Nuclear volume and number in long-term in vitro cultures of Puccinia graminis

Eight cultures of Puccinia graminis f. sp. tritici which had been serially subcultured in the Cereal Rust Laboratory for 2–13 years were examined for nuclear volume, number of nuclei per cell, and dimensions of hyphae. The cultures were of two types: compact with short, wide hyphal cells (ca. 10 × 22 μm); and fluffy with long, narrow cells (ca. 5 × 80 μm). The compact cultures tended to sporulate; the fluffy ones did not. By using volume of hyphal nuclei to estimate ploidy, the cultures were judged to be of three types: compact with two haploid nuclei per cell; compact with one diploid nucleus per cell; and fluffy with two diploid nuclei per cell. The compact cultures originated from American uredial isolates of races 17, 23, 38, and 56. The fluffy cultures originated only from University of Sydney uredial isolate 334, race 126Anz6,7, or as variants of compact cultures from American race 56. A uninucleate fluffy culture which had been initiated from uredial isolate 334 by Australian workers had a nuclear volume four times those of binucleate compact cultures, suggesting that the nucleus of each cell was tetraploid. A culture of P. graminis f. sp. secalis initiated at the Cereal Rust Laboratory was compact and had mostly four nuclei per cell, each presumed to be haploid. Generally, compact cultures had more characteristics in common with the parasitic form of P. graminis than fluffy cultures did.