Two shell color types of the exotic bivalve Corbicula fluminea were collected in Kyoto city, Japan. DNA microfluorometry revealed that both types were diploids with non-reductional spermatozoa. Maternal chromosomes were found to be extruded as two polar bodies at the first meiosis, and the second meiosis could not be observed. Only the male pronucleus was present in the egg cytoplasm and became metaphase chromosomes at the first mitosis. The present study indicates that the diploid C. fluminea in Japan has the same mode of androgenetic reproduction as the triploid C. leana.

No abstract

For pearl culture, the pearl oyster is forced open and a nucleus is implanted into the gonad with a mantle graft. The outer mantle epithelial cells of the implanted mantle graft elongate and surrounding the nucleus a pearl sac is formed. Shell matrix proteins secreted by the pearl sac play an important role in the regulation of pearl formation. Recently, seven shell matrix proteins were identified from the pearl oyster Pinctada fucata. However, there is a paucity of information on the function of these proteins and their gene expression patterns. Our study aims to elucidate the relationship between pearl type, quality, and gene expression patterns of six shell matrix proteins (msi60, n16, nacrein, msi31, prismalin-14, and aspein) in the pearl sac based on real-time PCR analysis. After culturing for about 2 months, the pearl sac tissues were collected from 22 individuals: 12 with high quality (HP), nine with low quality (LP), and one with organic (ORG) pearl formation. The surface of each of the 12 HP pearls was composed only of a nacreous layer; in contrast, that of the nine LP pearls was composed of nacreous and prismatic layers. The six target gene expressions were detected in all individuals. However, delta threshold cycle (ΔC(T)) for msi31 was significantly higher in the HP than in the LP individuals (Mann-Whitney's U test, p=0.02). This means that the relative expression level of msi31, which constitutes the framework of the prismatic layer, was higher in the LP than in the HP individuals.

For pearl culture, nucleus and mantle grafts are implanted into the gonad of the host oyster. The epithelial cells of the implanted mantle graft elongate and surround the nucleus, and a pearl sac is formed. Shell matrix proteins secreted by the pearl sac play an important role in pearl formation. We studied the gene expression patterns of six shell matrix proteins (msi60, n16, nacrein, msi31, prismalin-14, and aspein) in the epithelial cells associated with pearl sac formation. There were differences in the expression patterns of the six genes in the epithelial cells, and the relative expression levels for msi60 and aspein differed between the mantle graft and pearl sac (48 days after implantation). Therefore, the gene expression patterns of the epithelial cells were genetically undetermined, and changed between before and after pearl sac formation. The gene expression patterns of the epithelial cells of the pearl sac may be regulated by the host oysters.

We propose a hypothesis of ploidy elevation in the androgenetic clam Corbicula fluminea, based on an abnormal process of fertilization in clams collected at the Tone River, Ibaraki Prefecture, Japan. Most eggs showed androgenesis, that is, extrusion of all maternal chromosomes as two polar bodies during the first meiotic division. Most eggs did not form a female pronucleus, but only a male pronucleus. However, some eggs proceeded to the second meiosis and formed both a female and a male pronucleus. The formation of the female pronucleus suggests the hypothesis that ploidy elevation in androgenetic clams may have occurred by aberrant meiosis due to an altered orientation of the meiotic spindle.