Abstract
We review knowledge about the roles of sex chromosomes in vertebrate hybridization and speciation, exploring a gradient of divergences with increasing reproductive isolation (speciation continuum). Under early divergence, well-differentiated sex chromosomes in meiotic hybrids may cause Haldane-effects and introgress less easily than autosomes. Undifferentiated sex chromosomes are more susceptible to introgression and form multiple (or new) sex chromosome systems with hardly predictable dominance hierarchies. Under increased divergence, most vertebrates reach complete intrinsic reproductive isolation. Slightly earlier, some hybrids (linked in ‘the extended speciation continuum') exhibit aberrant gametogenesis, leading towards female clonality. This facilitates the evolution of various allodiploid and allopolyploid clonal (‘asexual’) hybrid vertebrates, where ‘asexuality' might be a form of intrinsic reproductive isolation. A comprehensive list of ‘asexual' hybrid vertebrates shows that they all evolved from parents with divergences that were greater than at the intraspecific level (K2P-distances of greater than 5–22% based on mtDNA). These ‘asexual' taxa inherited genetic sex determination by mostly undifferentiated sex chromosomes. Among the few known sex-determining systems in hybrid ‘asexuals', female heterogamety (ZW) occurred about twice as often as male heterogamety (XY). We hypothesize that pre-/meiotic aberrations in all-female ZW-hybrids present Haldane-effects promoting their evolution. Understanding the preconditions to produce various clonal or meiotic allopolyploids appears crucial for insights into the evolution of sex, ‘asexuality' and polyploidy.This article is part of the theme issue ‘Challenging the paradigm in sex chromosome evolution: empirical and theoretical insights with a focus on vertebrates (Part II)’.
Highlights
Our understanding of speciation has evolved from being regarded as a long and steady process, governed by natural selection in various forms [1,2,3], to a view that includes dynamic and/or reticulate and potentially fast processes [4,5,6,7,8,9]
After a lead-in on intrinsic reproductive isolation and on sex chromosomes in speciation, we explore a gradient of divergences to review knowledge about the evolutionary impact of sex chromosomes under hybridization in vertebrates
We examine sex chromosomes by moving along various stages of increasing divergences and accumulating intrinsic reproductive isolation between hybridizing species until a stage is reached, when hybrid vertebrates evolve that rarely exhibit so-called ‘asexual’ reproductive modes
Summary
Our understanding of speciation has evolved from being regarded as a long and steady process, governed by natural selection in various forms [1,2,3], to a view that includes dynamic and/or reticulate and potentially fast processes [4,5,6,7,8,9]. The latter authors speculate that interspecific hybrids with different sex-determining systems may produce intersexes with reduced viability or fertility, directly contributing to postzygotic isolation [24] This suggests that even in early stages of divergence, undifferentiated, in this case non-homologous, sex chromosomes may over-proportionately contribute to the onset of emerging reproductive isolation [135]. While the systems described above (§2(a)) exemplify that genetic and evolutionary interactions by hybridization between incipient or even further separated species may result in hardly predictable outcomes, they demonstrate considerable evolutionary impact of sex chromosomes during early divergence Their introgression may even lead to the establishment of new sex chromosomes and sex determination systems. When closely related species differ in their sex determination systems, the outcomes might be more complex than in cases with the same or similar sex determination systems (table 2)
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