Origin and Possible Role of Males in Hermaphroditic AndrogeneticCorbiculaClams

Abstract

Hermaphroditic Corbicula leana clams reproduce by androgenesis and have been regarded as simultaneous hermaphrodites. To date, there has been no report on the occurrence of male clams in hermaphroditic Corbicula. In an irrigation ditch in Shiga Prefecture, we found that 78.2% of C. leana specimens were males and 21.8% were hermaphrodites. Microfluorometric analysis revealed that males were diploids and hermaphrodites were triploids. All males produced nonreductional and biflagellate spermatozoa. The sequence analysis of mitochondrial DNA (cytochrome b, 621 bp) for 31 specimens of C. leana showed that four male and nine hermaphrodites shared the same H2 mtDNA haplotype; H1 was detected from 17 males and H3 was detected from one hermaphrodite. Coexisting C. fluminea clams also have haplotypes H1 and H2. Phylogenetic tree by a neighborjoining method based on the partial sequence of cytochrome b revealed that the haplotypes (H1- 3) of C. leana were evidently different from those of dioecious C. sandai (S1 and S2) and C. japonica (J1 and J2). These results suggest that males may be derived from hermaphrodite C. leana clams. The role of males in hermaphroditic populations is unknown. However, if the spermatozoon from a male is able to fertilize an egg from a hermaphrodite and the nuclear genome of the egg is expelled as polar bodies, the sperm nucleus could form a zygote nucleus. This mode of reproduction would allow the replacement of the nuclear genome.

Androgenesis: a review through the study of the selfish shellfish Corbicula spp.

“…Within the genus Corbicula, biflagellate sperm are unique to androgenetic lineages (including males coexisting with androgens), as far as is known. Moreover, the situation of androdioecy, found recently in C. leana (Houki et al, 2011), is very rare in the animal kingdom (Weeks et al, 2006).…”

“…Self-fertilization has also been suggested in Corbicula (Ikematsu and Yamane, 1977;: embryos were found in the visceral mass, within the gametogenic follicles, whereas most embryos are usually incubated in the gills in hermaphrodites (for example, Glaubrecht et al, 2006). Recently, Houki et al (2011) found a Japanese population of C. leana that contains both males and hermaphroditic individuals. The cytological and genetic results suggest androdioecy, with androgenetic males fertilizing eggs of hermaphrodites.…”

“…Biflagellate unreduced sperm is only observed in androgenetic hermaphrodite Corbicula species (Table 2). Recently, males of C. leana coexisting with hermaphroditic androgenetic individuals were also found to produce biflagellate unreduced sperm (Houki et al, 2011). Most bivalves, however, have monoflagellate reduced sperm (the primitive or ancestral form).…”

“…Several hermaphroditic freshwater species of Corbicula produce unreduced spermatozoa: that is, sperm cells containing the same amount of DNA as somatic cells (Komaru et al, 1997, Houki et al, 2011see Table 2). On the other hand, the dioecious, sexually reproducing Corbicula species produce reduced sperm: the DNA content of spermatozoa is half of that of somatic cells.…”

“…Androgenetic Corbicula are hermaphroditic (except the recent discovery of males in Japan; Houki et al, 2011) and capable of selffertilization. If a newly colonized niche is suitable, asexual lineages will not suffer an ecological disadvantage and are expected to thrive and spread: only one individual is sufficient to establish a new population, and each individual can have as many as 90 000 offspring in one breeding season (McMahon, 1999).…”