Extending the rules of fish distant hybridization using autopolyploid complex

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.