Same-sex mating and the origin of the Vancouver Island Cryptococcus gattii outbreak

Abstract

Genealogy can illuminate the evolutionary path of important human pathogens. In some microbes, strict clonal reproduction predominates, as with the worldwide dissemination of Mycobacterium leprae, the cause of leprosy. In other pathogens, sexual reproduction yields clones with novel attributes, for example, enabling the efficient, oral transmission of the parasite Toxoplasma gondii. However, the roles of clonal or sexual propagation in the origins of many other microbial pathogen outbreaks remain unknown, like the recent fungal meningoencephalitis outbreak on Vancouver Island, Canada, caused by Cryptococcus gattii. Here we show that the C. gattii outbreak isolates comprise two distinct genotypes. The majority of isolates are hypervirulent and have an identical genotype that is unique to the Pacific Northwest. A minority of the isolates are significantly less virulent and share an identical genotype with fertile isolates from an Australian recombining population. Genotypic analysis reveals evidence of sexual reproduction, in which the majority genotype is the predicted offspring. However, instead of the classic a-alpha sexual cycle, the majority outbreak clone appears to have descended from two alpha mating-type parents. Analysis of nuclear content revealed a diploid environmental isolate homozygous for the major genotype, an intermediate produced during same-sex mating. These studies demonstrate how cryptic same-sex reproduction can enable expansion of a human pathogen to a new geographical niche and contribute to the ongoing production of infectious spores. This has implications for the emergence of other microbial pathogens and inbreeding in host range expansion in the fungal and other kingdoms.

The fatal fungal outbreak on Vancouver Island is characterized by enhanced intracellular parasitism driven by mitochondrial regulation

“…In addition, mtDNA copy number does not change significantly following intracellular replication, eliminating this as a possible explanation for the overrepresentation of mitochondrial hits (Table 1). Furthermore, the expression of many nuclear-encoded proteins that function in mitochondria is also up-regulated in the VIO strains (e.g., respiratory genes and mitochondrial proteins, Table S3), which is particularly relevant given early in vivo transcriptional profiling showing the high expression of several respiratory genes by this yeast at the site of a central nervous (12). Note that strain WM276 has been omitted from the graph, since published ST 50 values on this strain vary widely for reasons that are not currently understood (12,45).…”

“…Furthermore, the expression of many nuclear-encoded proteins that function in mitochondria is also up-regulated in the VIO strains (e.g., respiratory genes and mitochondrial proteins, Table S3), which is particularly relevant given early in vivo transcriptional profiling showing the high expression of several respiratory genes by this yeast at the site of a central nervous (12). Note that strain WM276 has been omitted from the graph, since published ST 50 values on this strain vary widely for reasons that are not currently understood (12,45). For strains that did not cause mortality of 50% or more within the 45-day timeframe of the experiment, we arbitrarily assigned an ST 50 of 55 days.…”

“…Previous amplified fragment length polymorphism (AFLP) and multilocus sequence typing (MLST) studies have demonstrated that the Vancouver Island outbreak (VIO) is mainly caused by a single, hypervirulent genotype of C. gattii (AFLP6A/molecular type VGIIa) (8). Intriguingly, this genotype is not restricted to Vancouver Island but is also shared by, for instance, the CBS6956 strain (also known as NIH444 or ATCC32609, isolated from a patient in Seattle in 1971) and CBS7750 (isolated from a Eucalyptus tree in San Francisco in 1992) (11,12), suggesting that a recent genetic change has occurred within the VIO lineage, potentially as a result of an unusual same-sex mating event (12). However, the underlying molecular cause for the hypervirulence of this lineage remains unknown.…”

Determinación in vitro de la pareja sexual en aislamientos del complejo Cryptococcus neoformans

“…The mating type locus has been associated with virulence, as suggested by Kwon-Chung et al (13), who found that MATα strains were more virulent in a mouse model than the MAT a isolates. Recently it was found that the cryptococcosis outbreak on Vancouver Island, Canada was caused by isolates that were highly virulent mating type α strains belonging to the C. gattii molecular type VGII/AFLP6 (14,15).…”

Australia in the global picture of the molecular epidemiology of Cryptococcus gattii molecular type VGII

“…At first, two distinct subtypes among the molecular type VGII isolates responsible for the Canadian outbreak were characterised by multilocus sequence typing (MLST), the clinical and environmental common highly virulent VGIIa subtype, and the rarer, less virulent VGIIb subtype 8 . The early finding that both subtypes have only mating type alpha isolates that were fertile, led to the suggestion that same-sex mating between two alpha cells was the driving force for the emergence of the outbreak 8 Presence or absence of genes among the different isolates is …”