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A unicellular green alga which can grow without difficulty in pure culture, and which can be induced to undergo a complete sexual life cycle under experi- mental conditions, would offer possibilities as a suitable organism for an attack on problems in biochemical genetics for which Neurospora and similar heterotrophs are unsuited. Mating strains of a few species of Chlamydomonas have been isolated (Moewus, 1933; Gerloff, 1940; Smith, 1946; Bold, 1949; Lewin, 1949), but for various reasons none has proved altogether satisfactory. Several workers experienced difficulty in obtaining sexual strains when random isolates of Chlamydomonas from soil were paired and examined for mating reactions, Schreiber (1925) reported a lack of success when as many as 70 clones were tested in this way. An easier method was therefore sought and the following procedure, based chiefly on the differential sensitivity of spores and vegetative cells to desiccation, was devised. Zygospores of C. rnoewusii Gerloff are only produced as a result of sexual fusion of gametes; this is probably true also for most species in this genus. Such spores, which are only slightly more resistant to heat than are vegetative cells, are nevertheless remarkably resistant to desiccation. The walls are SO impermeable that spores may remain viable after immersion for a week in acetone, whereas this treatment kills yegetative cells within a few seconds. Bold (1949) isolated mating types of C. chlarnydogama Bold from air-dry soil; Smith (1948) stated that from his experience 'dry soil is a better source of sexual material than is damp soil'. It seems probable that the mating strains obtained by these authors arose from such drought-resistant zygospores. lMETHOD An enriched soil culture containing vegetative cells and zygospores is treated with acetone. From surviving spores a crude culture is grown. This consists of algal strains presumably capable of again producing zygospores. Pure cultures of these can be obtained by the usual method of dilution streaking on agar. When tested, they may prove either to be homothallic, producing zygospores within the clone, or they may exhibit heterothallism, and only form zygospores when mixed with cells from a culture of opposite mating type. Finally, bacteria-free cultures can be obtained. Details of a preliminary experiment using this method are given below.

Isolation of Sexual Strains of Chlamydomonas