Fertility investigation in F1 hybrid and karyotype analysis of backcross progeny from Misgurnus anguillicaudatus and Paramisgurnus dabryanus

Dojo loach (Misgurnus anguillicaudatus) (DD, 2n = 50) and large-scale loach (Paramisgurnus dabryanus) (PP, 2n = 48) are freshwater fish species widely distributed across East Asia. Hybrids of these two loach species have disrupted gonadal development, but few studies have reported the actual fertility and chromosome number in gametes of reciprocal hybrid loaches (DD x PP and PP x DD, assigned as DP and PD, 2n = 49). In this study, selfing and backcrossing experiments between M. anguilicaudatus and P. dabryanus were conducted. The fertilization, hatching, deformity, and survival rates were examined along with the embryonic development and growth performance to analyze the actual fertility of the hybrid loach. The karyotypes of viable backcross offspring were studied to indirectly assess the chromosome number in eggs of DP and PD hybrids. The failure in fertilization among crosses using male hybrid proved that the male hybrid was sterile. The moderate fertilization rates (11.49%--49.16%), high hatching rates (82.78%-86.31%), high survival rates (78.67%-81.33%), and the normal embryonic development among crosses using female hybrid revealed that the female hybrid was fertile, albeit with relatively high deformity rates (12.68%-27.45%). The karyotype analysis of viable backcross offspring showed 49 chromosomes for DP x DD (assigned as DPD) and PD x DD (assigned as PDD) hybrids, and 48 chromosomes for DP x PP (assigned as DPP) and PD x PP (assigned as PDP) hybrids. This indicated that fertile gametes produced by female hybrid loach contained 24 chromosomes. This study provides valuable information for loach hybrid breeding and in-depth research on the precise mechanism of meiosis in these hybrid loach.