Migratory behaviour shapes spatial genetic structure of cyprinid fishes within the Lake Malawi catchment

Genetic differences among freshwater fish populations are dependent on life-history characteristics of the species, including the range of adult dispersal and the extent of homing to natal breeding grounds. However, the effects of variation in such characteristics on population genetic connectivity are rarely studied comparatively among closely related species. We studied population genetic structure within three congeneric cyprinid species from the Lake Malawi catchment that differ substantially in life-history traits and conservation status, using a combination of microsatellite and mitochondrial DNA markers. Mpasa (Opsaridium microlepis) is a large (70cm total length) migratory species that spawns in rivers, but as an adult is exclusively known from the main lake body. Sanjika (Opsaridium microcephalum), is a medium size (30cm total length) species that exists in lake breeding, river-lake migratory and apparently landlocked populations. Dwarf sanjika (Opsaridium tweddleorum) is a small non-migratory species (15cm total length) that persists in small tributaries surrounding the main lake and adjoining rivers. The results revealed striking differences among the three species in spatial genetic structuring. The river-lake migratory mpasa showed only weak yet significant population genetic structure within the main Lake Malawi catchment, suggesting that there is no strong natal homing. The habitat-generalist sanjika showed only weak spatial genetic differentiation at microsatellite loci within the Lake Malawi catchment, but moderate structure in mitochondrial DNA, potentially reflecting male-biased dispersal. The river-restricted dwarf sanjika showed strong genetic structure in both microsatellite and mitochondrial DNA, suggesting strictly limited dispersal at both adult and juvenile stages. We conclude that contrasting migration life histories have resulted in dramatically different patterns of population genetic structure among these congeneric species. The observed patterns demonstrate how divergent life-history evolution may strongly influence broader patterns of population genetic connectivity in freshwater fish, with consequences for management and conservation. Specifically the results suggesting gene flow among Lake Malawi populations of mpasa, an IUCN red-listed Endangered' species endemic to the lake catchment, imply that conservation initiatives operating at both local and catchment scales are needed to reverse local population decline.