrid female sterility in organisms with heterogametic females has received much less attention. We investigated the genetic basis of reproductive isolation in two closely related avian species, the common nightingale (Luscinia megarhynchos) and the thrush nightingale (L. luscinia), that hybridize in a secondary contact zone and produce viable hybrid progeny. In accordance with Haldane's Rule, hybrid females are sterile, while hybrid males are fertile, allowing gene flow to occur between the species. Using transcriptomic data from multiple individuals of both nightingale species, we identified genomic islands of high differentiation (F_ST ) and of high divergence (D_xy ), and we analysed gene content and patterns of molecular evolution within these islands. Interestingly, we found that these islands were enriched for genes related to female meiosis and metabolism. The islands of high differentiation and divergence were also characterized by higher levels of linkage disequilibrium than the rest of the genome in both species indicating that they might be situated in genomic regions of low recombination. This study provides one of the first insights into genetic basis of hybrid female sterility in organisms with heterogametic females.

Song discrimination by nestling collared flycatchers during early development

Pre-zygotic isolation is often maintained by species-specific signals and preferences. However, in species where signals are learnt, as in songbirds, learning errors can lead to costly hybridization. Song discrimination expressed during early developmental stages may ensure selective learning later in life but can be difficult to demonstrate before behavioural responses are obvious. Here, we use a novel method, measuring changes in metabolic rate, to detect song perception and discrimination in collared flycatcher embryos and nestlings. We found that nestlings as early as 7 days old respond to song with increased metabolic rate, and, by 9 days old, have increased metabolic rate when listening to conspecific when compared with heterospecific song. This early discrimination between songs probably leads to fewer heterospecific matings, and thus higher fitness of collared flycatchers living in sympatry with closely related species.

Incipient ecological speciation between successional varieties of a dominant tree involves intrinsic postzygotic isolating barriers

Whereas disruptive selection imposed by heterogeneous environments can lead to the evolution of extrinsic isolating barriers between diverging populations, the evolution of intrinsic postzygotic barriers through divergent selection is less certain. Long-lived species such as trees may be especially slow to evolve intrinsic isolating barriers. We examined postpollination reproductive isolating barriers below the species boundary, in an ephemeral hybrid zone between two successional varieties of the landscape-dominant Hawaiian tree, Metrosideros polymorpha, on volcanically active Hawai'i Island. These archipelago-wide sympatric varieties show the weakest neutral genetic divergence of any taxon pair on Hawai'i Island but significant morphological and ecological differentiation consistent with adaptation to new and old lava flows. Cross-fertility between varieties was high and included heterosis of F1 hybrids at the seed germination stage, consistent with a substantial genetic load apparent within varieties through low self-fertility and a lack of self-pollen discrimination. However, a partial, but significant, barrier was observed in the form of reduced female and male fertility of hybrids, especially backcross hybrids, consistent with the accumulation of genetic incompatibilities between varieties. These results suggest that partial intrinsic postzygotic barriers can arise through disruptive selection acting on large, hybridizing populations of a long-lived species.

Hybrid Dysfunction Expressed as Elevated Metabolic Rate in Male Ficedula Flycatchers

Studies of ecological speciation are often biased towards extrinsic sources of selection against hybrids, resulting from intermediate hybrid morphology, but the knowledge of how genetic incompatibilities accumulate over time under natural conditions is limited. Here we focus on a physiological trait, metabolic rate, which is central to life history strategies and thermoregulation but is also likely to be sensitive to mismatched mitonuclear interactions. We measured the resting metabolic rate of male collared, and pied flycatchers as well as of naturally occurring F1 hybrid males, in a recent hybrid zone. We found that hybrid males had a higher rather than intermediate metabolic rate, which is indicative of hybrid physiological dysfunction. Fitness costs associated with elevated metabolic rate are typically environmentally dependent and exaggerated under harsh conditions. By focusing on male hybrid dysfunction in an eco-physiological trait, our results contribute to the general understanding of how combined extrinsic and intrinsic sources of hybrid dysfunction build up under natural conditions.

Competition-driven build-up of habitat isolation and selection favoring modified dispersal patterns in a young avian hybrid zone

Competition-driven evolution of habitat isolation is an important mechanism of ecological speciation but empirical support for this process is often indirect. We examined how an on-going displacement of pied flycatchers from their preferred breeding habitat by collared flycatchers in a young secondary contact zone is associated with (a) access to an important food resource (caterpillar larvae), (b) immigration of pied flycatchers in relation to habitat quality, and (c) the risk of hybridization in relation to habitat quality. Over the past 12 years, the estimated access to caterpillar larvae biomass in the habitat surrounding the nests of pied flycatchers has decreased by a fifth due to shifted establishment possibilities, especially for immigrants. However, breeding in the high quality habitat has become associated with such a high risk of hybridization for pied flycatchers that overall selection currently favors pied flycatchers that were forced to immigrate into the poorer habitats (despite lower access to preferred food items). Our results show that competition-driven habitat segregation can lead to fast habitat isolation, which per se caused an opportunity for selection to act in favor of future "voluntarily" altered immigration patterns and possibly strengthened habitat isolation through reinforcement.

