Ploidy and Pronuclear Interaction in Northeastern Pacific Lasaea Clones (Mollusca: Bivalvia).

A natural population of the bivalve genus Lasaea from Victoria, British Columbia, Canada was karyologically characterized, and pronuclear interaction was studied in newly spawned eggs. Mitotic metaphases from 95 cells were enumerated, and chromosome numbers ranged from 58 to 108, with 90 to 100 being most frequent. Ten well-spread metaphases were karyotyped, and the chromosomes were classified into 32 triplet subgroupings on the basis of shared morphology and size, together with a variable number of supernumerary chromosomes. Northeastern Pacific Lasaea clones share broad karyological features with direct developing congeners, but detailed comparisons reveal that they have experienced different evolutionary mechanisms of polyploidy. The pronuclear interaction study generated two key pieces of evidence that establish that northeastern Pacific Lasaea clones do not reproduce by self-fertilization, but that parthenogenetic development is triggered by autosperm. The incorporated sperm nucleus disintegrates in the egg cortex and does not fuse with the egg "pronucleus," i.e., syngamy does not occur. Both polar bodies have a diploid chromosome number, a result inconsistent with meiosis, implying that they are products of mitotic divisions. Nonhybridizing lineages of northeastern Pacific Lasaea therefore represent true asexual clones, not inbred lines. Lasaea is the first bivalve genus in which asexual reproduction has been confirmed and is also the first molluscan genus in which pseudogamy (gynogenesis) has been detected.