This study advances understanding of hybrid compatibility in distantly related fish by utilizing reciprocal crosses between tetraploid Percocypris pingi (P4n) and a Schizothorax polyploid complex comprising tetraploid S. davidi (S4n) and hexaploidy S. prenanti (S6n). We generated four hybrid combinations (S6n × P4n, S4n × P4n, P4n × S4n, and P4n × S6n), establishing a gradient of maternal-to-paternal ploidy ratios (3:2, 2:2, 2:2, 2:3). Genetic and morphological analyses confirmed successful hybridization and excluded gynogenesis, as anticipated. Hybrid viability (S6n × P4n = S4n × P4n > P4n × S4n > P4n × S6n) exhibited a positive correlation with ploidy ratios, maternal-to-paternal egg volume ratios (P4n: 5.8 mm 3 ; S4n: 13.3 mm 3 ; S6n: 13.6 mm 3 ), and egg quality. The sufficiency of maternal factors (mRNAs/proteins stored in the egg, essential for early development) can explain the above results: high-ploidy sperm depleted maternal factors (e.g., histones), explaining the greater viability of P4n × S4n than P4n × S6n. Conversely, large eggs provided adequate maternal factors, enabling high viability in S6n × P4n and S4n × P4n. Hybrid developmental barriers only occurred in the early stages of development between ZGA (zygotic genome activation) and swimming stages. Viable offspring were produced in all crosses, including the unprecedented low-ploidy (♀, P4n) × high-ploidy (♂, S6n) combination, which is potentially attributable to the closer phylogenetic relatedness of the parental species, a moderate ploidy ratio (1:1.5), and good egg quality. These findings expand upon existing hybridization rules and provide new insights into the mechanisms governing compatibility in polyploid fish hybridization. • Multiple factors collectively effect hybrid viability. • Viable hybrids resulted from a low-ploidy female × high-ploidy male. • Hybrid developmental barriers occur between the ZGA and swimming stages.
Li et al. (Wed,) studied this question.