Vegetative Hyphal Fusion and Subsequent Nuclear Behavior in Epichloë Grass Endophytes

Abstract

Epichloë species (including the former genus Neotyphodium) are fungal symbionts of many agronomically important forage grasses, and provide their grass hosts with protection from a wide range of biotic and abiotic stresses. Epichloë species include many interspecific hybrids with allodiploid-like genomes, which may provide the potential for combined traits or recombination to generate new traits. Though circumstantial evidence suggests that such interspecific hybrids might have arisen from nuclear fusion events following vegetative hyphal fusion between different Epichloë strains, this hypothesis has not been addressed empirically. Here, we investigated vegetative hyphal fusion and subsequent nuclear behavior in Epichloë species. A majority of Epichloë strains, especially those having a sexual stage, underwent self vegetative hyphal fusion. Vegetative fusion also occurred between two hyphae from different Epichloë strains. Though Epichloë spp. are uninucleate fungi, hyphal fusion resulted in two nuclei stably sharing the same cytoplasm, which might ultimately lead to nuclear fusion. In addition, protoplast fusion experiments gave rise to uninucleate putative hybrids, which apparently had two markers, one from each parent within the same nucleus. These results are consistent with the notion that interspecific hybrids arise from vegetative hyphal fusion. However, we also discuss additional factors, such as post-hybridization selection, that may be important to explain the recognized prevalence of hybrids in Epichloë species.

Metarhizium: jack of all trades, master of many

“…Instead of undergoing meiosis, the vegetative cells continue dividing mitotically, and haploidy is restored by random chromosome loss resulting in cells with unique combinations of chromosomes from either parent nucleus. Many fungi that have never been observed to mate in their natural environment are still capable of parasexuality [ 215 , 216 ]. Furthermore, it has been shown that the thin-walled blastospores Metarhizium produces in insect haemolymph may facilitate genetic exchange between isolates co-infecting an insect [ 217 ].…”

Two genomes are better than one: history, genetics, and biotechnological applications of fungal heterokaryons

“…Whether these shifts resulted from selection acting on nuclei [ 14 ] or from more random processes of gain or loss in particular mycelial fractions requires further investigation. Recently, it has been shown that heterokaryons and subsequent hybrids can form between different strains of the grass endophyte, Epichloë , through vegetative hyphal fusion [ 15 ]. In fact, the existence of multiple genotypically distinct nuclei within the same mycelium may be more common than is recognized, particularly among filamentous ascomycetes [ 16 ].…”

Large Scale Screening of Epichloë Endophytes Infecting Schedonorus pratensis and Other Forage Grasses Reveals a Relation Between Microsatellite-Based Haplotypes and Loline Alkaloid Levels

“…lolii ) or from interspecific hybridizations between sexual and/or asexual Epichloë species (Craven, 2003). Hybridizations may occur within host plants that are co-infected by different strains through a process known as vegetative hyphal fusion (VHF) or anastomosis followed by nuclear fusion (Shoji et al, 2015). Interspecific hybrids have an allopolyploid-like genome which is the result of the combination of two or more parental chromosome sets.…”