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Comparative measurements were made of the nuclear Feulgen‐DNA content of a heterothallic and a self‐fertile isolate of the myxomycete Didymium iridis. Plasmodial nuclei of both isolates contain the diploid amount of DNA. The replicated diploid (4C) values for the heterothallic and the self‐fertile isolates are 5.66 and 5.95, respectively. Myxamoebae, however, are quite dissimilar in their nuclear DNA content. Those of the heterothallic isolates, Honduran 1–2 (A1) and Panamanian 2–4 (A7), have mean values of 3.81 and 3.69, whereas myxamoebae of the self‐fertile Philippine‐1 isolate were found to have a mean value of 6.07. Myxamoebae of the Ph‐1 isolate are, therefore, at the same ploidy level as the Ph‐1 Plasmodium. Mean DNA values for Ph‐1 sporangial nuclei were in category 4C. Measurement of the DNA content of mitotic metaphases in sporangia at T = 6 hr confirmed that the mean DNA content of both Ph‐1 myxamoebae and plasmodial nuclei is equivalent to 4C. It is concluded that nuclear phase alternance is lacking in the Ph‐1 isolate and that the Plasmodium of this isolate develops by apogamy.

Genetic and cytochemical investigations of the origin, development, nuclear activity, and ploidy level of Plasmodia obtained from selfed clones S‐2 and B1P‐33 of the heterothallic myxomycete, Didymium iridis, are presented. To demonstrate that selfing did not result from contamination of the clones, or mutations at the mating‐type locus, crosses were made between F1 clones and clones of known mating types. The data were inconsistent with these two possibilities. DNA was quantified by Feulgen‐DNA microspectrophotometry. All cellular phases studied (logarithmic amoebae, swarmers, and encysted amoebae) appear to be haploid, with the nuclear DNA being in the replicated (2C) state. The plasmodia are in all cases diploid; however, the data indicate that the selfed Plasmodia are in an extended G1 condition. The nuclear DNA content of these is therefore 2C, whereas that of the cross Plasmodium is 4C. Sporangial nuclei exhibit DNA in diploid replicated (4C) category.

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