PHYLOGEOGRAPHIC STRUCTURE IN MITOCHONDRIAL-DNA OF A SOUTH-EAST ASIAN FRESH-WATER FISH, HEMIBAGRUS-NEMURUS (SILUROIDEI, BAGRIDAE) AND PLEISTOCENE SEA-LEVEL CHANGES ON THE SUNDA SHELF

We postulated that the biogeographical history of South-east Asia cont ributed to extensive admixture during Pleistocene low sea levels of ge netic groups of an obligate freshwater fish (the river catfish, Hemiba grus nemurus) isolated during periods of high sea levels. During Pleis tocene glacial maxima, the sea level was lower than at present and the islands of the Sunda shelf (Sumatra, Borneo and Java) and the Asian m ainland were connected by lowlands traversed by rivers. Restriction fr agment length polymorphisms in mitochondrial DNA were documented for 1 40 putative H. nemurus analysed from 13 sampling sites resulting in th e definition of 35 haplotypes. The high level of haplotype differentia tion (mean P x 100 = 2.22, SD = 1.33) indicates that the subdivision o f the ancestral H. nemurus group was extensive and probably occurred e arly in the Pleistocene. The occurrence of some genetically divergent groups of the H. nemurus complex occurring in sympatry in widely separ ated locations supports the proposition that low sea levels aided the dispersion and mingling of genetic groups. Based on both genetic and m orphological evidence, the main H. nemurus line gave rise to three reg ional groups: (I) a morphologically distinct 'Indochinese' group compo sed of two mtDNA clades overlapping in east peninsular Malaysia; (2) a 'Sundaic' group composed of various lineages of differing morphology and genetic identity; (3) a genetically distinct 'Sarawak' group in we st Borneo, similar in morphology to the 'Sundaic' and 'Indochinese' gr oups, but including a small, golden colour morph as a distinct clade. The morphologically similar Sundaic forms from west Java, Sumatra and west Borneo show some degree of genetic divergence, but their phylogen etic relationships are poorly resolved. The most genetically and morph ologically distinct Sundaic clade, assigned to and hoevenii, colonized the Kapuas liver (west Borneo), east Sumatra and south peninsular Mal aysia. Contrary to our original hypothesis and present biogeographical theory, little exchange of genetic groups has apparently occurred bet ween the mainland and the Sunda Islands during recent glaciations.