Chapter 2: Protozoan Pseudofungi, Chromistan Pseudofungi, and Early True Fungi

Myxostelida organisms exist as multinucleate amoeboid plasmodia that ingest bacterial prey before forming sporangia through straightforward differentiation. Dictyostelida represent a more sophisticated cellular strategy where individual amoeboid cells coordinate through cyclic adenosine monophosphate signaling to form mobile multicellular structures called pseudoplasmodia before fruiting. Labyrinthulida occupy marine environments and secrete characteristic polysaccharide tube networks while moving within these self-constructed pathways, with the genus Labyrinthula causing economically damaging wasting disease in eelgrass populations. Plasmodiophorida function as obligate endoparasites of plant roots, with Plasmodiophora brassicae inducing the characteristic club root disease in cruciferous crops. The chapter then examines Kingdom Chromista pseudofungi, particularly Hyphochytriomycota distinguished by anterior flagella with tubular projections and Oomycota, which despite superficial fungal resemblance possess diploid nucleic states, cellulose-based cell wall composition, and produce thick-walled sexual spores called oospores. Oomycetes include agriculturally catastrophic pathogens such as Phytophthora infestans, causative agent of late blight that precipitated the Irish Potato Famine; Plasmopara viticola responsible for downy mildew epidemics in European grapevines; Peronospora tabacina causing blue mold disease in tobacco; and Albugo candida generating white rust pustules on crucifers. The chapter concludes with three early-diverging true fungal phyla retaining zoosporic dispersal stages. Chytridiomycota comprise primarily aquatic saprotrophs and parasites with uniflagellate motile cells, including the amphibian pathogen Batrachochytrium dendrobatidis implicated in global amphibian population declines. Blastocladiomycota exemplified by Allomyces demonstrate alternation between distinct multicellular haploid and diploid generations. Neocallimastigomycota function as obligate anaerobic inhabitants of herbivorous mammalian digestive systems, where they enzymatically degrade plant structural polysaccharides essential for nutrient extraction. Collectively, these lineages illustrate how modern phylogenetic reconstruction has revealed profound evolutionary distances among superficially similar organisms and underscore the ecological and economic significance of microscopic eukaryotic diversity.