Aspergillus: Sex and Recombination

Abstract

The genus Aspergillus is one of the most widespread groups of fungi on Earth, comprised of about 300-350 species with very diverse lifestyles. Most species produce asexual propagula (conidia) on conidial heads. Despite their ubiquity, a sexual cycle has not yet been identified for most of the aspergilli. Where sexual reproduction is present, species exhibit either homothallic (self fertile) or heterothallic (obligate outcrossing) breeding systems. A parasexual cycle has also been described in some Aspergillus species. As in other fungi, sexual reproduction is governed by mating-type (MAT) genes, which determine sexual identity and are involved in regulating later stages of sexual development. Previous population genetic studies have indicated that some supposedly asexual aspergilli exhibit evidence of a recombining population structure, suggesting the presence of a cryptic sexual cycle. In addition, genome analyses have revealed networks of genes necessary for sexual reproduction in several Aspergillus species, again consistent with latent sexuality in these fungi. Knowledge of MAT gene presence has then successfully been applied to induce sexual reproduction between MAT1-1 and MAT1-2 isolates of certain supposedly asexual aspergilli. Recent progress in understanding the extent and significance of sexual reproduction is described here, with special emphasis on findings that are relevant to clinically important aspergilli.

Ecology of aspergillosis: insights into the pathogenic potency of Aspergillus fumigatus and some other Aspergillus species

“…Recent advances in the fields of genomics, cell biology and population genetics have reshaped our view of how fungal pathogens reproduce and might be evolving. Some species, traditionally regarded as asexual, mitotic and largely clonal, are now being examined in the context of their (cryptic) sexuality (Heitman et al ., ; Varga et al ., ). In this regard, while most known Aspergillus species – approximately two‐thirds of the total number have not yet been demonstrated to possess a functioning sexual cycle (Dyer and O'Gorman, ), there has been a remarkable discovery of sexual stages (also known as teleomorphs) for aspergilli that were hitherto assumed to be asexual, such as A. fumigatus (O'Gorman et al ., ), A. flavus (Horn et al ., ) and Aspergillus parasiticus (Horn et al ., ).…”

“…self‐fertilization) of most sexual aspergilli (Lee et al ., ; Dyer and O'Gorman, ). In addition, it is known that some Aspergillus species can undergo a parasexual cycle that enables genetic recombination during mitosis (Pontecorvo et al ., ; Lee et al ., ; Varga et al ., ).…”

Evidence for microbial local adaptation in nature

“…The parasexual cycle includes haploid cell and nuclear fusion, mitotic recombination and subsequent reduction in the nuclear content—without meiosis—back to a haploid state; parasexuality is best studied in filamentous fungi (Pontecorvo ; Tinline & Macneill ; Varga et al . ). Prokaryotes generally recombine through lateral gene transfer (Ochman et al .…”

Genomic Instability in Fungal Plant Pathogens

“…In general, parasexual recombination occurs in filamentous fungi through the fusion of hypha (anastomosis) followed by a fusion of nuclei. The diploid-like nuclei that are formed are unstable; eventually, chromosomes are lost to form a haploid nucleus that contains chromosomes from the two parental nuclei and potentially even recombined chromosomes [24,25]. Parasexual recombination performed in vitro in F. oxysporum lead to the creation of a new combination of alleles in the daughter cells that are important for pathology [26].…”