Patterns of gene flow and selection across multiple species of Acrocephalus warblers: footprints of parallel selection on the Z chromosome

Background

Understanding the mechanisms and selective forces leading to adaptive radiations and origin of biodiversity is a major goal of evolutionary biology. Acrocephalus warblers are small passerines that underwent an adaptive radiation in the last approximately 10 million years that gave rise to 37 extant species, many of which still hybridize in nature. Acrocephalus warblers have served as model organisms for a wide variety of ecological and behavioral studies, yet our knowledge of mechanisms and selective forces driving their radiation is limited. Here we studied patterns of interspecific gene flow and selection across three European Acrocephalus warblers to get a first insight into mechanisms of radiation of this avian group.

Results

We analyzed nucleotide variation at eight nuclear loci in three hybridizing Acrocephalus species with overlapping breeding ranges in Europe. Using an isolation-with-migration model for multiple populations, we found evidence for unidirectional gene flow from A. scirpaceus to A. palustris and from A. palustris to A. dumetorum. Gene flow was higher between genetically more closely related A. scirpaceus and A. palustris than between ecologically more similar A. palustris and A. dumetorum, suggesting that gradual accumulation of intrinsic barriers rather than divergent ecological selection are more efficient in restricting interspecific gene flow in Acrocephalus warblers. Although levels of genetic differentiation between different species pairs were in general not correlated, we found signatures of apparently independent instances of positive selection at the same two Z-linked loci in multiple species.

Conclusions

Our study brings the first evidence that gene flow occurred during Acrocephalus radiation and not only between sister species. Interspecific gene flow could thus be an important source of genetic variation in individual Acrocephalus species and could have accelerated adaptive evolution and speciation rate in this avian group by creating novel genetic combinations and new phenotypes. Independent instances of positive selection at the same loci in multiple species indicate an interesting possibility that the same loci might have contributed to reproductive isolation in several speciation events.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0692-2) contains supplementary material, which is available to authorized users.

Climate adaptation and speciation: particular focus on reproductive barriers in Ficedula flycatchers

Climate adaptation is surprisingly rarely reported as a cause for the build‐up of reproductive isolation between diverging populations. In this review, we summarize evidence for effects of climate adaptation on pre‐ and postzygotic isolation between emerging species with a particular focus on pied (Ficedula hypoleuca) and collared (Ficedula albicollis) flycatchers as a model for research on speciation. Effects of climate adaptation on prezygotic isolation or extrinsic selection against hybrids have been documented in several taxa, but the combined action of climate adaptation and sexual selection is particularly well explored in Ficedula flycatchers. There is a general lack of evidence for divergent climate adaptation causing intrinsic postzygotic isolation. However, we argue that the profound effects of divergence in climate adaptation on the whole biochemical machinery of organisms and hence many underlying genes should increase the likelihood of genetic incompatibilities arising as side effects. Fast temperature‐dependent co‐evolution between mitochondrial and nuclear genomes may be particularly likely to lead to hybrid sterility. Thus, how climate adaptation relates to reproductive isolation is best explored in relation to fast‐evolving barriers to gene flow, while more research on later stages of divergence is needed to achieve a complete understanding of climate‐driven speciation.

Morphology-function relationships and repeatability in the sperm of Passer sparrows

Sperm performance is likely to be an important determinant of male reproductive success, especially when females copulate with multiple males. Understanding sperm performance is therefore crucial to fully understand the evolution of male reproductive strategies. In this study, we examined the repeatability of sperm morphology and motility measures over three breeding seasons, and we studied relationships between sperm morphology and function. We conducted this study in wild-derived captive house sparrows (Passer domesticus) and Spanish sparrows (P. hispaniolensis). Results for the two species were similar. As predicted from results in other passerine species, total sperm length was highly repeatable across ejaculates, and repeatability for the length of other components was moderate. The repeatability of sperm swimming speed across ejaculates was lower, but statistically significant, suggesting that sperm velocity may be a relatively dynamic trait. Surprisingly, swimming speed did not correlate with the relative length of the midpiece, and it correlated negatively with the relative length of the flagellum and with total sperm length. This pattern is the opposite of what theory predicts and differs from what has been found in house sparrows before. Also contrary to previous work, we found no evidence that total sperm length correlates with sperm longevity. These results therefore highlight the need for a better understanding of relationships between sperm morphology and function in passerine birds.

