Hybridization versus Randomly-Sorting Ancestral Alleles: Genetic Variation in Lake Malawi Cichlids

3 1. ABSTRACT Hybridization is believed to be an important mechanism for genetic diversity and speciation in many groups of organisms. One of the most important evolutionary signatures of hybridization is low phylogenetic resolution in rapidly evolving and closely related species. However, low phylogenetic resolution may also be caused by randomly-sorting ancestral alleles. The cichlid fishes of Lake Malawi, Africa, are a commonly studied model of rapid speciation and have mosaic genomes characteristic of young and diverse groups of species. Because of reproductively viable hybrid offspring used in laboratory studies as well as observed populations of hybrid origin in the lake, hybridization is believed to be the major source of the mosaic genomes of Lake Malawi Cichlids. To test this hypothesis, DNA was isolated from individuals of mbuna (rock-dwelling) and non-mbuna cichlid species from Lake Malawi, including individuals from various populations of two mbuna species Metriaclima zebra a n d Labeotropheus fuelleborni. The mitochondrial gene ND2 and the nuclear gene DLX2 were amplified using PCR technique and were sequenced. Sequences were aligned and then used to build phylogenetic neighbor-joining distance trees. This study is the first to examine genetic variation among a wide range of species as well as populations of two potentially hybridizing species from the lake in order to examine the relative roles of hybridization and ancestral polymorphisms in the mosaic genomes of Lake Malawi cichlids. 2. INTRODUCTION 2.a The mbuna of Lake Malawi Lake Malawi is located in southeast Africa and is one of the east African rift lakes (Figure 1). It is world-renowned for its cichlid fishes. The Lake Malawi cichlids are very diverse and have experienced rapid speciation; with between 700 and 1000 species evolving in 4 lake's short history of less than one million years, the Lake Malawi cichlids are one of the most diverse group of vertebrates on earth (Owen et al. 1990, Turner et al. 2001). At least 300 of these species belong to the rock dwelling fish of the lake called the mbuna – one of the most frequently studied group of fishes in the world (Konings 2001). The rocky habitats in which the mbuna are predominantly found are often separated by stretches of sandy bottom or open water creating isolated populations throughout the lake (Genner and Turner 2005). 2.b Speciation of the mbuna Speciation of organisms can occur in one of two ways: allopatrically or sympatrically. The mechanisms of speciation …