Extensive MHC variability in cichlid fishes of Lake Malawi

LAKE Malawi in East Africa harbours 500–1,000 endemic species of cichlid fishes, all presumably derived by adaptive radiation from a single founding population within the past two million years1–3. The species of this 'flock' differ strikingly in their ecology and behaviour4, moderately in their external morphology1 and very little in their molecular characteristics5,6. Here we describe high sequence variability of class II major histocompatibility complex genes in a sample of species from Lake Malawi. The variability provides a set of molecular markers for studying adaptive radiation and should be useful for estimating the size of the population that founded the species flock.