DRIVE-SELECTION EQUILIBRIUM - HOMOPOLYMER EVOLUTION IN THE DROSOPHILAGENE MASTERMIND

Interspecific sequence comparison of the highly repetitive Drosophila gene mastermind (mam) reveals extensive length variation in homopolyme r domains. The length variation in homopolymers is due to nucleotide m isalignment in the underlying triplet repeats, which can lead to slipp age mutations during DNA replication or repair. In mam, the length var iation in repetitive regions appears to be balanced by natural selecti on acting to maintain the distance between two highly conserved charge clusters. Here we report a statistical test of the null hypothesis th at the similarity in the amino acid distance between the charge cluste rs of each species arose by chance. The results suggest that at main t here is a juxtaposition of length variability due to molecular drive a nd length conservation maintained by natural selection. The analysis o f mam allows the extension of current theories of drive-selection inte raction to encompass homopolymers. Our model of drive-selection equili brium suggests that the physical flexibility, length variability, and abundance of homopolymer domains provide an important source of geneti c variation for natural populations.