SECONDARY HOMOTHALLISM IN BREMIA-LACTUCAE

Many isolates of B. lactucae are heterothallic; some seemed to be self-fertile. Single conidial lines were derived from 1 self-fertile isolate. The majority of these lines were self-fertile, but some were self-sterile. The self-fertile lines were stable over many asexual generations when subcultured using large numbers of conidia. Attempts to restore self-fertility by culturing self-sterile lines together were not successful. Both normal and abnormal patterns of gametangial development were observed by scanning electron microscopy of a self-fertile line. The pattern of segregation of self-fertile and self-sterile lines from a self-fertile isolate of B. lactucae is similar to that described for the homothallic isolates of Phytophthora drechsleri. It is suggested that self-fertility in B. lactucae, as in P. drechsleri, may be due to the chromosomal determinants of compatibility-type being trisomic following numerical non-disjunction at meiosis. The segregation of self-sterile lines from the self-fertile isolate may have involved at least transitory heterokaryosis.