Evolution of the fungal self-fertile reproductive life style from self-sterile ancestors

In most fungal ascomycetes, mating is controlled by a single locus (MAT), F ungi requiring a partner to mate are heterothallic (self-sterile); those no t requiring a partner are homothallic (self-fertile), Structural analyses o f MAT sequences from homothallic and heterothallic Cochliobolus species sup port the hypothesis that heterothallism is ancestral. Homothallic species c arry both MAT genes in a single nucleus, usually closely linked or fused, i n contrast to heterothallic species, which have alternate MAT genes in diff erent nuclei. The structural organization of MAT from all heterothallic spe cies examined is highly conserved; in contrast, the organization of MAT in each homothallic species is unique. The mechanism of conversion from hetero thallism to homothallism is a recombination event between islands of identi ty in otherwise dissimilar MAT sequences. Expression of a fused MAT gene fr om a homothallic species confers self-fertility on a MAT-null strain of a h eterothallic species, suggesting that MAT alone is sufficient to change rep roductive life style.