Cortaderia jubata (Gramineae): an autonomous apomict.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The literature relating to apomixis in members of the Gramineae is reviewed, particularly as it relates to the types of mechanisms found and to the occurrence of apomixis in the different systematic divisions of the family. Apomixis is more prevalent in the Subfamily Panicoideae than in the Pooideae, and the course of development of the unreduced embryo sac is distinctive in the two subfamilies. Throughout the grasses somatic apospory predominates over gonial apospory, and pseudogamy is much more prevalent than non-pseudogamy. In Cortaderia jubata the spore mother cell (or its derivatives) degenerate and nucellar cells develop as somatic embryo sacs. These follow a course of development similar to the Hieracium type. The synergid develops a prominent haustorium which usually protrudes beyond the micropyle. No functional pollen forms and embryogenesis begins well before anthesis. The species is, therefore, non-pseudogamous. Since no other non-pseudogamous grass also exhibits somatic apospory the situation in Cortaderia would appear to be unique among apomictic grasses. The sexual species of this genus are gynodioecious and exhibit a Polygonum type, 8-nucleate, 7-celled embryo sac, which the polar nuclei fuse at or near fertilization and the two synergids develop haustorial extensions through the micropyle during the organisation of the megagametophyte. The structure of the component cells of the somatic embryo sac is described at the ultrastructural level and compared with published accounts. The progress of nucellar degeneration is discussed, and its association with dilatations of the nuclear envelope is noted. A deeply lobed nucleus which is intimately associated with both the apomictic egg and the synergid is characteristic of the central cell. Wall ingrowths of transfer-cell type appear during the organization of the somatic embryo sac and occur all over the central cell, except adjacent to the antipodal cells. Sexual species of Cortaderia also show this feature prior to fertilization. The structure of the haustorium and synergid is described and their role in nutrient uptake and transport is discussed in the light of their morphology. Antipodal cells appear also to be adapted to a nutritional function. The formation walls between the coenocytic endosperm cells follows the pattern already described for this tissue, but wall formation between the cells of the megagametophyte follows a distinctive course. A preliminary account is given of the ultrastructure of the cells of the young globular embryo and the unusual presence of layers of dilated endoplasmic reticulum cisternae in the suspensor is recorded. Finally, notes are given on some miscellaneous topics revealed by the electron microscope.