Cytology and Breeding Behavior of Twin Plants of Bromus inermis

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Abstract
Twenty-eight sets of twin plants of Bromus inermis were successfully grown to maturity. Much of the original variation in vigor evident as seedlings had disappeared before the plants were established in the field, suggesting that the seedling differences in vigor were perhaps largely due to the position of the embryo relative to the endosperm. All sets of twins differed at least in minor characters as adult plants. Large differences were observed to occur in seed-set following both self- and open-pollination, both between and within twin sets. Percentages of pollen stainable with IKI varied from 0 to 99.4, the differences being approx. as large within as between sets. Pollen of a given plant generally varied somewhat in size. In the population examined three major classes were found with diams. approximating 50, 61-78, and 84-88 microns. Variations were noted in the frequency of irregular meiotic divisions between and within sets. Generally there was a tendency for this irregularity to decrease as seed-set following both self- and cross-pollination increased. The plants of the I1 progeny from some plants were materially reduced in vigor when compared to those of the Io progeny. Those from other plants were not materially reduced in vigor following inbreeding. This relationship was evident both within and between sets. The data indicate also that low self-fertility may not be closely related to the occurrence of plants of superior vigor in the I1 generation. Comparisons within 28, 56, and 70-chromosome plants and their progenies indicate close similarity of behavior throughout gametogenesis irrespective of the degree of polyploidy. Seed-set was greater, however, in the higher polyploids, other behaviors being approximately equal. It would appear evident that the twins were not monozygotic when considered with respect to the morphology and cytology of members of a given set. Behavior of the progenies following inbreeding and open-pollination was also in accord with this interpretation.