Evolution of Fish in Extreme Environments: Insights from the Magadi tilapia ( Alcolapia grahami )

Abstract

Cichlid fishes are well known for the ir spectacular adaptive radiations in the Great East African lakes. Yet, a number of reasons including their size, young age and recent hybridizati on render it to difficult to infer the evolutionary history of these huge species assemblages. The Soda tilapia form a small radiation (Genus Alcolapia ) that is endemic to an extreme environment - Lakes Magadi and Natron in Kenya and Tanzania. It provides an excellent system in which to investigate ecological aspects during the early stages of speciation. We used an integrated approach including population genomics based on RAD - seq data, geometric morphometrics, and stable isotope analyses to investigate th e eco - morphological diversification of Lake Magadi tilapia. Based on coalescent simulations and joint site frequency spectrum analyses we reconstructed their demographic history. The population in the isolated satellite lake Little Magadi has a characteris tically upturned mouth, an adaptation associated with feeding on prey from the water surface. Ecomorphological differences between geographically separated populations within Lake Magadi are more subtle, but coincide with ecological differences. All popula tions diverged simultaneously only about 1 100 (95% CI: 846 - 1632) generations ago. Differences in the amount of gene flow between populations and the effective population sizes have likely resulted in variable patterns of genome - wide differentiation that w as inferred from RAD - seq data.