Triploid hard clams Mercenaria mercenaria produced by inhibiting polar body I or polar body II

Two types of triploid hard clams Mercenaria mercenaria were produced by inhibiting polar body I (PB1) or polar body II (PB2) with cytochalasin B. Treatments were applied at 22-23 degrees C, with PB1 inhibition starting at 4-7min postfertilization and ending when PB2 was first observed in control groups, and with PB2 inhibition starting at 17-23min postfertilization and ending when 80% of control eggs released PB2. Triploid induction success was evaluated by chromosome counting in 2-4 cell embryos and by flow cytometry at larval and juvenile stages. PB2 inhibition produced more triploids (82%-100%) than PB1 inhibition (71%-83%), although the difference was not significant (p.088). Triploid percentages in PB1- or PB2-inhibited groups showed a small but insignificant decline during the first 6months. At month 3, PB1 and PB2 triploids were not different from their within-group diploids, but significantly larger than control diploids; PB1 triploids were significantly larger than PB2 triploids (p.003). At month 6, PB1 triploids were not different from either within-group or control-group diploids, while PB2 triploids were significant larger than both within-group and control diploid; PB1 triploids were smaller than PB2 triploids. At month 16, PB1 and PB2 triploids in one remaining replicate were not different from their within-group diploids. Overall, this study shows that triploids can be efficiently produced by PB1 or PB2 inhibition, and their growth performance relative to diploids is variable depending on age and replicates or parental genotype.