Detailed knowledge of early embryo development is a prerequisite for implementation of reproductive biotechnologies. We described the timing of mid-blastula transition in the sturgeon Acipenser ruthenus and hybrid of A. ruthenus and Acipenser gueldenstaedtii. To identify mitotic changes during cell division, the nuclei of embryo blastomeres were sampled every 30 min from 4 to 25 h post-fertilization and stained with 5 mg/ml 4'-6-diaminido-2phenylindole. We observed embryos of hybrids to lose synchrony after the eighth cell division, one cycle earlier than in purebred sterlet embryos. To reveal the timing of zygotic gene activation, developing embryos were dechorionated and injected with 10, 50, 100, 250, or 500 µg/ml a-Amanitin. Termination of development in embryos was recorded throughout the period from the 1000 cell stage until gastrulation, showing the process of transition from maternal to zygotic gene transcription in sterlet to be of extended duration and not directly linked to mid-blastula transition.

Asexual reproduction can be triggered by interspecific hybridization, but its emergence is supposedly rare, relying on exceptional combinations of suitable genomes. To examine how genomic and karyotype divergence between parental lineages affect the incidence of asexual gametogenesis, we experimentally hybridized fishes (Cobitidae) across a broad phylogenetic spectrum, assessed by whole exome data. Gametogenic pathways generally followed a continuum from sexual reproduction in hybrids between closely related evolutionary lineages to sterile or inviable crosses between distant lineages. However, most crosses resulted in a combination of sterile males and asexually-reproducing females. Their gametes usually experienced problems in chromosome pairing, but females also produced a certain proportion of oocytes with premeiotically duplicated genomes, enabling their development into clonal eggs. Interspecific hybridization may thus commonly affect cell cycles in a specific way, allowing the formation of unreduced oocytes. The emergence of asexual gametogenesis appears tightly linked to hybrid sterility and constitutes an inherent part of the extended speciation continuum.