Genetic and morphological evidence for reproductive isolation between sympatric populations of Galaxias (Teleostei : Galaxiidae) in South Island, NewZealand

New Zealand's South Island houses a nock of closely related stream-resident fish taxa (Galaxias vulgaris sensu lato), including a number of species re cently described on the basis of subtle morphological differences. The taxo nomic status of some members of the species complex remains uncertain. This study examines the degree of reproductive isolation between recently recog nized morphotypes from Southland (G. 'southern', flatheads; G. gollumoides, roundheads) which co-occur in Bushy Creek, a tributary of the Mataura R. A lthough these morphotypes are broadly sympatric in Southland and Stewart Is land, Bushy Creek is their only documented zone of contact. Molecular (micr osatellite, isozyme and mtDNA markers) and morphological analyses of 139 fi sh samples across a 500-m transect (seven stations) reveal a dine from pred ominantly G. 'southern' (N=85) to predominantly G. gollumoides (N=54), corr esponding with a gradual increase in stream gradient. Multivariate analyses of genotypic and morphological data independently reveal distinct clusters that are completely congruent with mtDNA type, suggesting an absence of mt DNA introgression. Our data support the separate species status of G. 'sout hern' and G. gollumoides under both biological and phylogenetic species con cepts. We suggest that the speciation of these taxa occurred in allopatry t hrough independent losses of diadromy, with sympatry resulting from seconda ry contact. (C) 2001 The Linnean Society of London.