Genetics and the Evolution of Prezygotic Isolation

Pervasive gene flow despite strong and varied reproductive barriers in swordtails

The evolution of reproductive barriers leads to the formation of new species. However, recent research has demonstrated that hybridization has been pervasive across the tree of life even in the presence of strong barriers. Using swordtail fishes (genus Xiphophorus), an emerging model system, we document overlapping mechanisms that act as barriers to gene flow between Xiphophorus birchmanni and Xiphophorus cortezi by combining genomic sequencing from natural hybrid populations, experimental laboratory crosses, behavioural assays, sperm measures and developmental studies. We show that assortative mating plays a role in maintaining subpopulations with distinct ancestry within natural hybrid populations. Using F2 hybrids we identify several genomic regions that strongly impact hybrid viability. Strikingly, two of these regions underlie genetic incompatibilities in hybrids between X. birchmanni and its sister species Xiphophorus malinche. Our results demonstrate that ancient hybridization has played a role in the origin of this shared genetic incompatibility. Moreover, ancestry mismatch at these incompatible regions has remarkably similar consequences for phenotypes and hybrid survival in X. cortezi × X. birchmanni hybrids as in X. malinche × X. birchmanni hybrids. Our findings identify varied reproductive barriers that shape genetic exchange between naturally hybridizing species and highlight the complex evolutionary outcomes of hybridization.

Speciation

What drives the emergence of new species has fascinated biologists since Darwin. Reproductive barriers to gene flow are a key step in the formation of species, and recent advances have shed new light on how these are established. Genetic, genomic, and comparative techniques, together with improved theoretical frameworks, are increasing our understanding of the underlying mechanisms. They are also helping us forecast speciation and reveal the impact of human activity.

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The Role of Hybridization in Species Formation and Persistence

Hybridization, or interbreeding between different taxa, was traditionally considered to be rare and to have a largely detrimental impact on biodiversity, sometimes leading to the breakdown of reproductive isolation and even to the reversal of speciation. However, modern genomic and analytical methods have shown that hybridization is common in some of the most diverse clades across the tree of life, sometimes leading to rapid increase of phenotypic variability, to introgression of adaptive alleles, to the formation of hybrid species, and even to entire species radiations. In this review, we identify consensus among diverse research programs to show how the field has progressed. Hybridization is a multifaceted evolutionary process that can strongly influence species formation and facilitate adaptation and persistence of species in a rapidly changing world. Progress on testing this hypothesis will require cooperation among different subdisciplines.

Synthesis and Scope of the Role of Postmating Prezygotic Isolation in Speciation

How barriers to gene flow arise and are maintained are key questions in evolutionary biology. Speciation research has mainly focused on barriers that occur either before mating or after zygote formation. In comparison, postmating prezygotic (PMPZ) isolation-a barrier that acts after gamete release but before zygote formation-is less frequently investigated but may hold a unique role in generating biodiversity. Here we discuss the distinctive features of PMPZ isolation, including the primary drivers and molecular mechanisms underpinning PMPZ isolation. We then present the first comprehensive survey of PMPZ isolation research, revealing that it is a widespread form of prezygotic isolation across eukaryotes. The survey also exposes obstacles in studying PMPZ isolation, in part attributable to the challenges involved in directly measuring PMPZ isolation and uncovering its causal mechanisms. Finally, we identify outstanding knowledge gaps and provide recommendations for improving future research on PMPZ isolation. This will allow us to better understand the nature of this often-neglected reproductive barrier and its contribution to speciation.

Coupling of Barriers to Gene Exchange: Causes and Consequences

Coupling has emerged as a concept to describe the transition from differentiated populations to newly evolved species through the strengthening of reproductive isolation. However, the term has been used in multiple ways, and relevant processes have sometimes not been clearly distinguished. Here, we synthesize existing uses of the concept of coupling and find three main perspectives: (1) coupling as the build-up of linkage disequilibrium among loci underlying barriers to gene exchange, (2) coupling as the build-up of genome-wide linkage disequilibrium, and (3) coupling as the process generating a coincidence of distinct barrier effects. We compare and contrast these views, show the diverse processes involved and the complexity of the relationships among recombination, linkage disequilibrium, and reproductive isolation, and, finally, we emphasize how each perspective can guide new directions in speciation research. Although the importance of coupling for evolutionary divergence and speciation is well established, many theoretical and empirical questions remain unanswered.

Pervasive gene flow despite strong and varied reproductive barriers in swordtails

One of the mechanisms that can lead to the formation of new species occurs through the evolution of reproductive barriers. However, recent research has demonstrated that hybridization has been pervasive across the tree of life even in the presence of strong barriers. Swordtail fishes (genus Xiphophorus) are an emerging model system for studying the interface between these barriers and hybridization. We document overlapping mechanisms that act as barriers between closely related species, X. birchmanni and X. cortezi, by combining genomic sequencing from natural hybrid populations, artificial crosses, behavioral assays, sperm performance, and developmental studies. We show that strong assortative mating plays a key role in maintaining subpopulations with distinct ancestry in natural hybrid populations. Lab experiments demonstrate that artificial F1 crosses experience dysfunction: crosses with X. birchmanni females were largely inviable and crosses with X. cortezi females had a heavily skewed sex ratio. Using F2 hybrids we identify several genomic regions that strongly impact hybrid viability. Strikingly, two of these regions underlie genetic incompatibilities in hybrids between X. birchmanni and its sister species X. malinche. Our results demonstrate that ancient hybridization has played a role in the origin of this shared genetic incompatibility. Moreover, ancestry mismatch at these incompatible regions has remarkably similar consequences for phenotypes and hybrid survival in X. cortezi × X. birchmanni hybrids as in X. malinche × X. birchmanni hybrids. Our findings identify varied reproductive barriers that shape genetic exchange between naturally hybridizing species and highlight the complex evolutionary outcomes of hybridization.