MEGAGAMETOPHYTE ORGANIZATION IN A POLYEMBRYONIC LINE OF LINUM USITATISSIMUM

A haploid-diploid twin-producing polyembryonic line “RA91” of Linum usitatissimum L., flax, known to produce twin embryos with a frequency in excess of 32% was employed to determine the cytological factors relating to the production of haploid embryos using light and transmission electron microscopy. Potentially polyembryonic flax megagametophytes contain at maturity either a conventional 3-celled egg apparatus (36% of megagametophytes), or contain supernumerary eggs forming either a 4- (50%) or 5-celled (14%) egg apparatus. In megagametophytes with supernumerary eggs, one egg cell appears to occupy the conventional position, whereas additional supernumerary eggs surmount the synergids and are smaller cells presumably derived from division of the true egg. Although supernumerary eggs appear similar in ultrastructure, they differ in location, size, and unlike typical egg cells, appear to have a complete, contiguous cell wall adjacent to the synergid. Such supernumerary cells are postulated as the origin of such haploid embryos, with the diploid member formed by conventional fertilization events with the micropylar egg.