Challenges and Future Perspectives in the Systematics of Kickxellomycotina, Mortierellomycotina, Mucoromycotina, and Zoopagomycotina

This review presents a phylogenetically based classification of four subphyla of the early-diverging fungi: Kickxellomycotina, Mortierellomycotina, Mucoromycotina, and Zoopagomycotina. The Kickxellomycotina contains four orders: Asellariales (symbionts with isopods and Collembola), Dimargaritales (haustorial mycoparasites), Harpellales (symbionts of insect larvae), and Kickxellales (saprobes). Mortierellomycotina contains a single order, Mortierellales, that includes both saprobes and root endophytes. Zoopagomycotina also has one order, Zoopagales, that contains species of obligate animal parasites or mycoparasites. Mucoromycotina has two orders, Endogonales (saprobes and ectomycorrhizal fungi) and Mucorales (primarily saprobes). The Mucorales is by far the most diverse order and includes 14 families and two distinct but unnamed clades. In addition to providing a phylogenetic and taxonomic overview of these subphyla, we provide information on growth and axenic cultivation of these fungi. We also discuss DNA barcoding, environmental sampling, genome sequencing, and phylogenetic analyses as they relate to these subphyla and other early-diverging fungal lineages. Evidence suggests that genome sequencing from a wide array of taxa in these four subphyla combined with innovative environmental sampling approaches is likely to revolutionize our understanding of these fungi and also the fungal tree of life.