Reciprocal Crosses in Streptomyces Coelicolor.

interaction of two complementary growth-factor dependent mutants, i.e., Tzp lemen ta ry auxotrophs, of Streptomyces coeticolor to form grovvth-factor independent progeny, i.e., prototrophs, was interpreted as gene recombination by SERMO~TI and SPADA-SERMONTI (1955) and by BRAENDLE and SZ+BALSKI (1959yi but as heterokaryosis by BRADLEY (1957). The former workers concluded that gene recombination was operative because (1) nonparental auxotrophs were also isolated from mixed cultures of the two parental types and ( 2 ) all of the recombinant classes tested were found to be stable. Conversely, BRADLEY ( 1958a) obtained infrequent dissociates regularly from nonparental auxotrophic recombinants and from prototrophic recombinants; only parental dissociates were stable. Recently SERMONTI and SPADA-SERMONTI (1959) derived diverse types from recombinant clones. In order to elucidate the recombinatiohal process, BRAENDLE and SZYBALSKI (1957) carried out “reciprocal crosses” using streptomycin susceptibility as an unselected marker. They found that when streptomycin resistance was coupled with proline and glutamate independence, and streptomycin sensitivity was coupled with methionine and hi’stidine independence, all of the recombinants were drug resistant. Contrariwise, when streptomycin resistance was coupled with methionine and histidine independence, and streptomycin sensitivity with proline and glutamate independence, almost all of the recombinants were inhibited by streptomycin. Although this correlation could have resulted from gene recombination, interpretations based on the data of BRAENDLE and SZYBALSKI were inconclusive because progeny arising from the cross involving the two streptomycin resistant parental strains were not analyzed. This investigation was concerned with reciprocal crosses using streptomycin susceptibility and actinophage susceptibility as unselected markers. The progeny of sensitive by sensitive crosses and of sensitive by resistant crosses were sensitive. Conversely, the progeny of resistant by resistant crosses were resistant. These results prove that prototrophs derived from mixed cultures of complementary auxotrophs are not simple haplophase recombinants. These findings are consistent with the hypotheses that these prototrophs are heterokaryons or heterozygous diploids.