Genomic Organization and Mechanisms of Inheritance in Arbuscular Mycorrhizal Fungi: Contrasting the Evidence and Implications of Current Theories

Most plants, including the majority of crop species, associate with a specific group of soil fungi called arbuscular mycorrhizal (AM) fungi. These fungi facilitate the uptake of plant resources and are increasingly acknowledged as being critical to ecosystem function. This high level of ecological success is not expected for species that reproduce asexually, as they are expected to be vulnerable to accumulation of deleterious mutations (Muller 1932; Kondrashov 1988) and pathogenesis (Hamilton 1980; Lively 1987). Rather, AM fungi have flourished since the origin of plants (Pirozynski and Malloch 1975; Redecker et al. 2000), and appear to be the oldest asexual multicellular eukaryotes. While the ecological importance and evolutionary novelty of these fungi have become clear, the basic genetics of these fungi remain enigmatic. In fact, their genetic structure seems unusual in two ways. Firstly, individual cells always contain many nuclei, into the thousands or tens of thousands. And secondly, individual cells also contain very high levels of genetic variation, with as many as 13 variants at what would be expected to be single copy regions of the genome. It is likely that the presence and maintenance of this high level of standing variation is related to the long-term persistence and ecological success of these asexual species. However, basic issues such as the arrangement of this variation between, or within, nuclei remain controversial.