Differences in early development between parental species and asexual hybrids forms of European spined loaches (genus Cobitis)

Introduction Hybridization and polyploidy are increasingly recognized as major drivers of evolutionary innovation. By merging and multiplying genomes, they induce profound regulatory and structural perturbations that can alter phenotypes, yet the mechanisms by which they influence development remain debated. In particular, while theory predicts that polyploidy increases cause the increase in cell size resulting in slower development, empirical findings are mixed. Moreover, the independent effects of polyploidization and hybrid genome admixture on early ontogeny are rarely disentangled. Here, we used the Cobitis taenia-C. elongatoides hybrid complex, a natural system comprising two sexual diploids and both diploid and triploid asexual hybrids to investigate how hybridization and polyploidy affect early development. Methods We examined two key developmental processes: (1) the timing of embryonic progression from the first cleavage to somitogenesis, and (2) migration of primordial germ cells (PGCs), which are crucial for reproductive competence. By comparing both hybrid types to each of their sexual progenitors, we were able to isolate the effects of genome merging versus genome multiplication. Results We found distinct developmental delays linked to effects of all, the parental genome, hybridization and ploidy. Initial cleavage dynamics was mostly attributable to differences between the two parental species, while later delays were more pronounced in hybrids, regardless of ploidy-highlighting hybridization-driven developmental shifts. Only at subsequent stages did ploidy-specific effects emerge, prolonging development in triploids against all diploid forms. In contrast, PGC number and migration were primarily influenced by ploidy, with triploids showing higher PGC counts than diploids. Conclusion Our findings reveal that hybridization and polyploidy exert both independent and interactive effects on early ontogeny. The Cobitis complex thus offers a powerful model for exploring developmental consequences of genome restructuring.