Mixed arbuscular mycorrhizae from the Triassic of Antarctica

Abstract

Arbuscular mycorrhizae are the most ubiquitous of mycorrhizal fungi, that have formed mutualistic relationships with virtually almost all major groups of vascular plants. Five genera of arbuscular endomycorrhizal fungi are currently delineated, but fossil arbuscular mycorrhizae have been allied with only two, Glomus and Sclerocystis. A Triassic arbuscular mycorrhiza described inhabiting the roots of Antarcticycas was originally allied with Glomus. It is now known to be a mixed colony comprised of fungi attributable to the suborders Glomineae and Gigasporineae of the Glomales, described as two new species. The fossil Gigasporinean mycorrhiza is characterized by irregularly swollen intercellular and intracellular hyphae that are coiled extensively within the cells. Arbuscules have thick trunks and narrow branches. In the Glominean form, hyphal diameter is more uniform, with coiling rarely present. Arbuscules have thin trunks and fine branches. Vesicles may be lateral or terminal. Spores are not present; therefore, the probability of more than one species of each suborder being represented cannot be conclusively demonstrated. This provides the first fossil representative of the Gigasporineae and supports current rDNA estimates of the age of the lineage. Moreover, it is the first reported instance of a mixed colony of arbuscular endomycorrhizae in the fossil record.