Evolution of the Mating Type Locus: Insights Gained from the Dimorphic Primary Fungal Pathogens Histoplasma capsulatum , Coccidioides immitis , and Coccidioides posadasii

Abstract

Sexual reproduction of fungi is governed by the mating type (MAT) locus, a specialized region of the genome encoding key transcriptional regulators that direct regulatory networks to specify cell identity and fate. Knowledge of MAT locus structure and evolution has been considerably advanced in recent years as a result of genomic analyses that enable the definition of MAT locus sequences in many species as well as provide an understanding of the evolutionary plasticity of this unique region of the genome. Here, we extend this analysis to define the mating type locus of three dimorphic primary human fungal pathogens, Histoplasma capsulatum, Coccidioides immitis, and Coccidioides posadasii, using genomic analysis, direct sequencing, and bioinformatics. These studies provide evidence that all three species possess heterothallic bipolar mating type systems, with isolates encoding either a high-mobility-group (HMG) domain or an ␣-box transcriptional regulator. These genes are intact in all loci examined and have not been subject to loss or decay, providing evidence that the loss of fertility upon passage in H. capsulatum is not attributable to mutations at the MAT locus. These findings also suggest that an extant sexual cycle remains to be defined in both Coccidioides species, in accord with population genetic evidence. Based on these MAT sequences, a facile PCR test was developed that allows the mating type to be rapidly ascertained. Finally, these studies highlight the evolutionary forces shaping the MAT locus, revealing examples in which flanking genes have been inverted or subsumed and incorporated into an expanding MAT locus, allowing us to propose an expanded model for the evolution of the MAT locus in the phylum Ascomycota.In the fungal kingdom, sexual reproduction is regulated by a specialized genomic region known as the mating type (MAT) locus (12, 13). This important genomic feature has been most extensively studied in the largest of the fungal phyla, the Ascomycota. Most commonly, the sexual members of this phylum have a bipolar mating type system, where strains are one of two mating types. For mating to occur in these heterothallic species, cells of differing mating types must come together. In those species where the MAT locus has been characterized at a molecular level, this process is regulated by a bipolar system: cells normally carry one of two different alleles of the MAT locus. These alleles, known as idiomorphs, contain unrelated sequences that encode different transcription factors. In the euascomycetes, which include the medically important dimorphic pathogens and the majority of the ascomycete molds, one MAT locus allele encodes a high-mobility-group (HMG) domain-type transcription factor, and the other allele encodes an ␣-box domain transcription factor. When two isolates of a species with differing MAT loci come together under appropriate conditions, sexual reproduction can proceed. Importantly, for many ascomycete species, a sexual cycle has never been observed, yet analysis of the genomic s...

The Evolution of Sex: a Perspective from the Fungal Kingdom

“…Sexual reproduction has been described for H. capsulatum (142,146), B. dermatitidis (194), and most dermatophyte species (307). While successful mating has not yet been observed for P. marneffei, C. immitis, or C. posadasii, genes related to sexual reproduction have been identified in the genomes of these dimorphic fungal species (70,189,311). Data from population genetic studies also support extant sexual reproduction (30,61,62,138,189).…”

“…15). It is clear for the fungal kingdom that additional genes have been acquired into the mating type locus and/or evicted from the mating type locus (70). Thus, the last common ancestral MAT locus could have harbored the three genes for TPT/HMG/RNA helicase and that these genes were retained within the MAT locus in the microsporidian lineage but are no longer within MAT in the zygomycete lineage (Fig.…”

“…6). Further analysis revealed that the significantly larger MAT locus in Coccidioides species results from the capture of the flanking APN2 and COX13 genes, which now exhibit higher sequence divergence (70,189 .html]), a MAT1-2 HMG gene flanked by SLA2, APN2, and COX13 was identified by using Blastn and tBlastx searches (W. Li and J. Heitman, unpublished data). The orientation of these genes is the same as that of the most closely related species, H. capsulatum.…”

Complex Mechanisms Regulate Developmental Expression of thematA(HMG) Mating Type Gene in HomothallicAspergillus nidulans

“…Several lines of study indicate that mating type genes in fungi share structural and functional features with the more complex sex chromosomes of algae, plants, and animals (Kronstad and Staben 1997;Marais and Galtier 2003; Heitman 2004, 2005;Debuchy et al 2010). It has also been suggested that Mat-HMG in fungi represents a structural ortholog of the human SRY regulator (sexdetermining region Y) Heitman 2004, 2005;Fraser et al , 2007Heitman 2006).The genetic organization of mating type genes at mat loci determines sexual identity and mating strategy in filamentous fungi (Nelson 1996;Coppin et al 1997; …”

Frequency and Genetic Diversity of the MAT1 Locus of Histoplasma capsulatum Isolates in Mexico and Brazil

“…The classical studies of sexual compatibility in H. capsulatum were performed by mating fungal specimens in culture plates. However, this procedure is difficult because H. capsulatum isolates rapidly lose the ability to mate in vitro (5); therefore, molecular methods were developed to identify the mating type in this microorganism (6)(7)(8).…”