REPRODUCTIVE POTENTIAL AND GENETICS OF TRIPLOID PACIFIC OYSTERS, CRASSOSTREA-GIGAS (THUNBERG)

The reproductive potential and genetics of triploidy were studied in t he Pacific oyster. DNA content in sperm from triploids showed a single peak at 1.5c as determined by flow cytometry. In eggs from triploids, trivalents were the dominant form of synapsed chromosomes,, although the degree of synapsis varied considerably within and among females. S ome eggs went through complete synapsis and formed 10 trivalent chromo somes; most had a mixture of 11-13 trivalents, bivalents, and univalen ts. Factorial matings were produced from diploid (D) and triploid (T) parent oysters, creating four crosses: DD, DT, TD, and TT (female firs t). Gametes from triploids were fully capable of fertilization. After fertilization, eggs from triploids went through two meioses and releas ed two polar bodies as diploid eggs did. Karyological analyses showed that average ploidy of the resultant embryos was 2.0 n for DD, 2.46 n for DT, 2.52 n for TD, and 2.88 n for TT. Survival of fertilized eggs to metamorphosis and settlement was about 21% for DD, but considerably lower on other crosses: 0.0007% for DT, 0.0463% for TD, and 0.0085% f or TT. Nine months after matings, all survivors from DT crosses were d iploid. Survivors from TD crosses consisted of 33% diploids, 57% tripl oids, and 10% tetraploids. Survivors from the TT crosses consisted of 90% triploids, 4% diploids, and 6% mosaics. We hypothesize that differ ences in ploidy composition between DT and TD embryos and survivors we re caused by pro-egg segregations that favor the retention, rather tha n loss, of extra chromosomes in the egg. The reproductive potential of triploids and evolutionary implications are discussed.