A COMPARATIVE STUDY OF SEXUAL REPRODUCTION IN FOUR SPECIES OF CHLAMYDOMONAS

TRAINOR, FRANCIS R. (U. Connecticut, Storrs.) A comparative study of sexual reproduction in four species of Chlamydomonas. Amer. Jour. Bot. 46 (2): 65-70. 1959.-This paper reports results of comparative experiments on some factors affecting sexual reproduction in 4 species of Chlamydomonas. Sexuality can be controlled in 3 of these species (C. chlamydogama, C. eugametos and C. reinhardti) by the level of ammonium nitrate in the harvesting medium. It was demonstrated that periodicity of illumination was of importance for high zygospore yields with C. chlamydogama and C. eugametos. Other variables, e.g., various aspects of nitrogen level, illumination and inter. fertility, were investigated and the results summarized in tabular form. A REVIEW of the literature on sexuality in Chlamydomonas immediately presents the reader with a rather complicated picture. Individual investigators using one species appear to contradict results obtained by other investigators with another species. Furthermore, on occasion, these results have differed when more than one investigator studied the same species and even identical strains. The cause of many of these controversial results apparently lies in the complexity of the mating reaction itself. With such influential factors as age of culture, periodicity and intensity of illumination, medium, nitrogen requirement and temperature used for growth and mating reactions, one finds considerable disagreement regarding the exact range within which one would obtain maximum sexuality. With these considerations in mind, the writer undertook a comparative study of sexuality in several species of the genus Chiamydomonas, in the belief that carefully controlled experiments with several previously studied species maintained under the same conditions might explain and/or substantiate some facts reported already with individual species. It was hoped that, as a result, either similarities or differences in the over-all reaction might be easily recognized and that the factors controlling manifestation of the mating reaction in these species might become evident. Since the year 1931 many articles concerning Chiamydomonas species have been published by F. Moewus. In view of the report of Ryan (1955), the author will not include further reference to Moewus' publications in this paper. Recently, reinvestigations of sexual phenomena in C. eugametos have been made (Forster and Wiese, 1954a, 1954b; Bernstein and Jahn, 1955). 1 Received for publication March 27, 1958. This paper is a summary of a dissertation submitted to the Graduate School of Vanderbilt University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. The writer is especially indebted to Professor H. C. Bold, now of the University of Texas, for his encouiragerment and suggestions. Because they were unable to obtain strains of C. eugametos from Moewus in the 1930's and 1940's, attempts were made by many investigators to isolate mating types of other Chlamydomonas species which could be used for studies of sexuality and genetics. Gerlofi (1940) isolated complementary mating types which he thought were almost identical with C. eugametos, but he named the organism C. moewusii. When Moewus' C. eitgametos strains became available for study by other investigators, it was found that they would mate with the C. moewusii strains of Provasoli (Nybom, 1953; Bernstein and Jahn, 1955). In addition, Nybom stated that C. eugametos and C. moewusii 44show no conspicuous morphological or physiological differences." Lewin (1954) and Bernstein and Jahn (1955) believe that it is probable that the two names are synonyms. However, in their comparative study of sexuality in C. moewusii and C. eugametos, the latter investigators indicate physiological differences in response to ageing, nitrogen deficiency and light intensity. Tsubo (1956) described a species, Chlamydomonas sp. 24, which resembles both C. eugametos and C. moewusii in some respects, but, according to Tsubo, Lewin has shown that the new species was not compatible with C.