Hybridization and emergence of virulence in opportunistic human yeast pathogens

Abstract

Hybridization between different species can result in the emergence of new lineages and adaptive phenotypes. Occasionally, hybridization in fungal organisms can drive the appearance of opportunistic lifestyles or shifts to new hosts, resulting in the emergence of novel pathogens. In recent years, an increasing number of studies have documented the existence of hybrids in diverse yeast clades, including some comprising human pathogens. Comparative and population genomics studies performed on these clades are enabling us to understand what roles hybridization may play in the evolution and emergence of a virulence potential towards humans. Here we survey recent genomic studies on several yeast pathogenic clades where hybrids have been identified, and discuss the broader implications of hybridization in the evolution and emergence of pathogenic lineages. © 2017 The Authors. Yeast published by John Wiley & Sons, Ltd.

Genomic evidence for a hybrid origin of the yeast opportunistic pathogenCandida albicans

“…Advances in Next-Generation sequencing have recently allowed the identification of many hybrid yeasts with clinical relevance (Pryszcz et al 2014;Hagen et al 2015;Pryszcz et al 2015;Schröder et al 2016;Mixão et al 2019). Hybridization between diverged lineages is known to have an important role in the adaptation to new environments, or even in the emergence of new pathogens, as it is hypothesized to be the case of C. metapsilosis and C. inconspicua (Pryszcz et al 2015;Mixão and Gabaldón 2018;Mixão et al 2019). The hybrid nature of these strains was discovered by noting the presence of highly heterozygous genomes with a large divergence between alternative haplotypes and showing characteristic non-homogeneous distributions of heterozygous variants, resulting in highly heterozygous blocks separated by regions of low heterozygosity, likely resulting from LOH events.…”

“…In this regard, C. tropicalis have been shown to undergo a parasexual cycle under laboratory conditions (Seervai et al 2013), although it is unclear how widespread is this ability among Candida species. This raises once more the question of the importance of hybridization for the emergence of yeast pathogens (Mixão and Gabaldón 2018) and poses the intriguing question of whether C. albicans ability to colonize and infect humans was an emerging phenotype enabled by this hybridization event.…”

“…Hybrids are chimeric organisms originated from the hybridization between two diverged lineages (whether from the same or distinct species), and consequently have genomes with high levels of heterozygosity. Diploid hybrids comprise pairs of homeologous chromosomes, which divergence at the sequence level is initially similar to the one of the two parentals at the time of hybridization (Mixão and Gabaldón 2018). These genomes can subsequently evolve through recombination-mediated conversion of homeologous sequences, resulting in LOH (Mixão and Gabaldón 2018).…”

“…Diploid hybrids comprise pairs of homeologous chromosomes, which divergence at the sequence level is initially similar to the one of the two parentals at the time of hybridization (Mixão and Gabaldón 2018). These genomes can subsequently evolve through recombination-mediated conversion of homeologous sequences, resulting in LOH (Mixão and Gabaldón 2018). Hybridization may originate organisms with unique phenotypic features, which may contribute to the successful adaptation to new niches (Gladieux et al 2014).…”

“…inconspicua (Pryszcz et al 2015;Mixão and Gabaldón 2018;Mixão et al 2019). Hence, a possible scenario for the observed genomic patterns in C. albicans is that the divergence observed in highly heterozygous regions is not exclusively the consequence of continuous accumulation of mutations within the lineage, but also, to a large degree, a footprint of an ancient hybridization event between two diverged lineages.…”