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 years^1-3. The species of this 'flock' differ strikingly in their ecology and behaviour⁴, moderately in their external morphology¹ and very little in their molecular characteristics^5,6. 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.