Genome sizes of Epichloë species and anamorphic hybrids

Abstract

Several Neotyphodium species (asexual fungal symbionts of grasses) have been characterized genetically as heteroploids (nonhaploids) derived by interspecific hybridization involving sexual EpichloëSpecies (Clavicipitaceae, Hypocreales). Examples include Neotyphodium coenophialum with at least three ancestral Epichloë species, and a rare endophyte of perennial ryegrass that appears to be a hybrid of Epichloë typhina with the common perennial ryegrass endophyte, Neotyphodium lolii. In contrast, N. lolii appears to be haploid and closely related to Epichloë festucae. Hypothesizing that hybrid origins should result in larger genome sizes, we estimated genome sizes for two Epichloë and three Neotyphodium spp. Using quantitative Southern blot hybridization we estimated genome sizes of 29 ± 4 Mb for two sexual haploids, E. festucae isolate E189 and E. typhina E8. Estimates for hybrid anamorphs were 57 ± 7 Mb for N. coenophialum e19, and 55 ± 7 Mb for N. lolii X E. typhina Lp1. Electrophoretic karyotyping of these four isolates corroborated the genome size estimates. Also, the electrophoretic karyotype of N. lolii isolate e45 indicated a genome size similar to that of the Epichloë species, in keeping with the apparent non-hybrid nature of N. lolii. Genome size differences were reflected in the variation of conidium lengths and widths suggesting that large conidial size may be an indicator of heteroploidy and, therefore, possible hybrid origin of Neotyphodium species.