SEXUALITY IN ASCOBOLUS STERCORARIUS. I. MORPHOLOGY OF THE ASCOGONIUM; PLASMOGAMY; EVIDENCE FOR A SEXUAL HORMONAL MECHANISM

Abstract

DURING A recent genetic study (Bistis, 1956) of Ascobolus stercorarius (Bull.) Schr6t. it became apparent that any attempt to interpret certain of the genetic data would require a more detailed account of the life cycle, particularly the stages of the sexual process up to and including plasmogamy. This species is heterothallic (Dowding, 1931; Bistis, 1956); consequently each ascus fusion nucleus must be, heterozygous for the compatibility factors. Since mutation at the mating-type locus has never been observed, ascogonia produced by single ascospore cultures are presumed to -be homocaryotic. In matings, plasmogamy, which brings together the complementary nuclei, is of the gamete-gametangial type; there are no antheridia. Although there is a considerable literature on the morphology of the ascogonium, the nature of the fertilizing element was correctly determined only by Miss Dowding. She reported that the transfer of oidia of one mating-type onto a mycelium of a compatible strain results in the development of fertile apothecia. Possibly here, as in several other heterothallic ascomycetes lacking an antheridium, the fertilizing element may be any cell containing nuclei of opposite mating-type. A detailed account of the sexual process up to and including plasmogamy is presented in this paper. In addition, evidence for hormonal control of certain stages in this process will be given. MATERIALS AND METHODS.-The-cultures used in this investigation were single-ascospore isolates obtained through the courtesy of Dr. Clare Yu and Mrs. Jane Rhein. The medium used for vegetative culture consisted of yeast extract (0.4 per cent), glucose (1.0 per cent), and Difco Bacto-agar (2.5 per cent). For the production of fruiting bodies the medium (Yu, 1954) consisted of yeast extract (0.025 to 0.3 per cent), cellulose (a disk of 9 cm. Whatman No. 7 filter paper in each dish), and Difco Bacto-agar (2.5 per cent). The method devised for studying the interaction of oidia and mycelium has these basic steps: (1) an inoculum taken from a three-day-old culture of the a mating-type isolate (ascogonial parent) is placed at the center of a 9-cm. dish containing 25 cc. of the yeast extract-cellulose medium, (2) three days later a small block of fresh yeast-extract agar