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.

Keywords: comparative genomics; emergence of virulence; hybridization; pathogens; yeast.

Figure 1

Schematic representation of several different mechanisms that can lead to genome stabilization in hybrids. Shaded ovals represent cells. Chromosomes are painted in different colours with different tones of the same colour indicating homeologous pairs of chromosomes. In some cases insets highlight specific regions with genes indicated as coloured boxes. From top to bottom, two haploid cells from different species cross and form a diploid hybrid. Four non‐exclusive, alternative evolutionary paths to genome stability are shown: (a) whole‐genome duplication; (b) total or partial chromosome loss; (c) gene loss; and (d) gene conversion and loss of heterozygosity

Figure 2

Evolutionary tree depicting the principal fungal clades with hybridization events in the origin of emergent pathogens. Clades where hybridization events have already been reported are indicated with the diamond symbol (◆). Hybrids or hybrid parent species are in bold. Basidiomycota and Ascomycota phyla are presented with blue and yellow backgrounds, respectively. Dark yellow highlights the CTG clade. Species already described as possible plant pathogens are in green, while those already described as possible human pathogens are in orange. The tree was reconstructed based on a set of four marker genes able to resolve fungal phylogenies (Capella‐Gutierrez et al., 2014). Genes were aligned and trimmed following PhylomeDB pipeline (Huerta‐Cepas et al., 2014), and the tree was reconstructed using the raxmlHPC‐PTHREADS‐SSE3 option of RAxML v8.2.4 (Stamatakis, 2014) set with PROTGAMMALG substitution model

Figure 3

Schematic trees with representation of the different hybridization events already described in (a) Cryptococcus and (b) Candida parasilosis s.l. clades. For each tree, hybrid parent species are in bold and connected with coloured curve lines. In each tree, different colours represent different hybridization events. The tree representing Cryptococcus clade was adapted from Hagen et al. (Hagen et al., 2015), while the one representing C. parapsilosis clade was adapted from Pryszcz et al. (Pryszcz et al., 2015). [Colour figure can be viewed at wileyonlinelibrary.com]