Megastrombilus development in Juniperus ashei (Cupressaceae)




Wakefield, Andrea

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Juniperus ashei Buchh. (Cupressaceae) is a common gymnospermous tree in central Texas. Its range extends from the mountains of northwestern Coahuila northeast through Texas, eastern Oklahoma, northwestern Arkansas into southern Missouri. Despite the widespread abundance of the species and its importance as a major allergen source, very little is known of its life cycle. This study provides a baseline examination of the life cycle. Megastrobilus initiation occurs in mid October in central Texas. The microscopic megastrobili develop through late October, November, early December, finally becoming visible just before pollination in late December through early February. Megasporogenesis is concurrent with pollination. Immediately after pollination the micropyle closes and the cone scales fuse to form the berry like cone. As the pollen germinates and pollen tube growth begins, the functional megaspore undergoes mitosis to form a free-nuclear megagametophyte. In early to mid April when the free-nuclear megagametophyte has reached its maximum size, of about 2000 free nuclei, it begins to cellularize. Cellularization is rapid and upon completion an archegonial complex of five to eight archegonia develops at the micropylar end of the megagametophyte. As the archegonial complex matures the pollen tubes reach the neck cells of the archegonia. Fertilization occurs from late April to mid May. One to all the archegonia in a complex may be fertilized generating many potential embryos. The fusion nucleus moves to the bottom of the archegonium where it undergoes three free-nuclear divisions followed by cellularization eventually producing a 12 celled three-tiered proembryo. Initially the suspensor cells and the embryonic suspensor cells of each proembryo do not carry embryo cells at their tips. Embryo cells eventually form as a few or just one line of cells out compete the others and reaches deepest into the gametophyte. The leading embryo rapidly divides giving rise to a secondary suspensor which pushes back any remaining competitors. Embryo competition is completed by late July. The new sporophyte grows and differentiates reaching maturity by late October to early November. The mature megastrobili are shed in November, December, and January. Juniperus ashei seeds are primarily bird distributed.



Ashe juniper, Junipers, Gymnosperms


Wakefield, A. (2004). Megastrombilus development in Juniperus ashei (Cupressaceae) (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.


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