Abstract
The fungal kingdom displays a fascinating diversity of sex-determination systems. Recent advances in genomics provide insights into the molecular mechanisms of sex, mating type determination, and evolution of sexual reproduction in many fungal species in both ancient and modern phylogenetic lineages. All major fungal groups have evolved sexual differentiation and recombination pathways. However, sexuality is unknown in arbuscular mycorrhizal fungi (AMF) of the phylum Glomeromycota, an ecologically vital group of obligate plant root symbionts. AMF are commonly considered an ancient asexual lineage dating back to the Ordovician, approximately 460 M years ago. In this study, we used genomic and transcriptomic surveys of several AMF species to demonstrate the presence of conserved putative sex pheromone-sensing mitogen-activated protein (MAP) kinases, comparable to those described in Ascomycota and Basidiomycota. We also find genes for high mobility group (HMG) transcription factors, homologous to SexM and SexP genes in the Mucorales. The SexM genes show a remarkable sequence diversity among multiple copies in the genome, while only a single SexP sequence was detected in some isolates of Rhizophagus irregularis. In the Mucorales and Microsporidia, the sexM gene is flanked by genes for a triosephosphate transporter (TPT) and a RNA helicase, but we find no evidence for synteny in the vicinity of the Sex locus in AMF. Nonetheless, our results, together with previous observations on meiotic machinery, suggest that AMF could undergo a complete sexual reproduction cycle.
Highlights
Sexual reproduction drives natural selection and adaptation of organisms by generating genetic diversity through recombination
arbuscular mycorrhizal fungi (AMF) pheromone-sensing pathway The genomes of Rhizophagus spp. and G. cerebriforme analyzed in this study possess orthologous genes coding for all the proteins in the S. cerevisiae sex pheromone-sensing pathway (Figure 2 and Table S3), except for the scaffolding STE5 gene
Using genomic and transcriptomic surveys of several Rhizophagus spp. and G. cerebriforme, we clearly demonstrated the presence of conserved putative sex pheromone-sensing mitogen-activated protein (MAP) kinases comparable to those described in Ascomycota and Basidiomycota
Summary
Sexual reproduction drives natural selection and adaptation of organisms by generating genetic diversity through recombination. This allows beneficial mutations to spread, and purges deleterious mutations in eukaryotic genomes [1,2]. A recent study has challenged this view by showing that AMF have conserved a set of genes encoding the complete meiotic machinery [10]. These findings suggest that AMF could undergo meiotic recombination events, even though the existence of a cryptic sexual cycle still remains to be demonstrated. Only one author has described multinuclear propagation structures, in Gigaspora decipiens, which are different from generally observed chlamydospores, that could cautiously be interpreted as products of putative sexual development [11,12]
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