Myxogastria

Lore & Background

The class Myxogastria was introduced in 1899 by Thomas Huston Macbride (as Myxogastres), based on the family Myxogastridae. Swedish mycologist Elias Magnus Fries had earlier described many slime molds as Myxogasteres in 1829. The name Myxomycota, now considered a synonym, derives from Ancient Greek words for 'mucus' and 'fungus'. The phylogeny of Myxogastria remains unsettled due to conflicting molecular and developmental data, and some consider it a separate kingdom.

All Myxogastria species pass through a life cycle that begins with haploid spores, which germinate to produce either myxamoebae (in dry conditions) or myxoflagellates (in moist conditions). Two compatible cells fuse to form a diploid zygote, which develops into a multinucleate plasmodium. The plasmodium is the trophic phase, absorbing nutrients via phagocytosis and capable of enormous growth; one artificially cultivated cell of Physarum polycephalum reached 5.5 square meters. Under adverse conditions, myxamoebae can form long-lived microcysts.

The classification of Myxogastria is based on a 2019 revision by Leontyev and coauthors, which divides the class into two subclasses: Lucisporomycetidae (bright-spored) and Columellomycetidae (dark-spored). These subclasses contain eight orders and 14 families. Reclassification is complicated by the morphological plasticity of myxogastrids, which are susceptible to environmental influences; only a few characteristics are diagnostic for a small number of species.

Reader's Guide

Myxogastria represents a significant group of slime molds that bridge the gap between protists and fungi in biological classification. Their complex life cycle—alternating between microscopic unicellular stages, a giant multinucleate plasmodium, and elaborate fruiting bodies—makes them a model for studying cell biology, development, and evolution. The class is distributed worldwide but shows higher biodiversity in temperate regions, particularly in open forests, though they also inhabit extreme environments such as deserts, snow-covered areas, and underwater. Corticolous myxomycetes live on tree bark, sometimes high in the canopy.

Taxonomically, Myxogastria is challenging due to morphological plasticity and environmental influences, leading to historical duplications in species descriptions. The 2019 revision by Leontyev and coauthors integrated molecular phylogenetics with morphology to establish a more stable classification of nine orders and 13 families. Despite this, the relations among orders remain unclear, and the group's phylogeny is unsettled. Estimates of species diversity have increased with environmental sampling, suggesting 1200–1500 species, though only 446 species are common across multiple locations. The class continues to be incompletely described, with new taxa being discovered regularly.

Did You Know?

• Although monocellular, Myxogastria can reach up to 1 meter across.
• Spores of Myxogastria have been known to germinate after 75 years in herbarium specimens.
• One artificially cultivated cell of Physarum polycephalum attained a surface area of 5.5 square meters.
• Myxogastria are found in extreme regions such as deserts, under snow blankets, and underwater.