LAUSR

Summary Fungi have evolved diverse lifestyles and adopted pivotal new roles in both natural ecosystems and human environments. However, the molecular mechanisms underlying their adaptation to new lifestyles are obscure. Here, we hypothesize that genes shared across all species with the same lifestyle, but absent in genera with alternative lifestyles, are crucial to that lifestyle. By analysing dozens of species within four genera in a fungal order, with each genus following a different lifestyle, we find that genus‐specific genes are typically few in number. Notably, not all genus‐specific genes appear to derive from de novo birth, with most instead reflecting recurrent loss across the fungi. Importantly, however, a subset of these genus‐specific genes are shared by fungi with the same lifestyle in quite different evolutionary orders, thus supporting the view that some genus‐specific genes are necessary for specific lifestyles. These lifestyle‐specific genes are enriched for key functional classes and often exhibit specialized expression patterns. Genus‐specific selection also contributes to lifestyle transitions, and is especially associated with intensity of pathogenesis. Our study, therefore, suggests that fungal adaptation to new lifestyles often requires just a small number of core genes, with gene turnover and positive selection playing complementary roles.