Genetic heterogeneity of rat-derived Pneumocystis.

Abstract

Two distinct levels of genetic diversity have been reported for Pneumocystis populations harbored by rat colonies. The first, more dramatic level of diversity, was exhibited by 2 populations that were found to co-exist within individual rat lungs and retain genetic identity. These 2 populations were originally called ‘prototype and variant’[1, 2], based on fidelity to and divergence from the small ribosomal subunit gene sequences originally reported for rat derived Pneumocystis [3, 4]. Subsequently, these populations have been awarded ‘special form’ status. Pneumocystis carinii f.sp. carinii and Pneumocystis carinii f.sp. ratti are now used to designate ‘prototype’ and ‘variant’ populations, respectively [5, 6] (discussed below). The second level of diversity was more subtle and characterized by low levels of genetic polymorphisms among karyotypic forms of the ‘prototype’ population. The levels of divergence between ‘prototype’ and ‘variant’ were characterized as Class II and those among the ‘prototype’ populations as Class I using the genetic ranking system described by Stringer [7]. This system of classification is based on sequence comparisons at 5 genetic loci: thymidylate synthase, mitochondrial large subunit rRNA, β-tubulin, arom, and internal transcribed spacer regions (ITS) in the nuclear ribosomal RNA locus. The highest level of divergence was observed among Pneumocystis populations from different mammalian hosts and ranged between 15–50% divergence at the selected loci, with the ITS regions being most divergent. Class II divergence ranged from 4–7% for all genes, but within the ITS regions a 20–30% divergence was observed. This level of divergence approaches that seen among bonafide species of other microbes. Class I sequences differed by 0–0.8% in the 4 gene sequences and by 2–4% in the ITS regions. Differences among Pneumocystis populations obtained from human beings have been shown to only vary at a Class I level, as well as …