Biogeography of a southern hemisphere freshwater fish: how important is marine dispersal?

Galaxias maculatus is one of the world's most widely distributed freshwater fish. This species has a marine-tolerant juvenile phase, and a geographical range extending through much of the southern hemisphere. We conducted phylogeographic analyses of 163 control region haplotypes of G. maculatus, including samples from New Zealand (five locations), Tasmania (one location) and Chile (one location). A lack of genetic structure among New Zealand samples suggests that marine dispersal facilitates considerable gene flow on an intra-continental scale. The discovery of a Tasmanian-like haplotype in one of 144 New Zealand samples indicates that inter-continental marine dispersal occurs but is insufficient to prevent mitochondrial DNA differentiation among continents. The sister relationship of Tasmanian and New Zealand clades implies that marine dispersal is an important biogeographical mechanism for this species. However, a vicariant role in the divergence of eastern and western Pacific G. maculatus cannot be rejected.

Extreme intraspecific mitochondrial DNA sequence divergence in Galaxias maculatus (Osteichthys: Galaxiidae), one of the world's most widespread freshwater fish.

Biogeographic controversies surrounding the widespread freshwater fish, Galaxias maculatus, were addressed with DNA sequence data. Mitochondrial cytochrome b and 16S rRNA sequences were obtained from representatives of six populations of this species. Substantial levels of cytochrome b (maximum 14.6%) and 16S rRNA sequence divergence (maximum 6.0%) were detected between western Pacific (Tasmania-New Zealand) and South American (Chile-Falkland Islands) haplotypes. A considerable level of divergence was also detected between Tasmanian and New Zealand haplotypes (maximum 5.1%) and within and among Chilean and Falkland Island G. maculatus (maximum 3. 8%). The phylogenetic structure of haplotypes conflicts with the accepted pattern of continental fragmentation. Molecular clock calibrations suggest that haplotype divergences postdate the fragmentation of Gondwana. These findings point to marine dispersal rather than ancient vicariance as an explanation for the wide distribution. The phylogenetic structure of South American haplotypes was not consistent with their geographic distribution. We consider factors such as population divergence, population size, dispersal, secondary contact, and philopatry as potential causes of the high level of mtDNA nucleotide diversity in this species.