Haploid vegetative mycelia ofArmillaria gallicashow among-cell-line variation for growth and phenotypic plasticity

Abstract

Vegetative mycelial cells of Armillaria are expected to have diploid nuclei. Cells from a single mycelium therefore would not be expected to differ from one another for ecologically relevant quantitative traits. We isolated two sets of basidiome cell lines (from spores and stipe cells) and one set of vegetative cell lines (from an attached rhizomorph) from a single contiguous Armillaria gallica mycelium. We isolated a second set of vegetative cell lines from the soil 20 cm from the above basidiome-rhizomorph complex. In all four sets of cell lines in situ DAPI-DNA measurements showed cells are haploid and quantitative-trait analyses of cell lines grown at different water potentials revealed high levels of among-cell-line genetic variation for both growth and phenotypic plasticity. Haploidy and the existence of ecologically relevant genetic variation within vegetative individuals are unexpected and mean that a process similar to evolutionary adaptation could take place within the soma of a genetic individual. We believe this is a key to understanding how large A. gallica mycelia survive exposure to variation in ecological conditions during lives that potentially span several tree (host) generations.

A termite symbiotic mushroom maximizing sexual activity at growing tips of vegetative hyphae

“…Intragenic recombination has been reported from EF1a and other DNA regions among Termitomyces isolates from 31 colonies of Macrotermes natalensis in South Africa, which apparently belonged to a breeding population but curiously lacked apparent mushroom fruiting (de Fine Licht et al 2006 ). Matings between compatible haploid monokaryons in two honey mushrooms, Armillaria gallica (Peabody et al 2005 ) and Armillaria tabescens (Grillo et al 2000 ), resulted in a haploid dikaryon and subsequently a persistent diploid monokaryon by karyogamy. Tissue of fruiting bodies, however, contained haploid dikaryotic cells and was shown to be genetically mosaic, implying somatic genetic recombination during haploidization.…”

Mosaic fungal individuals have the potential to evolve within a single generation

“…Our earlier studies 7 , 8 are consistent with the existence of HGM in A. gallica ; but analyses were limited to isozyme loci, mating-type loci, and a restriction fragment length polymorphism (RFLP) of IGS-1 in stipes. IGS-1 analyses were instructive but limited to one locus in one tissue type of a single individual 7 , 8 . In the present study RFLP analyses were expanded to include five loci in three tissue types from seven geographically distinct sites in southeastern Massachusetts, USA.…”

“…Raynham spore nuclei (N = 100), soil mycelium nuclei (N = 100), and rhizomorph hyphal filament nuclei (N = 100) were shown to be haploid in an earlier study of A. gallica growth responses to water potential 8 . To confirm haploidy of the current study’s cultured rhizomorph hyphal filament lines, multiple nuclei were measured for nuclear DNA content in each of 10 lines in the Raynham and Bridgewater genets.…”

“…However, A. gallica ’s life cycle is unusual among basidiomycetes, because after spore formation a second diploidization-haploidization event occurs. After compatible primary mycelia (n) fuse to reestablish dikaryons (n + n), nuclear fusion produces diploid nuclei (2n) that undergo a second (vegetative-stage) haploidization at some point prior to mushroom formation 4 – 8 . The resulting haploid nuclei (n) persist in vegetative stages (soil mycelia, rhizomorphs) and in mushroom stipes 7 .…”

“…It has been proposed that the vegetative-stage diploidization-haploidization in A. gallica ’s life cycle produces haploid genetic mosaics 4 – 8 , and that, if haploid genetic mosaicism (HGM) is found in other Armillaria species, it might account for some of the broad range of ecological, morphological, and pathogenic diversity that characterizes the genus. Armillaria species occur in boreal, temperate, and tropical forests where they can act as beneficial soil-borne decomposers, mycorrhizal symbionts with trees and several orchid species, or economically important root rot pathogens infecting over 600 plant species 9 .…”