Houki Shouji, Yamada Mitsuya, Honda Takako, Komaru Akira
Atmosphere and Ocean Research Institute, The University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8564, Japan.
Zoolog Sci. 2011 Jul;28(7):526-31. doi: 10.2108/zsj.28.526.
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.
雌雄同体的日本蚬通过雄核发育进行繁殖，被视为同步雌雄同体。迄今为止，尚未有关于雌雄同体日本蚬中出现雄体的报道。在滋贺县的一条灌溉沟渠中，我们发现78.2%的日本蚬标本为雄体，21.8%为雌雄同体。微量荧光分析表明，雄体为二倍体，雌雄同体为三倍体。所有雄体均产生不减数且具双鞭毛的精子。对31个日本蚬标本的线粒体DNA（细胞色素b，621 bp）序列分析显示，4个雄体和9个雌雄同体共享相同的H2线粒体DNA单倍型；17个雄体检测到H1，1个雌雄同体检测到H3。共存的河蚬也有H1和H2单倍型。基于细胞色素b部分序列通过邻接法构建的系统发育树表明，日本蚬的单倍型（H1 - 3）明显不同于雌雄异体的三代蚬（S1和S2）和日本蚬（J1和J2）。这些结果表明，雄体可能源自雌雄同体的日本蚬。雄体在雌雄同体种群中的作用尚不清楚。然而，如果雄体的精子能够使雌雄同体的卵子受精，并且卵子的核基因组作为极体被排出，精子核就可以形成合子核。这种繁殖方式将允许核基因组的替换。
