A structural peculiarity observed in the sexual embryo sacs of Dichanthium intermedium

Abstract

In an acetocarmine squash study of the ovules of the compilospecies Dichanthium intermedium, an opening was consistently observed on the wall of the eight-nucleate sexual embryo sac directly below the micropyle. Such an opening was never observed in the four-nucleate aposporous sac. It was postulated that the opening could develop because of a difference in the mode of thickening of the wall of the two types of embryo sacs. The opening would provide a path of least resistance to the growing pollen tube and enhance the chances of fertilization, thus favoring sexuality in a facultatively apomictic plant.

APOMIXIS, POLYPLOIDY, AND SPECIATION IN DICHANTHIUM

“…Seed production may be initiated in either the sexual or asexual embryo sacs, through either fertilization or pseudogamous parthenogenesis. The sexual embryo sac usually lies closer to the micropyle than does the asexual one, and Saran and de Wet (1969) demonstrated a weak spot on the sexual sac that seems to offer a path of least resistance to the pollen tube. Hybrid offspring among biotypes of Dichanthium agamospecies usually range from almost fully sexual to essentially obligate apomictic, and the haploids that are occasionally produced are either sterile or reproduce sexually.…”

“…However, gametophytic apomixis and sexual reproduction are not operational alternatives (Harlan et al, 1964). Apomictic and sexual embryo sacs are always produced, side by side, even in the most apomictic biotypes of the agamospecies (Saran and de Wet, 1969). Depending on the genotype and the environment, either one of these embryo sacs may develop parthenogenetically, or get fertilized and develop sexually (Knox, 1967;Saran and de Wet, 1970).…”

Cytotaxonomical studies in Tribulus terrestris and T. alatus (Zygophyllaceae)

“…Sexual and apomictic reproductions are not genetic alternatives, although apomixis may behave as either a dominant or recessive trait against sexuality (Harlan et al 1964, de Wet andStalker 1974). Successful natural populations are almost always facultatively apomictic (de Wet and Harlan 1970), with the degree of sexuality controlled by the genotype as well as environment (Knox 1967, Saran andde Wet 1969). Intrapopulation variation in degree of apomictic and sexual reproduction may explain the presence of some dwarf plants in group 111.…”

Apomixis and sexuality in the Potentilla argentea complex III. Euploid and aneuploid derivatives (including trisomics) of some apomictic biotypes

“…In Rosaceae, and in Potentilla especially, many cases of mitotic instability (e.g., BRITTON andHULL 1956, 1957;HASKELL and TUN 1961;LARSSON, personal communication;ASKER 1970 d and unpublished work) as well as meiotic instability of different kinds (THOMAS 1942, Dow-RICK 1958, SKALINSKA and CZAPIK 1958OLDEN 1965;CARLBOM 1969) have been reported. Abnormal mitoses might for instance precede embryosac as well as pollen formation, thus giving rise to gametophytes with a changed degree of ploidy.…”

“…In these genera, apomictic and sexual embryosacs are easy to distinguish, as they are four-and eightnucleate, respectively. Depending on the genotype and the environment, either one of these embryosacs may develop parthenogenetically, or become fertilized (SARAN and DE WET 1970). This interesting apomictic system must be kept in mind when studying other groups.…”