LAUSR

Abstract Misgurnus anguillicaudatus and Paramisgurnus dabryanus are two loach species widely sympatrically distributed in China. Hybrids of the two species have been detected in the wild and produced by artificial hybridization experiments, but their sexual development has not been studied. In this study, the phenotypic traits of purebred and hybrid groups were evaluated in the aspects of survival, growth, gonad development and sex-related genes (dmrt1a, dmrt1b, cyp19a1a and cyp19a1b) expression. Hybrids and purebreds showed the same high levels of fertilization, hatching success and early survival. Growth of hybrids was intermediate between those of parental species, showing hybrid heterosis compared to M. anguillicaudatus. Cyp19a1a and dmrt1s were expressed in the ovary and testis but expression levels were sexually dimorphic. Although the exact timing of dmrt1s and cyp19a1a expression during the gonad development varied between groups, in general their expressions were correlated with the differentiation and formation of testes and ovaries respectively. Histological analysis showed that in purebreds ovaries differentiate/form much earlier than testes: 15/35 and 35/80 days after hatching (dah) respectively. In hybrids, however, both differentiation and formation are simultaneous: 35 and 80 dah respectively. Adult hybrids possessed mature gametes, but they appeared to exhibit weaker reproductive ability than either of the parental species. Comprehensive gene expression and histological analysis demonstrate that gonad development in hybrids is retarded than in parental species.