Gamete Compatibility and Early Development of Hybrids and Backcross Progenies Between Crassostrea gigas 'Haida No.1' and C. ariakensis

To investigate the heterosis of interspecific hybridization between Crassostrea gigas 'Haida No.1' (H) and C. ariakensis (A) and the vigor of backcross progenies (Ha) of hybrids Ha, we conducted a 3×3 factorial crossing design to generate nine experimental groups, HaHa (hybrid Ha ♀×hybrid Ha♂), HaH (hybrid Ha ♀×H♂), HaA (hybrid Ha ♀×A♂), HHa (H ♀×hybrid Ha♂), HH (H ♀×H♂), HA (H ♀×A♂), AHa (A ♀×hybrid Ha♂), AH (A ♀×H♂) and AA (A ♀×A♂). Early phenotypic traits and vigor of progenies were systematically analyzed. The results demonstrated that backcross significantly improved gamete compatibility between hybrid Ha and parents. Fertilization and hatching rates of HaH, HaA, HHa and AHa were markedly higher than those of HA and AH. During the planktonic larval stage, HaH and HaA exhibited parental single-parent backcross heterosis in growth traits while HHa showed survival advantages over both parents and HA. In contrast, AHa exhibited complete heterosis disadvantage. Although HaHa exhibited superior fertilization rate, hatching rate, growth velocity and survival rate, no statistically significant difference was observed compared to H, A or HA. In this study, we successfully obtained backcross progenies HaH, HHa and HaA with measurable heterosis. Our findings provided valuable insights into the genetic improvement of C. ariakensis and C. gigas.