Microsporogenesis, pollination, pollen germination and male gametophyte development in Taxus brevifolia

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 2-year-old foliage although they may occur on foliage from 1 to 5-years-old. This delayed staminate strobilus development may be an adaptation to the low light environment where T. brevifolia grows. Microsporogenesis occurred in the autumn preceding pollination. Successive divisions produced isobilateral tetrads visible as early as mid-October. Overwintering staminate strobili usually contained separate microspores. In 1996 to 1999, pollination occurred in March and April in two natural forest sites on southern Vancouver Island, British Columbia, Canada. The low amounts of airborne pollen and prolonged pollination period indicated low pollination success within T. brevifolia. Female receptivity was measured by the presence of a pollination drop. Protandry up to 18 d was observed. In vitro pollen germination was moderate to good, ranging from 65 to 88% depending on the tree and year. DAPI fluorescence staining showed successful male gametophyte development in vitro. The microspore divided forming a tube nucleus and generative cell within 3 d of culture. The generative cell then divided forming a sterile nucleus and spermatogenous nucleus after 17 d. The spermatogenous nucleus acquired a cell wall then divided forming two equal sperm after 24 d. (C) 2000 Annals of Botany Company.