title: Balancing selection on allorecognition genes in the colonial ascidian Botryllus schlosseri.
abstract: Allorecognition is the capability of an organism to recognize its own or related tissues. The colonial ascidian Botryllus schlosseri, which comprises five genetically distinct and divergent species (Clades A-E), contains two adjacent genes that control allorecognition: fuhcsecand fuhctm. These genes have been characterized extensively in Clade A and are highly polymorphic. Using alleles from 10 populations across the range of Clade A, we investigated the type and strength of selection maintaining this variation. Both fuhc genes exhibit higher within-population variation and lower population differentiation measures (FST) than neutral loci. The fuhc genes contain a substantial number of codons with >95% posterior probability of dN/dS> 1. fuhcsecand fuhctmalso have polymorphisms shared between Clade A and Clade E that were present prior to speciation (trans-species polymorphisms). These results provide robust evidence that the fuhc genes are evolving under balancing selection.
authors.affiliation: Division of Science and Mathematics, Centre College, 600 W. Walnut Street, Danville, KY 40422, United States. Electronic address: marie.nydam@centre.edu.
authors.forename: Marie L
authors.initials: ML
authors.lastname: Nydam
authors.affiliation: Division of Science and Mathematics, Centre College, 600 W. Walnut Street, Danville, KY 40422, United States; Centre for Infectious Disease Research, P.O. Box 34681, Lusaka, 10101, Zambia. Electronic address: emily.stephenson.rotary@gmail.com.
authors.forename: Emily E
authors.initials: EE
authors.lastname: Stephenson
authors.affiliation: Division of Science and Mathematics, Centre College, 600 W. Walnut Street, Danville, KY 40422, United States. Electronic address: claire.waldman@centre.edu.
authors.forename: Claire E
authors.initials: CE
authors.lastname: Waldman
authors.affiliation: Department of Molecular, Cellular, and Developmental Biology, University of California Santa Barbara, Santa Barbara, CA 93106, United States. Electronic address: anthony.detomaso@lifesci.ucsb.edu.
authors.forename: Anthony W
authors.initials: AW
authors.lastname: De Tomaso
mesh_terms: D000818:Animals; D019143:Evolution, Molecular; D049810:Genetic Speciation; D005828:Genetics, Population; D006648:Histocompatibility / Q000235:genetics; D006649:Histocompatibility Antigens / Q000235:genetics / Q000378:metabolism*; D007107:Immune System; D007109:Immunity / Q000235:genetics; D011110:Polymorphism, Genetic; D012641:Selection, Genetic; D017634:Self Tolerance; D014561:Urochordata / Q000276:immunology*
chemical_list: D006649:Histocompatibility Antigens
keywords: Allorecognition; Ascidian; Balancing selection; Botryllus schlosseri; fuhc(sec); fuhc(tm)