Population structure and colour variation of the cichlid fish Labeotropheus fuelleborni Ahl along a recently formed archipelago of rocky habitat patches in southern Lake Malawi

Extremely fine-scale genetic partitioning has recently been detected among populations of Lake Malawi's rock-dwelling cichlids through the study of mi crosatellite loci. Understanding the mechanisms of genetic differentiation that operate in this rapidly speciating group requires further investigatio n of the geographic patterns of gene flow and the congruence between morpho logical and genetic divergence. In pursuit of this goal, genetic variation at four microsatellite loci and variation in male breeding coloration were examined in several populations of Labeotropheus fuelleborni from southern Lake Malawi. Significant genetic differentiation exists among populations ( overall F-ST=0.063; p=0.0002). While migration appears unrestricted within continuous rocky patches, deep waters and sandy bays more than 2 bm wide ac t as strong barriers to gene flow. Dispersal of L. fuelleborni appears to f ollow a stepping-stone model in which the distribution of habitats often co nstrains migration to one dimension. It is hypothesized that clinal colour variation in the study area has resulted from the secondary contact of dive rgent lineages, although reproductive isolation between colour variants is not apparent. Relative to shoreline populations, reduced levels of gene flo w among populations inhabiting isolated, deep-water islands provides greate r opportunities for drift, adaptation to local conditions, or sexual select ion to effect genetic differentiation in this species.