Allozymic variation in the clam genus Eurhomalea (Bivalvia: Veneriidae) along southern South American coast Variación alozímica en el género de almejas Eurhomalea (Bivalvia: Veneriidae) a lo largo de la costa sur de Sudamérica

The correspondence between allozymic variation and specific differentiation was studied in four populations corresponding to the three nominal, allopatric species of clam genus Eurhomalea (E. rufa, E. lenticularis, E. exalbida) described for southern South America. Allozyme variation scored in 12 loci was high as indicated by heterozygosity levels (15.8-20.7 %) and by the presence of only three monomorphic loci (Hk-2, Icd-2, and Xdh-1). These high estimates of allelic variability were influenced by the low levels of interspecific genetic similarity (I = 0.64) and for the high conspecific values of genetic identity observed. The high estimates of substructuring found at the species level (FST = 0.39) contrasted with the low differentiation (FST = 0.027) and high migration rate (Nm = 9) existing among conspecific samples. Diagnostic allele fixation coinciding with specific recognition was recorded at locus Hk-1 whereas nearly-fixed differences at loci (Adh, α-Gpd, Icd-1, Ldh, Odh, Pgm-3) differed sharply in frequency among species. The Wagner procedure and the neighborjoining algorithm produced a similar tree topology highly related to the geographic distance and to their taxonomic recognition. The lack of coincidence between patterns of allozymic variation and the two distinctive shell morphs (flat and globoid) occurring in E. exalbida from Ushuaia bay do not support their taxonomic recognition.