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

Abstract

Two groups of twin and triplet sets are considered. Twenty-four pairs of twins and two sets of triplets (group I) were grown to maturity from seedlings. The frequency of aberrants in progenies from three sets of twins was found to differ significantly. No such difference occurred between the progenies from the other twenty-one sets of twins or between those from the two sets of triplets. Forty-six per cent of a nonrandom population (group II) of twin and triplet sets differed in morphological characteristics. No significant differences were found between the number and the vigor of seedlings 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 two of the twin and one of the triplet sets. The gross morphology of the predominant plant type in progenies from seven twin and two triplet sets was greatly different. It was concluded from the breeding behavior and the chromosome number in plants of progenies, and from two sets of twins, that multiple embryos had arisen in the following manners: (1) from the egg and a synergid of an embryo sac carrying the reduced chromosome number, one member of the egg apparatus developing parthenogenetically because of the stimulative effect of fertilization of the other; (2) from two 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 two egg cells or from an egg and synergid that arose by somatic apospory, one of which was fertilized, the other developing parthenogenetically; 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.