title: Molecular Polymorphism in the MTA and MTB Mating Type Genes of Tetrahymena thermophila and Related Asexual Species.
abstract: Each of the seven mating types of Tetrahymena thermophila is determined by a pair of large genes, MTA and MTB, whose expression peaks at early conjugation. Each protein consists of a mating-type specific domain and a common transmembrane domain. To assess variation in natural populations, regions of both domains from wild isolates expressing mating types V and VII were analyzed. Corresponding regions of amicronucleates incapable of mating also were examined. MTA and MTB showed high haplotype diversity, with greater sequence variation in MTB. Mating type VII was less variable than mating type V, suggesting more recent origin. No polymorphism distinguished between mat1- and mat2-like alleles encoding different arrays of mating types, nor did polymorphisms give evidence of population structure. MTA and MTB variants have different phylogenies, suggesting independent rather than concerted evolution, and are under weak purifying selection. Codon usage is less biased than for housekeeping genes, and reassigned glutamine encoding stop codons are preferentially used. Amicronucleate T. thermophila and closely related nsp15 and nsp25 have higher levels of nucleotide and amino acid substitution, consistent with cox1 distances. The results suggest that complete sequencing of mating type genes of wild isolates coupled with functional analysis will be informative.
authors.affiliation: Department of Biological, Geological and Environmental Sciences, Cleveland State University, 2121 Euclid Avenue, Cleveland, Ohio, 44115, USA.
authors.forename: Laurie
authors.initials: L
authors.lastname: Booth
authors.affiliation: Department of Biological, Geological and Environmental Sciences, Cleveland State University, 2121 Euclid Avenue, Cleveland, Ohio, 44115, USA.
authors.forename: Benjamin
authors.initials: B
authors.lastname: Wolfe
authors.affiliation: Department of Biological, Geological and Environmental Sciences, Cleveland State University, 2121 Euclid Avenue, Cleveland, Ohio, 44115, USA.
authors.forename: F Paul
authors.initials: FP
authors.lastname: Doerder
mesh_terms: D000483:Alleles; D003062:Codon; D030561:Databases, Nucleic Acid; D005091:Exons; D020043:Genes, Essential; D017125:Genes, Protozoan; D014644:Genetic Variation; D005973:Glutamine / Q000235:genetics; D006239:Haplotypes; D010802:Phylogeny; D011110:Polymorphism, Genetic; D012098:Reproduction / Q000235:genetics; D016808:Tetrahymena thermophila / Q000235:genetics*
chemical_list: D003062:Codon; D005973:Glutamine
keywords: Amicronucleate; evolution; macronucleus; purifying selection; recombination; sequence polymorphism; sex