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Studies on pollen of selected species in Pinaceae Ho, RongHui

Abstract

The study of the exine pattern of pollen grains might permit the separation of various taxa and -assist the interpretation of their relationships. Thirty-seven specimens from twenty-six species in eight Pinaceae genera were studied by means of light, scanning electron and transmission electron microscopy. The fine pollen surface structure of twenty-two species (out of twenty-six) and the variations of pollen morphology in Tsuga heterophylla and T. mertensiana were demonstrated for the first time. The pollen grains of four Pseudotsuga species were found to be elliptical-shaped when turgid, and cup-shaped when dry. They varied in size, but all had the same scabrate sculpture. The proximal face of grains bore an annular thickening, and a triradiate streak which divided this face into three sectors. Under the transmission electron microscope, surface replicas revealed the exine areolae which consisted of intersecting ribs protruding from the pollen surface. Minute protrusions emanated from intersections of ribs. The diameter and the density of the minute protrusions varied from species to species. Pollen of the species examined here could be distinguished from size-frequency curves of mature grains, but not on the basis of fine structure. In Pseudotsuga menziesii, the diameter of the minute protrusions of pollen grains harvested prior to natural shedding was found to be much greater than that of mature grains. The reason might be that the early harvest of pollen cuts off additional supplies of the necessary precursors for the polymerization and the formation of the exine. The application of acetone during specimen preparation might cause expansion of the protrusions because of incomplete polymerization. Of eight Larix species examined, pollen-size distribution curves overlapped each other. Pollen grains of the eight species had the same morphology and exine sculpture as described for Pseudotsuga, with the exception that the mature pollen size was smaller and the triradiate streak was less well developed. Hence, species were not distinguishable by these criteria. The similarity of the exine sculpture in species of Pseudotsuga pollen may be due to similarities in chromosomal makeup after the chromosome reorganization. It is also possible that Pseudotsuga and Larix have evolved from common ancestors, with the parallel development of similar characters from their common ancestor. Pollen grains of the species in Abies, Cedrus, Keteleeria, Picea and Tsuga examined were found to be distinct from one another, based on pollen morphology and fine structure. The sculpture pattern of the exine was microverrucate, except in Keteleeria which was rugulate on the proximal face. On the other hand, the four species of Pinus pollen were found to be indistinguishable based on fine structure. The exine pattern on the proximal surface of pollen grains was microverrucate. Each granule of the microverrucate sculpture was also ornamented with microverrucoid sculpture. The similarity in fine structure of Pinus pollen might be attributed to the genetic similarity in the species of this genus. Pollen morphology of Tsuga mertensiana completely differed from that of T. heterophvlla. The difference may have been due to varying rates of evolutionary change, to the modification of reproductive structure for their continued existence, or to introgression. The author recommends splitting these two species into separate genera. The higher were the rank of taxa, the greater were the differences in pollen morphology. Based on morphology, pollen in Pinaceae was proposed to be grouped as follows: (1) Pseudot- suga-Larix: (2) Abies-Keteleeria-Tsuga (pollen of saccate type e.g., T. mertensiana); (3) Cedrus-Picea; (4-) Pinus; (5) Tsuga (pollen of biconvex-lens type e.g., T. heterophylla).

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