Influence of glaciation on divergence patterns of the endemic minnow, Zacco pachycephalus, in Taiwan

The genetic subdivision of the endemic minnow, Zacco pachycephalus, in 12 r ivers of Taiwan was studied by allozyme electrophoresis. Among 26 loci surv eyed, six were significantly differentiated among sites. Allozyme genotype frequencies within samples accord with Hardy-Weinberg expectations. A highl y divergent genetic structure among sites (F-ST = 0.497) with low genetic v ariability within samples (H = 0.006-0.062, mean = 0.03) suggests that loca l populations originated from a small number of founders. UPGMA and Fitch t rees derived from Nei's genetic distance and hierarchical analysis of F-ST values reveal that samples of Z. pachycephalus can be divided into northern , middle, and southern subgroups and that most genetic variation (87%) is d istributed among rather than within groups. It is proposed that there were two isolated refugia during the most recent glacial period, and that popula tions of the southern group (Group I) originated from a southern refugium i n Taiwan below the boundary of the Kaoping River, and other groups (Group I I), including northern and middle subgroups, originated from a refugium loc ated on the extinct landbridge between Taiwan and mainland China. The postg lacial division between northern and middle subgroups may be due to hydrolo gical differences between these two regions.