Auxospore formation in two marine clones of the diatom genus Coscinodiscus

Auxospore formation has been observed in two marine clones of the diatom genus Coscinodiwcus. In each of these clones the variation in valve morphology and structure before and after auxospore formation is appreciable. This variability suggests a reappraisal of the taxonomic characters within the genus will be required if the kinds and types of variation noted are substantiated in other clones. The clones are monecious and the 32 uniflagellate colorless monads, inferred from indirect evidence to be male gametes, form within an undifferentiated mother celL. The method of release of the male gametes differs in the two clones; otherwise their development is similar. The zygote is recognizable and the formation of large diameter cells within the zygote membranes has been observed. THE CYTOLOGICAL events associated with auxospore formation in marine centric diatoms has recently received considerable attention by von Stosch (1956, 1953). These studies show that auxospore formation in the marine centric diatoms investigated is a sexual process. Although the sexual nature of this process had long been suspected (Geitler, 1935) adequate documentation was first provided by von Stosch (1951) in Melosira. Since 1951 the number of observations with different genera and species has steadily increased (von Stosch, 1954, 1956, 1958), leaving little doubt that auxospore formation in the centric diatoms is a sexual process. In this laboratory two marine clones of diatom genus Coscinodiscus (C. concinnus W. Smith and C. asteromphalus Ehrenberg) produced auxospores within a few months of each other, and this report summarizes the results of observations on these two clones. The report is divided into two sections. The first deals with the changes in valve pattern and structure observed during the life cycle of these diatoms and reveals the variability in taxonomic characters. The second deals with observations on the production of "male gametes" and auxospores (zygotes).