Estimation of linkage disequilibrium and interspecific gene flow in Ficedula flycatchers by a newly developed 50k single-nucleotide polymorphism array

With the access to draft genome sequence assemblies and whole-genome resequencing data from population samples, molecular ecology studies will be able to take truly genome-wide approaches. This now applies to an avian model system in ecological and evolutionary research: Old World flycatchers of the genus Ficedula, for which we recently obtained a 1.1 Gb collared flycatcher genome assembly and identified 13 million single-nucleotide polymorphism (SNP)s in population resequencing of this species and its sister species, pied flycatcher. Here, we developed a custom 50K Illumina iSelect flycatcher SNP array with markers covering 30 autosomes and the Z chromosome. Using a number of selection criteria for inclusion in the array, both genotyping success rate and polymorphism information content (mean marker heterozygosity = 0.41) were high. We used the array to assess linkage disequilibrium (LD) and hybridization in flycatchers. Linkage disequilibrium declined quickly to the background level at an average distance of 17 kb, but the extent of LD varied markedly within the genome and was more than 10-fold higher in ‘genomic islands’ of differentiation than in the rest of the genome. Genetic ancestry analysis identified 33 F₁ hybrids but no later-generation hybrids from sympatric populations of collared flycatchers and pied flycatchers, contradicting earlier reports of backcrosses identified from much fewer number of markers. With an estimated divergence time as recently as <1 Ma, this suggests strong selection against F₁ hybrids and unusually rapid evolution of reproductive incompatibility in an avian system.

Failed sperm development as a reproductive isolating barrier between species

Hybrid male sterility is a common reproductive isolating barrier between species. Yet, little is known about the actual developmental causes of this phenomenon, especially in naturally hybridizing species. We sought to evaluate the developmental causes of hybrid male sterility, using spadefoot toads as our study system. Plains spadefoot toads (Spea bombifrons) and Mexican spadefoot toads (S. multiplicata) hybridize where they co-occur in the southwestern USA. Hybrids are viable, but hybrid males suffer reduced fertility. We compared testes size and developmental stages of sperm cell maturation between hybrid males and males of each species. We found that testes of hybrid males did not differ in mean size from pure-species males. However, hybrids showed a greater range of within-individual variation in testes size than pure-species males. Moreover, although hybrids produced similar numbers of early stage sperm cells, hybrids produced significantly fewer mature spermatozoids than pure-species males. Interestingly, an introgressed individual produced numbers of live sperm comparable to pure-species males, but the majority of these sperm cells were abnormally shaped and non-motile. These results indicate that hybrid incompatibilities in late sperm development serve as a reproductive isolating barrier between species. The nature of this breakdown highlights the possibilities that hybrid males may vary in fertility and that fertility could possibly be recovered in introgressed males.

Incipient post-zygotic barrier in a model system of ecological speciation with gene flow

The role of post-zygotic isolation in nonallopatric ecological speciation is still mostly unknown and information on the nature and strength of these barriers in well-known speciation models is essential for a deeper understanding of such processes. The Galician ecotypes of the marine snail Littorina saxatilis represent one of the best studied cases of nonallopatric ecological speciation. Here, we test the existence of incipient post-zygotic isolation by comparing the fertility of male hybrids with that of both pure forms [ridged and banded (RB) and smooth and unbanded (SU) ecotypes]. We analysed the degree of sperm DNA fragmentation (SDF) of individuals morphologically classified as RB, SU and hybrids, sampled from two locations. SDF analyses were chosen to study sperm quality because, in other animal species, SDF rates correlate with important parameters for speciation research, such as fertilization and abortion rates and viability of adult progeny. In the present work, hybrids showed significantly higher SDF rates than RB and SU males in one location and significantly higher variances in both locations. These results suggest the existence of an incipient post-zygotic barrier, the strength of which may vary across the Galician shore, and highlight the potential of SDF analyses for speciation research.

Low fertility of wild hybrid male flycatchers despite recent divergence

Postzygotic isolation may be important for maintaining species boundaries, particularly when premating barriers are incomplete. Little is known about the course of events leading from minor environmental mismatches affecting hybrid fitness to severe genetic incompatibilities causing sterility or inviability. We investigated whether reduced reproductive success of hybrid males was caused by suboptimal sperm traits or by more severe genetic incompatibilities in a hybrid zone of pied (Ficedula hypoleuca) and collared flycatchers (F. albicollis) on the island of Öland, Sweden. About 4 per cent hybridization is observed in this population and all female hybrids are sterile. We found no sperm in the ejaculates of most sampled hybrid males, and sperm with abnormal morphology in two hybrids. Furthermore, none of the hybrids sired any offspring because of high levels of hatching failure and extra-pair paternity in their nests. These results from a natural hybrid zone suggest that the spermatogenesis of hybrid males may become disrupted despite little genetic divergence between the parental species.