Evidence for cryptic sex in Escovopsis , a mycoparasite in the fungus-growing ant symbiosis

Abstract

The Red Queen hypothesis for the maintenance of sexual reproduction proposes that sex should be favored during parasite-host interactions. The presence of sexual reproduction in Escovopsis sensu lato (s.l.) (Hypocreaceae, Ascomycota), obligate specialized parasites on the fungal gardens of fungus-growing ants, has been debated; previous analyses have concluded that sex is likely absent based on Escovopsis s.l. appearing to lack a complete mating-type (MAT) locus, which controls sexual compatibility in fungi. Using 39 previously sequenced genomes, we found that computational annotation of these loci was inconsistent. Through manual annotation, we show that all sequenced Escovopsis s.l. have a complete MAT1 locus. The MAT1 locus is found in the typical genomic context for Hypocreaceae fungi, contains the expected genes (one in the MAT1-2 idiomorph and three in the MAT1-1 idiomorph), are highly conserved at a structural and functional level, and are under strong purifying selection. Using a Phi test, we also find evidence for recombination in one closely related group of samples. Past phylogenomic analyses of Escovopsis s.l. have generated two distinct topologies, and we find that the MAT1 genes also have differing topologies. Further, phylogenetic network analyses show large-scale gene tree discordance between early diverging Escovopsis s.l. taxa and some outgroups, supporting a potential past hybridization event. Taken together, these data suggest that Escovopsis s.l. undergoes cryptic sex, changing our understanding of the ecology and evolution of the model fungus-growing ant symbiosis and opening many exciting research avenues on the dynamics of sex and infection in this system.