Reproductive biology of Pacific yew (Taxus brevifolia)




Anderson, Erika Dee

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Taxus brevifolia Nutt., commonly known as Pacific or Western yew, is a conifer native to the Pacific Northwest of North America. Contrary to other Taxus species, T. brevifolia staminate strobili are usually located on two-year old foliage though they may occur on foliage from one to five years old. This delayed staminate strobilus development may be an adaptation to the low light environment where T. brevifolia grows. Microsporogenesis was found to occur in the fall preceding pollination. Isobilateral tetrads were visible as early as mid-October. Over-wintering staminate strobili usually contained separate microspores. In 1996 through 1999, pollination occurred in March and April in two natural forest sites on southern Vancouver Island, British Columbia. Low amounts of airborne pollen and a prolonged pollination period indicated low pollination success within T. brevifolia. Female receptivity was measured by the presence of a pollination drop and protandry up to 18 days was observed. In vitro pollen germination was moderate to good, ranging from 65% to 88%. DAPI fluorescence staining showed successful male gametophyte development in vitro. The phenology of megasporogenesis and free nuclear mitosis within the megagametophyte was variable and this development occurred anytime between early February until the end of June. One megaspore mother cell developed from the sporogenous tissue and underwent meiosis forming a linear tetrad of megaspores. Though up to three of the megaspores may be functional, the chalazal megaspore developed faster than the others and became the dominant megaspore. Cellularization of the megagametophyte began in mid-April and continues until early June. The presence of an ephemeral ventral canal nucleus was confirmed. Fertilization was observed in June in 1996. The mature egg cell cytoplasm and sperm structure was used to infer paternal inheritance of plastids and biparental inheritance of mitochondria. To examine this further, DNA was extracted from hybrid embryos of T. brevifolia and T. x media Rehd. Paternal contribution of mitochondria was confirmed using the probe rpS 14-cob. The T. x media parents produced two bands of 526 and 970 by in length, whereas the T. brevifolia parents produced only one band (526 bp). The chloroplast probes were not effective at amplifying Taxus DNA although appropriate sized bands were produced in Pinus contorta. Proembryos occurred from mid-May to mid-June. Sixteen nuclei were present before cellularization. Early embryos were present from mid-May to mid-August. Simple polyembryony was observed up to the massive embryo stage and differential growth of the embryonal cells was interpreted as incomplete cleavage polyembryony. Mid-embryos were present from mid-June to late August and had a distinct protoderm and focal zone. Late embryos were visible from mid-July onwards. Starch began accumulating at the early embryo stage, whereas, proteins and lipids accumulated in the late embryo stage. The presence of a red aril corresponded to increased amounts of lipid in the megagametophyte cells. Individual seeds matured from July until November. The seed efficiency ranged from 0% to 16% and averaged 5%. Pre-zygotic loss was the most common fate of ovules, followed by post-zygotic loss. Possible causes of this poor seed efficiency are poor pollination success, insect damage or light limitation.



Pacific yew, Reproduction, Vancouver Island