Analysis of the phylogenetic relationships and evolution of the cell walls from yeasts and fungi

The fungal cell wall is a coherent structure formed by microfibrillar polysaccharides and amorphous material made of other polysaccharides and proteins. We performed a phylogenetic analysis of covalent proteins and enzymes that synthesize fungal wall polysaccharides to determine the possible evolution of the wall structure. It is suggested that the components that made up the archaic walls were structural ones, forming a primitive girdle that retained noncovalently bound proteins in the periplasm and allowed cell growth in hypotonic media. The following hypothetical series of events in fungal wall evolution is suggested: (1) Construction of a primitive wall made of chitin and chitosan by division 2 chitin synthases and chitin deacetylases, respectively. (2) Appearance of class II chitin synthase genes (CHS) after separation of Microsporidia. (3) Capture of a gene encoding beta-1,3-glucan synthase from an organism related to Plantae or Chromista by horizontal transfer after separation of Chytridiomycota. (4) Appearance or horizontal capture from Chromista of genes involved in beta-1,6-glucan synthesis after separation of Zygomycota. (5). Appearance of class III CHS genes. (6) After split of Dikarya phyla, appearance in Ascomycota of class I CHS genes and the capacity to synthesize covalently bound wall proteins.