The role of homeodomain transcription factors in fungal development

Homeodomain (HD) transcription factors are well established as regulators of development in animals since the identification of homeobox gene clusters in Drosophila melanogaster. However, in fungi this class of transcription factors remains poorly characterized. In fungi genes with HD domains were first identified in mating type loci, which regulate sexual compatibility and development. In both Ascomycota and Basidiomycota several HD transcription factors have previously been shown to play an important role in multicellular development and fructification, including hom1 and hom2 in S. commune. Furthermore, our preliminary results suggest hom3 in S. commune is also involved in fructification. We used a phylogenetic analysis of 222 previously published genomes to provide an evolutionary framework for HD transcription factor function in fungi. On average fungi with a yeast-like lifestyle carry fewer HD transcription factors than multicellular fungi. Particularly HD genes related to complex multicellular development and fructification are lacking in yeast-like fungi. Ascomycota and Basidiomycota both feature distinct HD gene groups, with Basidiomycota carrying more genes and featuring broader diversification. However, a single group of conserved HD genes across both Basidiomycota and Ascomycota has a role in fructification in both Phyla, suggesting shared regulatory pathways. Most HD transcription factor groups we identified do not have any characterized members and as such potential functions cannot be predicted. This emphasizes the necessity of powerful molecular tools in fungi to reliably combine in silico functional prediction with in vivo characterization.