Embryo-sac formation in diploid and polyploid species of roseæ

Abstract

Embryo-sac for mation was studied in 193 vars. of 104 Linnean spp. of Rosa, Platyrhodon, and Hesperhodos. I. Diploid and regular polyploid spp. of the tribe.[long dash]The meiotic di visions are normal. The 1st division is reductional, an equal number of chromosomes passing to each pole; the 2nd is equational, resulting in regular axial quartettes of which the micropylar cell usually develops into the normal 8-celled embryo-sac. Triple fusion takes place and after fertilization normal endosperm is formed which, with embryo development, is reduced to a single layer. Throughout the tribe several embryo-sacs are produced in each ovule, one of which ultimately sur vives in the regular spp. No evidence of apomixis was found in the diploid or regular polyploid spp. of Rosa.[long dash]II. Irregular polyploid spp. of Rosa.[long dash]Here an entirely different mechanism prevails. The reduction divisions of the embryo-sac mother cells are regular but unequal, giving rise to a mechanism of [female] gametogenesis so far unparalleled in plants or animals, and differing from that of [male] gametogenesis in the same sp. as widely as both differ from the normal in the regular spp. In the 1st meiotic division, the 7 bivalent chromosomes alone reduce while the whole of the univalent chromosomes collect at the micropylar pole without splitting, thus giving rise to an unequal reduction division of chromosomes of 21 and 7 in the tetraploid, 28 and 7 in the pentaploid, and 35 and 7 in the hexaploid spp. The 2d meiotic division is equational and regular, resulting in unequal quartets in an axial row of 2 large micropylar and 2 small chalazal cells. The upper micropylar cell develops into an 8-celled embryo-sac, the egg-cells of which contain 3, 4 or 5 times as many chromosomes as the [male] gamete. The [female] heritage of these irregular polyploid spp. is consequently that of a regular polyploid sp. higher than itself, i.e., an irregular tetraploid produces the [female] gametes of a regular hexaploid, a pentaploid those of an octoploid, and a hexaploid those of a decaploid sp., while the [male] heritage of all these irregular polyploid spp. is that of a simple diploid sp. Experimental genetical evidence shows that these irregular polyploid spp have an alternative method of reproduction by apomictical seeds. When "selfed" or agamised there is no varietal segregation, the progeny being identical with the mother parent. Self-fertility has not been demonstrated, but the spp. are facultatively sexual and hybridize freely when cross-fertilized with other varieties or other spp. Cyto-logical evidence of this apomixis is scanty, although occasional divisions are found in the embryo-sac mother cells in which the reduction division is suppressed and these cells contain the full somatic number of chromosomes. So far, however, the few good counts available in embryo-sacs show only the reduced number of chromosomes. The constant presence of 2 or more mature embryo-sacs in each ovule, peculiar to these spp., may be significant, since one may be sexual and the other apomictical, the latter functioning in the absence of fertilization. Though large amounts of material were examined, no traces of the formation of embryo-sacs from nucellar or integumental tissue were found in Rosa.