CROSSING OVER IN CHLAMYDOMONAS REINHARDI

Abstract

THE UNICELLULAR green alga Chlamydomonas has been used for genetic investigations by several workers who have studied the inheritance of sexuality (Moewus, 1938; Smith and Regnery, 1950; Lewin, 1953), and the production and the inheritance of several types of morphological and physio-logical mutant strains (Lewin, 1952, 1953; Nybom, 1953; Sager, 1954, 1955; Eversole and Tatum, 1956; Eversole, 1956). The most extensive contribution to the present knowledge of Chlamydoomonas genetics has been made by Moewus. One of the most interesting results of his work was the finding that during meiosis crossing over in C. eugametos occurred at the four-strand stage at room temperature but only at the two-strand stage at temperatures below 5?C. (Moewus, 1938). Since these results are of considerable theoretical interest as to the mechanism of crossing over, investigations were begun in an effort to determine whether crossing over at the two-strand stage could be detected in another species, C. reinhardi. The studies reported here involve the recovery of genetically useful mutant strains, the establishment of linked markers, and the analysis of tetrads to distinguish between crossing over at the twoand at the four-strand stage. MATERIALS AND METHODS.-Tlaploid (+) and (-) cultures of C. reinhardi (strain 137C) obtained from G. M. Smith were used for this investigation [mating type (+) and (-) is enclosed in parentheses in order to avoid confusion with + which is used to designate the wild-type alleles of mutant genes]. Stocks were maintained in pure culture on agar slants and were grown at 26?C. under daylight fluorescent lamps at a constant light intensity of 150 f.-c. The life cycle of C. reinhardi has been described recently by Sager (1955). Media.-A mineral nutrient medium composed of 10 per cent Beijerinck's solution (Bold, 1942), 2 per cent agar, and M/150 phosphate buffer (pH 6.8-7.0) in distilled water supports growth of both the (+) and (-) wild-type strains of C. reinhardi in the light. This minimal medium was used throughout except where otherwise indicated. Detection of mutants and types of mutant strains obtained.-After ultraviolet irradiation of vegetative cells mutant strains were recovered by means of the layering method of Lederberg and Tatum (1946) or the surface irradiation technique of Wyckoff (1930). The supplemented medium used with both of these techniques was prepared by the