Altering sexual reproductive mode by interspecific exchange of MAT loci.

Abstract Sexual fungi can be self-sterile (heterothallic, requiring genetically distinct partners) or self-fertile (homothallic, no partner required). In most ascomycetes, a single mating type locus ( MAT ) controls the ability to reproduce sexually. In the genus Cochliobolus , all heterothallic species have either MAT1 - 1 or MAT1 - 2 (but never both) in different individuals whereas all homothallic species carry both MAT1 - 1 and MAT1 - 2 in the same nucleus of an individual. It has been demonstrated, previously, that a MAT gene from homothallic Cochliobolus luttrellii can confer self-mating ability on a mat -deleted strain of its heterothallic relative, Cochliobolus heterostrophus . In this reciprocal study, we expressed, separately, the heterothallic C. heterostrophus MAT1 - 1-1 and MAT1 - 2 - 1 genes in a mat -deleted homothallic C. luttrellii strain and asked if this converts homothallic C. luttrellii to heterothallism. We report that: (1) A C. luttrellii transgenic strain carrying C. heterostrophus MAT1 - 1 - 1 and a C. luttrellii transgenic strain carrying C. heterostrophus MAT1 - 2 - 1 can mate in a heterothallic manner and the fertility of the cross is similar to that of a wild type C. luttrellii self. Full tetrads are always found. (2) A C. luttrellii transgenic strain carrying C. heterostrophus MAT1 - 1 - 1 can mate with the parental wild type C. luttrellii MAT1 - 1 ; MAT1 - 2 strain, indicating the latter is able to outcross, a result which was expected but has not been demonstrated previously. (3) A C. luttrellii transgenic strain carrying C. heterostrophus MAT1 - 2 - 1 cannot mate with the parental wild type C. luttrellii MAT1 - 1 ; MAT1 - 2 strain, indicating outcrossing specificity. (4) Each transgenic C. luttrellii strain, carrying only a single C. heterostrophus MAT gene, is able to self, although all pseudothecia produced are smaller than those of wild type and fertility is low (about 4–15% of the number of wild type asci). These data support the argument that in Cochliobolus spp., the primary determinant of reproductive mode is MAT itself, and that a heterothallic strain can be made homothallic or a homothallic strain can be made heterothallic by exchange of MAT genes. The selfing ability of transgenic C. luttrellii strains also suggests that both MAT1 - 1 - 1 and MAT1 - 2 - 1 genes of C. heterostrophus carry equivalent transcription regulatory activities, each capable of promoting sexual development when alone, in a suitable genetic background.