Scolytine bark beetles are the most destructive pests of conifers; they sometimes aggregate in such large numbers that they actually kill their hosts. They maintain close relationships with yeasts and fungi, in particular those that are assumed to aid in digestive, detoxification processes and pheromone production. In this study, 403 yeast strains were isolated from the guts, ovaries, eggs and frass of nine bark beetle species in the genus Dendroctonus Erichson. The beetles were collected from 10 conifer species at 34 locations in Mexico, Guatemala and the USA. Yeast identification was based on partial DNA sequences from 18S rDNA, 26S rDNA and internal transcribed spacer (ITS1), as well as morphological and physiological characteristics. A combined phylogenetic analysis delimited 11 clades with sequences similar to Candida arabinofermentans, C. ernobii, C. membranifaciens (including C. lessepsii, Pichia mexicana and P. scolyti), C. oregonensis, C. piceae, Kuraishia capsulata (including K. capsulata and K. cf. molischiana), Pichia americana, P. canadensis, P. glucozyma, P. guilliermondii and an undescribed species of Candida. Nucleotide divergences between the major clades were at least 5% while, with the exception of 30 isolates, yeasts within clades differed from named reference species at fewer than 1% of the nucleotide sites. There do not appear to be obligate relationships between particular yeasts and specific anatomical partitions, nor between particular yeasts and bark beetle species. Some yeasts do appear to be preferentially associated with bark beetles feeding on different conifer genera and therefore host plant defences may limit yeast community diversity in Dendroctonus.

It is well known that agricultural practices change the physical and chemical characteristics of soil. As a result, microbial populations can also be affected.

An in vitro evolution model was used to study changes in the genetic diversity of 24 strains of Pichia guilliermondii isolated from the midgut of bark beetles of the genus Dendroctonus. The genetic diversity of P. guilliermondii strains over 400 generations was analysed using multilocus enzyme electrophoresis (MLEE) and random amplified polymorphic DNA polymerase chain reaction (RAPD-PCR) markers. Resemblance relationships among strains were observed by cluster analysis. From the MLEE and RAPD markers, it was shown that the effective number of alleles, polymorphism and expected heterozygosity varied over the generations. The average heterozygosity among generations was statistically significant. Both the genetic diversity and the average heterozygosity were statistically significant among generations. The reduction in the population size from 10 9 to 10 5 yeast mL -1 associated with each transfer in P. guilliermondii strains and the clonal population structure observed along 400 generations suggest that genetic diversity changes and the observed replacement of genotypes are a consequence of a genetic drift process and not of the reproductive mode.