Abstract

Some species in the genus Pinctada are important resources for the pearl industry, but some of them are on the verge of extinction. We studied the evolutionary relationship and identification of some species in Pinctada based on sequences of the internal transcribed spacers (ITS1 and ITS2) of nuclear ribosomal DNA. The length of ITS1 ranges from 410 to 482 bp, with P. margaritifera and P. maxima being the longest, and P. fucata, P. fucata martensii, P. imbricata and P. nigra the shortest. The length of ITS2 ranges from 210 to 249 bp, with P. albina and P. nigra being the longest, and P. margaritifera and P. maxima the shortest. Homogeneity test on the pattern of nucleotide substitution indicates that the GC contents in P. margaritifera and P. maxima are significantly higher, and chromosomal rearrangements may have occurred in P. chemnitzi. This finding suggests that P. margaritifera and P. maxima are likely to be primitive species and P. chemnitzi appears to be a recent species. Phylogenetic analysis shows that the pearl oysters studied constitute three clades: clade I with P. fucata, P. fucata martensii and P. imbricata, clade II with P. albina, P. nigra, P. chemnitzi and P. radiata, and clade III with P. margaritifera and P. maxima. The insignificant genetic dif-ferentiation among the species in clade I indicates that they may be conspecific, with P. imbricata being the senior synonym. In clade II, the low genetic divergence between P. albina and P. nigra suggests that they may represent two subspecies. The ITS1 sequence of P. radiata in GenBank is almost identical to that of P. chemnitzi as determined in the present study, and thus we suspect that the specimen used for the P. radiatasequence in GenBank was misidentified. Clade III has a basal position , suggesting that species in this clade are more primitive than the others. This is congruent with the results revealed by the homogeneity test on the nucleotide substitution pattern.