Evidence of Recombination in Mixed-Mating-Type and α-Only Populations of Cryptococcus gattii Sourced from Single Eucalyptus Tree Hollows

Abstract

Disease caused by the pathogenic yeast Cryptococcus gattii begins with the inhalation of an infectious propagule. As C. gattii is heavily encapsulated, this propagule is most likely to be a basidiospore. However, most C. gattii strains are infertile in laboratory crosses, and population studies indicate that recombination and dispersal are very restricted. In addition, strains of the ␣ mating type predominate, which would not be expected in a mating population. C. gattii comprises four genetically distinct molecular genotypes, designated VGI to VGIV. C. gattii molecular type VGI has a strong association with Eucalyptus camaldulensis and can be found in high numbers in E. camaldulensis hollows. Previous work on isolates obtained from E. camaldulensis suggested that environmental populations of C. gattii are highly fragmented, have limited ability to disperse, and are confined to individual tree hollows. In the current study, we examined large numbers of isolates from three separate hollows for evidence of recombination. In two hollows, the ␣ and a mating types were present in approximately equal numbers. The third hollow had ␣ cells only and was from a region where a isolates have never been found. Statistical analysis of multilocus genotypes revealed recombining subpopulations in the three Eucalyptus hollows. Recombination was equally present in the ␣-a and ␣-only populations. This is consistent with recent studies that have found evidence suggestive of ␣-␣ mating in C. gattii and Cryptococcus neoformans and raises the possibility this may be a widespread phenomenon, allowing these fungi to recombine despite a paucity of a mating partners.

The Evolution of Sex: a Perspective from the Fungal Kingdom

“…First, the discovery and characterization of ␣AD␣ hybrids produced by the mating of two ␣ isolates of different serotypes (A and D) provide definitive evidence that unisexual mating has occurred in nature (164). Second, recent studies of C. gattii populations from tree hollows in Australia provided evidence that recombination is occurring within populations that are exclusively of the ␣ mating type (264). Third, population genetic studies of C. neoformans isolates from tree hollows in India provide evidence for recombination in otherwise ␣ unisexual populations, and one parsimonious hypothesis is that this involves unisexual reproduction (105).…”

Cryptococcus gattii Infections

“…AFLP analysis of clinical isolates revealed that they are geographically restricted, highly clonal, and of low fertility (53,116,117). Subsequent extensive testing of isolates from single tree hollows by AFLP analysis revealed that recombination is occurring in both MATa and MAT␣ or MAT␣-only populations (118). Isolates of VGII from Australia are highly fertile and recombining (117), with a recent study also suggesting recombination within the MAT␣-MATa population, especially in Western Australia (88).…”

“…Eucalypts are in fact only one of Ͼ50 different species of tree that can provide an ecological niche for C. gattii (106)(107)(108)(109)(110). Environmental isolates have been obtained from Africa (33,(110)(111)(112), Asia (76,77,108), Australia (26,110,(113)(114)(115)(116)(117)(118), Europe (110,119,120), North America (38,110,121), and South America (122). In addition, it has been shown that plant material promotes fertility (108,(123)(124)(125) and may render C. gattii strains more pathogenic (125).…”

Spores as Infectious Propagules ofCryptococcus neoformans

“…grubii and C. gattii strains do not reproducibly undergo monokaryotic fruiting under laboratory conditions. Population genetic studies provided evidence that they do so in nature (4,21,36,49), but this remains to be established under defined laboratory conditions.…”

“…However, if spores are the infectious propagule, and spores are produced by mating between a and ␣ cells, how and where would this occur in the largely ␣ unisexual population? A potential resolution was the discovery that ␣ isolates can produce hyphae, basidia, and basidiospores via monokaryotic fruiting (11,54), a modified sexual cycle that requires only one of the two mating types (4,12,21,(33)(34)(35)(36)49). This might be a route by which infectious propagules are produced in nature; however, serotype A C. neoformans var.…”