Recurrent loss of abaa, amaster regulator of asexual development in filamentous fungi, correlates with changes in genomic andmorphological traits

Abstract

Gene regulatory networks (GRNs) drive developmental and cellular differentiation, and variation in their architectures gives rise to morphological diversity. Pioneering studies in Aspergillus fungi, coupled with subsequent work in other filamentous fungi, have shown that the GRN governed by the BrlA, AbaA, and WetA proteins controls the development of the asexual fruiting body or conidiophore. A specific aspect of conidiophore development is the production of phialides, conidiophore structures that are under the developmental control ofAbaAand function to repetitively generate spores. Fungal genome sequencing has revealed that some filamentous fungi lack abaA, and also produce asexual structures that lack phialides, raising the hypothesis that abaA loss is functionally linked to diversity in asexual fruiting bodymorphology. To examine this hypothesis,we carried out an extensive search for the abaAgene across 241 genomes of species fromthe fungal subphylum Pezizomycotina.We found that abaAwas independently lost in four lineages of Eurotiomycetes, including fromall sequenced specieswithin the orderOnygenales, and that all four lineages that have lost abaA also lack the ability to form phialides. Genetic restoration of abaA from Aspergillus nidulans into Histoplasma capsulatum, a pathogenic species from the order Onygenales that lacks an endogenous copy of abaA, did not alter Histoplasma conidiation morphology but resulted in a marked increase in spore viability.We also discovered that species lacking abaA contain fewer AbaA binding motifs in the regulatory regions of orthologs of some AbaA target genes, suggesting that the asexual fruiting bodyGRNof organisms that have lost abaA has likely been rewired.Our results provide an illustration ofhowrepeated losses of a key regulatory transcription factor have contributed to the diversity of an iconic fungal morphological trait.