Evidence for a more recently evolved clade within a Candida albicans North American population

Candida albicans is diploid and displays a primarily clonal mode of reproduction. There is, however, evidence for meiosis and the degree to which this occurs in nature is unknown. Although random mating would act to obscure clonal lineages, previous studies have demonstrated that collections of North American isolates display three major partitions with no evidence of geographic clustering. To better understand the extent of sexuality and its role in the phylogeny of the species, a reference subset of 50 isolates representing this tripartite division was analysed using 1 minisatellite, 5 microsatellites (MSs) and 15 nuclear polymorphisms (NP). A total of 87 alleles were observed for 21 loci and 12/16 informative loci exhibited a departure from Hardy-Weinberg expectations (G(2) less than or equal to 0.05). We did not observe an absolute correlation between MSs and NP, although isolates with identical NP genotypes were correlated with a previously defined, predominant class (putative group I). The use of additional markers did not give increased support for the tripartite structure of the population. However, (9/19) group I isolates were found to be highly related, differing by only one or a few alleles, Designated subgroup A, the interpretation is that these isolates are related by descent and that they are of a more recent evolutionary origin, diverging from an ancestral group I clone. The reason for their relative abundance in the population is unknown; one possibility is that they may be under positive selection.