The effect of positive selection on a sexual reproduction gene in Thalassiosira weissflogii (Bacillariophyta): results obtained from maximum-likelihood and parsimony-based methods.

Maximum-Likelihood-based and parsimony-based methods were used to test for potential effects of positive selection on the sexually induced gene 1 (Sig1) in Thalassiosira weissflogii. The Sig proteins are thought to play a role in mediating sperm-egg recognition during the sexual reproduction phase. The results obtained from parsimony-based analyses showed that none of the amino acid sites were influenced by positive selection. Maximum-likelihood analyses indicated that positive selection was affecting a maximum of seven and a minimum of four amino acid sites in the polypeptide derived from Sig1. It was concluded that the results obtained from the maximum-likelihood-based method are more reliable than those obtained from the parsimony-based approach. This is apparently the first study that has shown that reproductive proteins in unicellular eukaryotes are influenced by positive selection.