Using the mating type genes (MAT-1 and MAT-2)
from
Cochliobolus heterostrophus (a pathogen of maize) as probes, MAT
genes
from C. carbonum (also a maize pathogen) and C. victoriae
(an oat pathogen) were cloned. Sequence analyses and functional tests
showed that C. carbonum MAT-1 and C. victoriae MAT-2
are structurally similar to their C. heterostrophus homologues,
and all three
genes function comparably when expressed in C. heterostrophus.
Gel blot analyses of DNAs from C. heterostrophus or C. carbonum,
probed with C. heterostrophus MAT-specific fragments,
revealed that field isolates from diverse geographical locations contained
either MAT-1 or MAT-2, but never both. The entire
currently existing collection of C. victoriae isolates, however,
contained only
MAT-2, and all members of the collection were found to be
either female sterile or completely infertile. Thus, crosses between
isolates of C. victoriae were not possible, although crosses
between C. victoriae and C. carbonum (but not
C. heterostrophus) were
readily made, confirming previous reports that these two species are
interfertile. These observations, combined with careful scrutiny
of the literature, have led to the hypothesis that MAT-1
female fertile isolates of C. victoriae have never existed. All
available
evidence is consistent with the notion that the entire C. victoriae
field population was derived from a single progenitor strain, which
arose as the result of horizontal transfer of genes for pathogenicity to
oats
into a strain of C. carbonum that was MAT-2 and female
sterile.