Clonality and Adaptation to the Niche

An extensive study using multilocus microsatellite typing and 169 isolates representing a number of geographically distinct regions lead to the conclusion that P. marneffei is one of the most clonal species of fungus characterized (Fisher et al., 2005). Ecological studies performed in a number of opportunistic pathogens have revealed an apparent link between asexuality and pathogenicity. Fungi such as C. neoformans, Candida albicans, and Aspergillus fumigatus, together with P. marneffei show extensive levels of clonality in the environment with very low levels of recombination (Fisher et al., 2005; Heitman, 2006). The hypothesis is that clonality is a hallmark of organisms that are highly adapted to their environments. Despite this, all of these pathogens possess the genes which are required for mating and mating has been clearly demonstrated for C. neoformans and recently for C. albicans (Hull & Johnson, 1999; Lengeler et al., 2002; Paoletti et al., 2005). A recent report documented the presence of the MAT1-1 a box and MAT1-2 high-mobility group mating-type genes in P. marneffei (Woo et al., 2006). These genes are closely related to their homologues in A. nidulans and A. fumigatus. P. marneffei has a heterothallic arrangement of mating-type loci as is the case for A. fumigatus, which means that the strains require partners of the opposite mating-type to engage in sexual reproduction. In addition, previous studies have demonstrated that the P. marneffei STE12 homologue stlA can complement the sexual defects of a steA mutant in A. nidulans (Borneman et al., 2001), suggesting that some sexual genes in P. marneffei are indeed functional. Thus, the question is why has P marneffei, an overwhelmingly clonal fungus, retained the sexual machinery. The simplest explanation is that P. marneffei has only recently lost the ability to mate. Alternatively, it has been suggested that limiting sexual reproduction appears to be a common virulence strategy, enabling the generation of clonal populations well adapted to host and environmental niches. As virulence is a polygenic trait, sexual reproduction might separate advantageous combinations of alleles (Heitman, 2006). Therefore, it is possible that by limiting sexual reproduction, P. marneffei has become restricted to the areas where it is well adapted and displays a high prevalence of infection in both human and bamboo rat populations for which it is adapted. Sexual reproduction between strains of the a mating-type is a contributor to the emergence of the hypervirulent clone of C. gattii on Vancouver Island in 1999 (Fraser et al., 2005; Kidd et al., 2005). A survey performed using a small number of clinical isolates has shown that P. marneffei isolates containing the MAT1-1 a box mating-type gene are twice as frequent as their opposite mating-type counterparts (Woo et al., 2006). Whether this is significant remains to be tested.

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