Sex chromosomes and speciation in birds and other <scp>ZW</scp> systems

Irwin, Darren E.

doi:10.1111/mec.14537

Cited by 114 publications

(162 citation statements)

References 124 publications

(316 reference statements)

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“…In contrast, prolonged long‐distance allopatry in Plegadis may have suppressed genome‐wide heterogeneity in divergence. We did find that divergence, based on d XY , was higher on the Z chromosome relative to autosomes, consistent with proliferating evidence for the importance of the Z chromosome in avian divergence and speciation (Irwin, ).…”

Section: Discussionsupporting

confidence: 89%

Evolutionary dynamics of hybridization and introgression following the recent colonization of Glossy Ibis (Aves: Plegadis falcinellus) into the New World

et al. 2019

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Geographic range shifts can cause secondary contact and hybridization between closely related species, revealing mechanisms of species formation and integrity. These dynamics typically play out in restricted geographic regions, but highly vagile species may experience major distributional changes resulting in broad areas of contact. The Glossy Ibis (Plegadis falcinellus) is a dispersive waterbird of the Old World and Australia that colonized eastern North America in the early 19th century and came into contact with the native White‐faced Ibis (P. chihi). Putative hybrids between the two species have been observed across North America. To examine the population genomic consequences of this natural invasion, we sequenced 4,616 ultraconserved elements from 66 individuals sampled across the distributions of falcinellus, chihi, and the Puna Ibis (P. ridgwayi) of South America. We found genomic differentiation among the three species. Loci with high sequence divergence were often shared across all pairwise species comparisons, were associated with regions of high nucleotide diversity, and were concentrated on the Z chromosome. We detected signals of genetic admixture between chihi and falcinellus in individuals both near and far from their core area of sympatry. Genomic cline analyses revealed evidence of greater introgression into falcinellus from chihi, but we found little evidence for selection against hybrids. We also found signals of admixture between ridgwayi and South American populations of chihi. Our results indicate vagile species can experience pervasive introgression upon secondary contact, although we suggest these dynamics may be more ephemeral than the stable hybrid zones often observed in less dispersive organisms.

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Section: Discussionsupporting

confidence: 89%

Evolutionary dynamics of hybridization and introgression following the recent colonization of Glossy Ibis (Aves: Plegadis falcinellus) into the New World

et al. 2019

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“…The simplest explanation, then, for why observed F stX / F stA ratios are often higher than expected is that the ratio of sex‐linked to autosomal effective sizes, N X / N A , is <0.75. In birds, for instance, Z/autosome ratios of nucleotide diversity imply N Z / N A ratios that are typically <0.75, with some as low as ~0.25 (Huang & Rabosky, ; Irwin, ; Mank, Vicoso, & Charlesworth, ; Wright et al., ). Under these circumstances, F stZ can be as much as fourfold higher than F stA during the early stages of divergence (when t /2 N is small; Figure ).…”

Section: Sex Chromosome Differentiation Without Gene Flowmentioning

confidence: 99%

Evaluating genomic signatures of “the large X‐effect” during complex speciation

2018

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The ubiquity of the "two rules of speciation"-Haldane's rule and the large X-effect-implies a general, special role for sex chromosomes in the evolution of intrinsic postzygotic reproductive isolation. The recent proliferation of genome-scale analyses has revealed two further general observations: (a) complex speciation involving some form of gene flow is not uncommon, and (b) sex chromosomes in male- and in female-heterogametic taxa tend to show elevated differentiation relative to autosomes. Together, these observations are consistent with speciation histories in which population genetic differentiation at autosomal loci is reduced by gene flow while natural selection against hybrid incompatibilities renders sex chromosomes relatively refractory to gene flow. Here, I summarize multilocus population genetic and population genomic evidence for greater differentiation on the X (or Z) vs. the autosomes and consider the possible causes. I review common population genetic circumstances involving no selection and/or no interspecific gene flow that are nevertheless expected to elevate differentiation on sex chromosomes relative to autosomes. I then review theory for why large X-effects exist for hybrid incompatibilities and, more generally, for loci mediating local adaptation. The observed levels of sex chromosome vs. autosomal differentiation, in many cases, appear consistent with simple explanations requiring neither large X-effects nor gene flow. Discerning signatures of large X-effects during complex speciation will therefore require analyses that go beyond chromosome-scale summaries of population genetic differentiation, explicitly test for differential introgression, and/or integrate experimental genetic data.

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“…Enriching the faster‐X discussion, Irwin () synthesizes theory concerning sex chromosome evolution and its impact on speciation in species with ZW sex determination. As in XY systems, selection favouring recessive adaptive mutations and relaxed selection against weakly deleterious variants are both expected to elevate divergence on the Z. Irwin () highlights the potential for sexually antagonistic traits, which should map preferentially to the sex chromosomes, to generate selection for mutations involved in mating preference. Comparing within‐species polymorphism to between‐species differentiation, Irwin () estimates that the ratio of effective population sizes on the Z and the autosomes ranges widely among bird species (from 0.135–0.806).…”

Section: Faster Evolution Of Sequencesmentioning

confidence: 99%

“…As in XY systems, selection favouring recessive adaptive mutations and relaxed selection against weakly deleterious variants are both expected to elevate divergence on the Z. Irwin () highlights the potential for sexually antagonistic traits, which should map preferentially to the sex chromosomes, to generate selection for mutations involved in mating preference. Comparing within‐species polymorphism to between‐species differentiation, Irwin () estimates that the ratio of effective population sizes on the Z and the autosomes ranges widely among bird species (from 0.135–0.806). He suggests that this observation, along with elevated differentiation on the Z, is attributable to a higher frequency or strength of positive selection on this chromosome.…”

Section: Faster Evolution Of Sequencesmentioning

confidence: 99%