珠母贝属的系统发育: 核rDNA ITS序列证据

珠母贝属（pinctada）的一些种类是生产海水珍珠的重要母贝，个别种类已濒临灭绝。本文利用核糖体DNA内部转录间隔区1（ITS1）和2（ITS2）序列对珠母贝属常见种类的系统发育和分类地位进行了分析。结果表明：ITS1长410-482bp，其中Pinctada maxima最长，P.fucata，P.fucata martensii,P.imbricata和P.nigra最短。ITS2长210-249bp，其中P.albina和P.nigra最长，P.maxima和P.margaritifera最短。碱基替换的同质性检测发现，P.maxima、P.margaritifera和P.chemnitzi的碱基替换格局存在显著性差异，前二者的GC含量显著高于其他种，在进化上可能比较原始；而P．chemnitzi可能发生过染色体重排事件，可能是新近形成的种。系统发育分析表明，所研究的种类可分成3个类群：类群Ⅰ包含P.fucata、P.fucatamartensii和P．imbricata；类群Ⅱ包含P.albina、P.nigra、P.chemnitzi和P.radiata;类群Ⅲ包含P.maxima和P.margaritifera。在类群Ⅰ中，我国的P．fucata、日本的P．fucatamartensii和澳大利亚的P．imbricata的种间遗传分化不明显，可能为同种，根据命名优先原则应以P.imbricata命名该种为宜。类群Ⅱ中P.albina和P．nigra可能是两个亚种，而GenBank中的P.radiata（AY144603）可能是P.chemnitzi的误定。类群Ⅲ（P.maxima和P.margaritifera）分化较早，与碱基替换格局的检测结果相符。

Phylogenetics of the common pearl oysters in the genus Pinctada: evidence from nrDNA ITS sequence

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 spe- cies 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 short- est. Homogeneity test on the pattern of nucleotide substitution indicates that the GC contents in P. margari- tifera 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.