Abstract
Expression patterns of the shell matrix protein genes MSI31 and MSI60 in the pearl sac epithelium were examined by in situ hybridization 38 days after implantation, and related to pearl quality. A pearl sac that produced a nacreous pearl showed very weak expression of MSI31 and strong expression of MSI60. A pearl sac, which yielded a prismatic pearl, strongly expressed MSI31 and very weakly expressed MSI60. In a complex pearl, whose surface consisted of a mosaic of both nacreous and prismatic layers, the expression pattern of MSI31 and MSI60 similarly corresponded to the underlying surface structures of the pearl. A nacreous pearl whose pearl sac showed strong MSI31 expression had an entirely nacreous surface composed of a laminar structure with unusual tablet growth at the corresponding site. MSI31 and MSI60 are the major components of the shell matrix proteins of the nacreous and prismatic layers. Clearly, high expression of MSI31 does not always result in prismatic secretion. These observations cannot be explained solely on the basis of the expression patterns of MSI31 and MSI60. We propose that, in addition to the MSI genes that form the prismatic and nacreous layers, upstream from these genes there are regulatory master genes that determine whether a nacreous layer (aragonite) or a prismatic layer (calcite) is formed.
Highlights
After the implantation of a nucleus prepared from a freshwater mussel shell and a small piece of mantle tissue from a donor oyster, into a recipient pearl oyster Pinctada fucata, the outer epithelium from the mantle graft migrates over the surface of the nucleus and completely envelopes it [1]
Expression of MSI31 and MSI60 in juveniles In the juvenile, MSI31 expression was observed in the inner epithelium of the mantle edge and MSI60 expression was observed in the pallial mantle [12] by in situ hybridization (ISH)
MSI60 was continuously expressed over the outer surface of mantle epithelium from the dorsal end, near the hinge, to the ventral pallial mantle
Summary
After the implantation of a nucleus prepared from a freshwater mussel shell and a small piece of mantle tissue from a donor oyster, into a recipient pearl oyster Pinctada fucata, the outer epithelium from the mantle graft migrates over the surface of the nucleus and completely envelopes it [1]. This epithelial tissue around the nucleus is termed the pearl sac. In cross-sections of high quality nacreous pearls, the prismatic layer appears as a very thin concentric circle [7]. Elucidation of changes in the expression patterns of genes for prismatic and nacreous layer formation during the early stage of pearl formation would be valuable for the control the pearl quality
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