Cytology and Breeding Behavior of Twin Plants of Bromus inermis

Abstract

1. 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 relative positions of the embryos to the endosperm. All sets of twins differed in at least minor characters as adult plants. 2. Large differences occurred in seed-set following both self- and open-pollination. This was evident both between and within twin sets. The percentages of pollen stainable with IKI varied from 0 to 99.4, the differences being approximately as large within as between sets. Pollen of a given plant generally varied somewhat in size. Three major classes were found in the population examined, these having diameters approximating 50, 61-78, and 84-88 μ. 3. Variations were noted in the frequency of irregular meiotic divisions between and within sets. Generally there was a tendency for the amount of this irregularity to be inversely related to the extent of seed-set following both self- and cross-pollination. 4. The plants of the I1 progeny from some plants were materially reduced in vigor when compared with those of the I0 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. 5. 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. 6. 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. The behavior of the progenies following inbreeding and open-pollination was also in accord with this interpretation.