contrasting codon usage patterns and purifying selection at the mating locus in putatively asexual alternaria fungal species对比密码子使用模式和净化选择在推定地交配轨迹链格孢属真菌无性繁殖的物种.pdfVIP

Contrasting Codon Usage Patterns and Purifying Selection at the Mating Locus in Putatively Asexual Alternaria Fungal Species 1 1¤ 2 3 1 Jane E. Stewart *, Masato Kawabe , Zaid Abdo , Tsutomu Arie , Tobin L. Peever 1 Department of Plant Pathology, Washington State University, Pullman, Washington, United States of America, 2 Departments of Mathematics and Statistics and Initiative of Bioinformatics and Evolutionary Studies, University of Idaho, Moscow, Idaho, United States of America, 3 Institute of Symbiotic Science and Technology, Tokyo University of Agriculture and technology (TUAT), Fuchu, Tokyo, Japan Abstract Sexual reproduction in heterothallic ascomycete fungi is controlled by a single mating-type locus called MAT1 with two alternate alleles or idiomorphs, MAT1-1 and MAT1-2. These alleles lack sequence similarity and encode different transcriptional regulators. A large number of phytopathogenic fungi including Alternaria spp. are considered asexual, yet still carry expressed MAT1 genes. The molecular evolution of Alternaria MAT1 was explored using nucleotide diversity, nonsynonymous vs. synonymous substitution (dn/ds) ratios and codon usage statistics. Likelihood ratio tests of site-branch models failed to detect positive selection on MAT1-1-1 or MAT1-2-1. Codon-site models demonstrated that both MAT1-1-1 and MAT1-2-1 are under purifying selection and significant differences in codon usage were observed between MAT1-1-1 and MAT1-2-1. Mean GC content at the third position (GC3) and effective codon usage (ENC) were significantly different between MAT1-1-1 and MAT1-2-1 with values of 0.57 and 48 for MAT1-1-1 and 0.62 and 46 for MAT1-2-1, respectively. In contr