Genetics of an interspecific cross in the genus Sordaria. I. Analysis of 8-spored asci homozygous for the mating type factor.

new heterothallic species of the genus Sordaria, Sordaria heterothallis ' Ze?ds and Maniotis (FIELDS and MANIOTIS, 1963) and Sordaria thermophila Fields (FIELDS 1968), are the subjects of this study. FIELDS (1963), in a comparative study of the two isolates concluded that they were distinct species on the basis of morphological and cultural studies, and from the results of hybridization experiments. FIELDS observed from these hybridization studies that although the two isolates reacted as distinct species they did show a certain degree of affinity. In detailing the degree of this affinity he noted the abundant production of perithecia from the interspecific matings. These perithecia produced asci which contained aborted protoplasts or occasionally delimited ascospores which did not mature. Perithecia produced on a long lasting medium, however, contained a few asci with 8 mature spores. No more than 3 asci containing 8 mature spores were ever found within a single perithecium. Some asci contained 2-7 mature spores, and in most matings ascospore production was scant or totally lacking. In the analysis of asci which contained 8 black spores FIELDS determined that in any one ascus the mating type, spore size, and pigmentation of the mycelium of all the spores which germinated were always like the S. heterothallis parent. He reported the recovery of two asci in tvhich 7 spores germinated, one ascus in which 6 spores germinated and 4 with 5 spores germinating. Strains from all these ascospores showed normal ascospore production when backcrossed with the S. heterothallis parent while in backcrosses with S. thermophila, ascospore production was less than that in the original interspecific cross. FIELDS concluded from these crosses that the genome of only one parent, S. heterothallis, took part in the formation of the 8-spored asci; apparently the S. thermophila genome did not participate in ascospore production but insufficient markers were available to completely exclude its participation. The present study was initiated to determine the origin and mode of formation of the asci with 8 mature ascospores, produced from the interspecific matings between these two heterothallic species.