| Summary: | Three replicates of hybrid crosses of tetraploid and diploid C gigas (Thunberg) with diploid C ariakensis (Fujita) were produced with controls. Larval survival and growth were documented. Cytological events were also monitored in oocytes from hybrid crosses following insemination. Among the four types of hybrid crosses, diploid C. gigas (female) x diploid C. ariakensis (male) (GA) was the most successful. Survival of GA was about the same as that of controls in two of three replications, although its growth rate was 25-30% lower. Crosses of tetraploid C. gigas (female) and diploid C. ariakensis (male) (GGA) had poor yield at day 2 post-fertilization (0.05%), but grew nearly as well as controls subsequently. The other two types of hybrids (i.e., diploid C. ariakensis [female] and tetraploid C. gigas [male] [AGG], diploid C ariakensis [female] and diploid C. gigos [male] [AG]) suffered very low yield at day 2 (0.01% and 0.003%) and grew very slowly. Spat were obtained from all replicates of GA crosses and one of three replicates of GGA, and proved to be hybrids by polymerase chain reaction/restriction fragment length polymorphism (PCR/RFLP) diagnosis. GGA hybrids were confirmed to be triploid by flow cytometry. No larvae survived to eyed stage in AGG or AG crosses. Cytological examination revealed that the vast majority (>99%) of oocytes from hybrid crosses had a prolonged meiotic prophase I or metaphase I at least through 180 min post-insemination.
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