Clues of sexual reproduction in the Arbuscular Mycorrhizal Fungi: a putatively ancient asexual land-plant symbiont
Arbuscular Mycorrhizal Fungi (AMF) are an ancient group of obligate land-plant symbionts which form a mutualistic symbiosis with the roots of over 80% of land plants and are found in the majority of terrestrial ecosystems. The filamentous hyphae of these fungi grow throughout the soil, scavanging for water and nutrients and transfer these to the plant in exchange for plant produced sugars. As a result, extensive underground networks can form where multiple fungal individuals can be connected to a single plant and a single fungus can be connected to multiple plants simultaneously. AMF do not have cells, rather nuclei flow freely through a common cytoplasm and genetically distinct individuals of a species are capable of anastomosis – or hyphal fusion – where cytoplasmic and nuclear exchange may occur. Curiously, these fungi have been once classified as ‘ancient asexuals’ due the lack of any observable sexual structures and an assumed purely clonal life-style. I present recent insights from the genome of the model AMFRhizophagus irregularis which contains evidence of a genetic tool kit which looks more like that of a sexually reproducing organism. I highlight the extreme expansion of a group of MATA-HMG genes which normally act as master- regulators of sex in fungi and present insights into the structure and function of these genes from a survey of local and global populations of R. irregularis individuals along with transcriptional evidence via QRT-PCR that a subset of these genes have a functional involvement in AMF partner recognition and possibly sexual reproduction.