Molecular drift of the bride of sevenless (boss) gene in Drosophila

Abstract

DNA sequences were determined for three to five alleles of the bride-of-sevenless (boss) gene in each of four species of Drosophila. The product of boss is a trans-membrane receptor for a ligand coded by the sevenless gene that triggers differentiation of the R7 photoreceptor cell in the compound eye. Population parameters affecting the rate and pattern of molecular evolution of boss were estimated from the multinomial configurations of nucleotide polymorphisms of synonymous codons. The time of divergence between D. melanogaster and D. simulans was estimated as ∼1 Myr. that between D. teissieri and D. yakuba as ∼0.75 Myr, and that between the two pairs of sibling species as ∼2 Myr. (The boss genes themselves have estimated divergence times ∼50% greater than the species divergence times.) The effective size of the species was estimated as ∼5 × 106. and the average mutation rate was estimated as 1-2 × 10-9 /nucleotide/generation. The ratio of amino acid polymorphisms within species to fixed differences between species suggests that ∼25% of all possible single-step amino acid replacements in the boss gene product may be selectively neutral or nearly neutral. The data also imply that random genetic drift has been responsible for virtually all of the observed differences in the portion of the boss gene analyzed among the four species.