The Origin of Multiple Macrogametophytes in Poa pratensis

Abstract

The origin of multiple macrogametophytes in Poa pratensis was studied and the results presented in relation to known breeding behavior of plants from both plural and single embryos. Eggs developed following meiosis may result in haploid-diploid and diploid-diploid relationships in the plural embryos and in the mature plants. The origin of these plural embryos is (1) from two members of the egg apparatus and (2) from two macrospore mother cells. Embryo sacs developed from somatic or nucellar cells may form diploid-diploid, diploid-triploid, and diploid-tetraploid combinations of plural embryos and mature plants. The diploid-diploid combinations are considered to arise from two somatic egg cells or from an egg and a synergid of a single embryo sac that develop parthenogenetically. The diploid-triploid combination probably arises in a similar manner; the egg of one embryo sac develops parthenogenetically, whereas that of the second develops as a result of fertilization by a male gamete carrying approximately the reduced chromosome number. Triploid-triploid combinations could form by the fertilizing of two cells (two eggs or an egg and a synergid). The diploid member of diploid-tetraploid plural embryos might originate in the manner indicated previously for other combinations, the tetraploid by the parthenogenetic development of the egg with a somatic doubling of the chromosomes of the nuclear complement at the time of the zygotic division, or fertilization with an unreduced or restitution male nucleus, or with a high polyploid gamete. The chromosome complement of the male gamete would have to equal that of the egg nucleus. It is considered quite improbable that origin of tetraploids involving gametes would be of common occurrence. Plural embryos were found to arise from reduced macrospores and nucellar cells in the same ovule. The embryos resulting from these would be diploid-diploid or diploid-triploid, depending upon whether fertilization of only the egg of the reduced macrogametophyte or fertilization of all of the several members, reduced and unreduced, occurred. "Supernumerary embryo sacs" might develop from nucellar "bud" cells of the wall of the gametophytic cavity. These embryo sacs may be imperfect and nonfunctional, or they may be complete and account for some of the instances wherein proembryos or well-developed embryos are located at various positions other than at or near the micropylar portion of the seed. The observations are incomplete for such a manner of origin of plural embryos. Macrosporogenesis in the haploid plant was quite similar to that reported for the higher polyploids of the species. The sterility of this plant is undoubtedly due to genetic rather than to evident cytologic disturbances.