Summary: Cytological aspects of monokaryotic variants produced by axenic cultures of Pers. f.sp. Erikss. & E. Henn. were studied. Mitotic nuclei of variants at the end of stage I of division (late prophase to metaphase) contained approximately the same number of chromatinic bodies as haploid nuclei in dikaryotic hyphae of the parental wild type. However, an examination of spores produced by pathogenic variants showed that the size of uredospores and teliospores, the number of nuclei per spore cell, and the size of nuclei in teliospores, were all suggestive of the monokaryotic variants being diploid rather than haploid. Diploidy was also consistent with the occasional breakdown of both non-pathogenic and pathogenic monokaryons to form dikaryotic somatic cells; hyphal anastomosis was observed within one such region of dikaryotic cells produced by a non-pathogenic variant. On balance, it is concluded that nuclei in both pathogenic and non-pathogenic monokaryotic variants are diploid, and that mitotic chromosome counts do not distinguish between haploid and diploid nuclei. Diploidy is discussed in relation to naturally occurring monokaryotic rust fungi and to somatic variation in populations of dikaryotic rusts.


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