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Abstract
The transition from sexual reproduction to asexuality is often triggered by hybridization. The gametogenesis of many hybrid asexuals involves premeiotic genome endoreplication leading to bypass hybrid sterility and forming clonal gametes. However, it is still not clear when endoreplication occurs, how many gonial cells it affects and whether its rate differs among clonal lineages. Here, we investigated meiotic and premeiotic cells of diploid and triploid hybrids of spined loaches (Cypriniformes: Cobitis) that reproduce by gynogenesis. We found that in naturally and experimentally produced F1 hybrids asexuality is achieved by genome endoreplication, which occurs in gonocytes just before entering meiosis or, rarely, one or a few divisions before meiosis. However, genome endoreplication was observed only in a minor fraction of the hybrid’s gonocytes, while the vast majority of gonocytes were unable to duplicate their genomes and consequently could not proceed beyond pachytene due to defects in bivalent formation. We also noted that the rate of endoreplication was significantly higher among gonocytes of hybrids from natural clones than of experimentally produced F1 hybrids. Thus, asexuality and hybrid sterility are intimately related phenomena and the transition from sexual reproduction to asexuality must overcome significant problems with genome incompatibilities with a possible impact on reproductive potential.
Highlights
Species are fundamental evolutionary units, presumably evolving in a continuum from intermixing populations to independent entities isolated from other species by preand postzygotic barriers [1,2]
To test if the germ cells of diploid as well as triploid hybrids showed evidence of genome duplication, we determined the number of bivalents in oocytes during the diplotene stage
We used fluorescent in situ hybridization (FISH)-based identification of particular chromosomes using species polymorphic satellite markers, which is known to occur on two chromosomes in C. taenia and a single chromosome in C. elongatoides (Supplementary Figure S3C,D,G,H) [43]
Summary
Species are fundamental evolutionary units, presumably evolving in a continuum from intermixing populations to independent entities isolated from other species by preand postzygotic barriers [1,2]. Their formation is frequently accompanied by interspecific hybridization, which may have positive [1,3,4,5] as well as negative impacts [1,6,7] and appears to be a mighty evolutionary force. With incompatibilities accumulating among their genomes, the crossing of parental species often affects the fertility of hybrids leading to their sterility [8,9]. Hybridization affects interactions between the two admixed genomes but it can modify gametogenic pathways and induce the switch of a hybrid’s reproduction towards asexuality [5,12,13,14,15,16]
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