Evolutionary Aspects of Ascospore and Conidium Ontogeny in Scoleconectria cucurbitula

Morphological changes and nuclear behavior during ascospore ontogeny in a collection of Scoleconectria cucurbitula with two-spored asci were characterized, and asexual states observed in artificial culture. Following meiosis, two single-celled, uninucleate ascospores were formed within the developing ascus; the other two post-meiotic nuclei did not take part in ascospore development. Ascospores progressed through two-celled, four-celled, and multi-celled stages, finally becoming highly elongate and transversely septate, with numerous uninucleate cells. While within the ascus, ascospore cells differentiated into phialides and produced numerous conidia. Synanamorphs formed in culture included a phialidic hyphomycetous state, a pycnidial state, and a state producing spores within hyphal cells by a process resembling free cell formation. It is suggested that intra-ascal germination obviates the need for greater numbers of ascospores, and that the two-spored form of S. cucurbitula likely arose from the four-spored form. Intra-ascal germination appears correlated with loss of ascus rings and possibly with loss of forcible spore discharge in certain Nectria species. The possible role of heterochrony in the evolution of phialidic ascospore germination in S. cucurbitula and allied fungi is discussed.