Breeding Behavior and Chromosome Numbers in Progenies from Twin and Triplet Plants of Poa pratensis

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Abstract
Two groups of twin and triplet sets are considered. 24 pairs of twins and 2 sets of triplets (group 1) were grown to maturity from seedlings. The frequency of aberrants in progenies from 3 sets of twins was found to differ significantly. No such difference occurred among the progenies from the other 21 sets of twins or between those from the 2 sets of triplets. 46% of a non-random population (group II) of twin and triplet sets differed in morph. characteristics. No significant differences were found in number and vigor of seedlings arising from members of multiple sets. Likewise, no difference occurred between the same characters for identical compared with dissimilar sets. The frequency of aberrant plants was significantly different in progenies from 2 of the twin and 1 of the triplet sets. The gross morphology of the predominant plant type in progenies from 7 twin and 2 triplet sets was greatly different. It is concluded from the breeding behavior and the chromosome number in plants of progenies, and from 2 sets of twins, that multiple embryos had arisen in the following ways: (1) from the egg and a synergid of an embryo sac carrying the reduced chromosome number, one member of the egg apparatus de-veloping parthenogenetically because of the stimulative effect of fertilization of the other; (2) from 2 embryo sacs in which the eggs (or an egg and a synergid) carrying the gametophytic chromosome number had been fertilized; (3) from the unfertilized eggs, or from an egg and a synergid, of embryo sacs that arose by somatic apospory; (4) from eggs (or the egg and a synergid of one sac) of embryo sacs developed through somatic apospory, one forming a diploid embryo and the other a tetraploid by somatic doubling of the dividing zygote; (5) from 2 egg cells or from an egg and synergid that arose by somatic apospory, one of which was fertilized, the other developing parthenogetically; and (6) from the fertilized egg derived from a reduced macrospore and from an egg of an aposporous embryo sac. The observations are discussed in relation to known breeding behavior of biotypes of the species and to the morphological and chromosomal variation in naturally occurring populations of the species.

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