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Investigation into the effects of self-pollination on pseudotsuga menziesee (mirb.) franco Orr-Ewing, Alan Lindsay

Abstract

Extensive investigations have shown that the genetic systems which prevent self-fertilization in many of the Angiosperms are based upon the incompatibility of pollen and style. Past studies have also shown that many of the Gymnosperms, including Pseudotsuga, produce little or no viable seed after self-pollination. The causes of this seed failure, however, had not been determined. It was considered that a cytological study of development within the ovules after the self-pollination of Pseudotsuga menziesii should show whether seed failure was caused by comparable incompatible systems. Controlled self- and cross-pollinations were accordingly made on two trees at Vancouver in 1952 and on three trees each, at Victoria and Lake Cowichan in 1954. Cone samples from both pollinations were removed at various dates from each tree and the ovules embedded for later sectioning. The remaining cones were collected at maturity and the seed extracted. It was found that the yields of viable seed varied greatly in individual trees and the investigation was accordingly divided into two parts. The cytological study was confined to two of the trees which had produced very little viable seed after selfing. The second part of the investigation was concerned with the effect of self-pollination on the progeny. The cytological study showed that neither germination of the pollen nor its subsequent development until the time of syngamy were in any way inhibited and both compared favourably with development in the ovules from cross-pollinated cones. Proembryo formation and early embryonic development were normal but the embryos in both trees collapsed approximately ninety days after self-pollination. This collapse appeared to be caused by some failure in the vital relationship between the young embryos and their surrounding gametophytes. The latter appeared healthy and comparable in every respect with those in which embryos from cross-pollination were growing vigorously. It is considered that embryo collapse after self-pollination is probably an inbreeding effect caused by the action of lethal and semi-lethal genes when brought together in a homozygous state. This explanation could account for the very variable effects of self-pollination on the trees studied. Some viable seed was also obtained from cones isolated and not pollinated. The seedlings are diploid and the possible causes of agamospermy are discussed. A study of the inbred progeny shows that self-pollination usually results in seedlings which are smaller and less vigorous than those from controlled cross-pollination. The seedlings from wind-pollination were intermediate and the small size of some suggests that self-pollination may have occurred. The practical aspects of self-pollination in relation to both the natural and artificial regeneration of forest land are briefly discussed.

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