Population structure of two beetle-associated yeasts: comparison of a New World asexual and an endemic Nearctic sexual species in the Metschnikowia clade.

The genetic structure of two related yeast species, one sexual and one asexual, was compared using polymorphic DNA markers. Although both yeasts propagate by asexual budding of haploid cells, Metschnikowia borealis reproduces sexually when compatible strains come in contact. To what extent this has occurred in nature was not known. As Candida ipomoeae is a closely related, asexual species, the two yeasts provide an excellent model system to assess the role of sexual reproduction in a biogeographic context. Natural isolates of the two species were characterized using several polymorphic DNA markers. As predicted for an organism whose reproduction is strictly clonal, C. ipomoeae exhibited low haplotype diversity, high linkage disequilibrium, and high population differentiation. In contrast, M. borealis had unique haplotypes in most isolates, lower population differentiation, and little linkage disequilibrium, demonstrating that sexual recombination is prevalent. Geographic gradients were identified in both species, indicating that historical and climatic factors both play a role in shaping the populations. The spatial structure is also thought to be influenced by the ecology of the small floricolous beetles (family Nitidulidae) that vector the yeasts. For example, Hawaiian strains of C. ipomoeae show evidence of having undergone a genetic bottleneck, most likely when the vector was introduced to the islands. The two haplotypes found in Hawaii were nearly identical and were also found in North and Central America. M. borealis had a more continuous distribution where the genetic markers follow latitudinal and longitudinal gradients.