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.

Department of Biology and Ecology Faculty of Science University of Ostrava Chittussiho 10 710 00 Ostrava Czech Republic

Department of Zoology Faculty of Science Charles University Prague 128 00 Prague Czech Republic

Institute of Zoology MD 2028 Academiei 1 2001 Chisinau Moldova

Laboratory of Fish Genetics Institute of Animal Physiology and Genetics of the CAS Rumburská 89 277 21 Liběchov Czech Republic

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Sperm-dependent asexual species and their role in ecology and evolution

Achiasmatic meiosis in the unisexual Amazon molly, Poecilia formosa

Genome elimination from the germline cells in diploid and triploid male water frogs Pelophylax esculentus