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ABSTRACT
In the majority of sexual organisms, reproduction occurs almost exclusively through the combination of distinct and alternate forms, called sexes or mating types. In some fungi, there can be dozens to hundreds of alternate alleles that determine compatible mating types. Such extensive polymorphism is expected to be maintained by balancing selection, and in extreme cases may give rise to trans-specific polymorphism. Here, we analyzed sequences of two pheromone receptors in the Microbotryum fungal species complex (Basidiomycota), which has only two alternate mating types. Several lines of evidence strongly suggest that the pheromone receptors are two allelic sequences acting to determine the alternate A1 and A2 mating types required for mating in Microbotryum. Phylogenetic trees of pheromone receptors in the Microbotryum species complex indicated a trans-specific polymorphism: the Microbotryum sequences from a given mating type were all more similar to the pheromone receptors of distantly related classes of fungi than to the alternate pheromone receptor in the Microbotryum species. A phylogenetic tree built using other known pheromone receptors from basidiomycetes showed that trans-specific polymorphism is widespread. The pheromone receptor alleles from Microbotryum appeared as the oldest, being at least 370 million years old. This represents the oldest known trans-specific polymorphism known in any organism so far, which may be due to the existence of sex chromosomes, obligate sexuality, mitochondrial inheritance linked to the mating type, and a highly selfing mating system in Microbotryum.
IN sexual organisms, reproduction occurs almost exclusively through the combination of distinct and alternate forms, called sexes or mating types. Dimorphic sex chromosomes define mating compatibility in most mammals, while more polymorphic systems, involving dozens to hundreds of alleles, determinemating types in some plants and fungi (Richman 2000). Such extensive polymorphism at mating types is maintained by negative frequency-dependent selection, the most common form of balancing selection (Richman 2000): when a matingtype allele becomes rare, it has a selective advantage because the individuals carrying it can mate with a larger proportion of other individuals. Balanced polymorphism ofmultiple alleles is therefore frequent atmating-typeloci in a wide range of organisms, such as plants, insects, and fungi (May et al. 1999; Richman 2000), and it is also pervasive in various somatic self/nonself recognition systems, such as the vertebrate major histocompatibility complex (MHC) (Hughes and...