Frontiers in Marine Science (Dec 2015)
speciation through asexuality in fish: postzygotic reproductive isolation may be completed in spite of fertility of hybrids
Abstract
Speciation process often occurs in the face of hybridisation between forming species. Hybrid genomic constitution of many clonal organisms including fish, amphibians and reptiles also indicates tight linkage between hybridization, clonality and malfunction of sex in hybrids. To address the causal links between these phenomena, we investigated the evolution of asexuality and postzygotic reproductive isolation mechanisms (postRIMs) in the Cobitis loaches, which involves three sister species (C. taenia, C. pontica and C. tanaitica) and their distant relative, C. elongatoides. Interspecific crossings revealed that initiation of asexuality is causally linked to hybridization but does not require involvement of any particular genome. Rather, it depends on genetic distance between hybridizing parental taxa: crossings of distantly related elongatoides-taenia pair produced sterile males and fertile clonal females, while crossings of closely related species, C. pontica and C. taenia, produced fertile sexually reproducing hybrids of both sexes, with only rarely occurring clonal biotypes. Previous study of elongatoides-taenia hybrid zone together with current population genetic analysis of elongatoides-tanaitica hybrid zone demonstrated the completion of speciation: no BC or F2 hybrids have been revealed and inferred F1 hybrids were clonal. However, phylogenomic analysis revealed that gene flow between these species has been quite intensive in the past, leading to massive historical introgressions. Altogether, it appears that initiation of hybrid asexuality and the completion of speciation process through formation of postRIMs are interconnected phenomena. Both processes are linked to the genetic divergence of hybridizing taxa: initially, hybridization between little diverged species leads to recombinant and fertile hybrids allowing intensive gene flow. As the hybridizing taxa continue to diverge, clonally reproducing hybrid females and sterile males become dominant and the gene flow ceases. The speciation may therefore be completed through asexuality of hybrids The work was supported by grant no. 13-12580S provided by the Czech Science Foundation (www.gacr.cz). Further support was provided by the Academy of Sciences of the Czech Republic (www.cas.cz) by the grant no. RVO 67985904
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