No abstract

Interest in the possibility of sexual selection in plants has focused primarily on competition among pollen donors based on the speed of pollen-tube growth. However, when pollen arrives on stigmas, there is the opportunity for both races for access to ovules (exploitation competition) and interference with the germination and growth of pollen from other donors (interference competition). We considered whether this second form of competition might occur among pollen grains of wild radish in two experiments. In the first, interference likely occurred because the amount of pollen germination was less in mixed-donor than in single-donor pollinations. This result was duplicated in a second experiment, which also showed that interference occurred only when pollen grains from different donors were in direct contact with each other. In addition, in the second experiment, the opportunity for interference affected the frequency of seeds sired by different pollen donors. Because pollen loads are often mixed in nature, interference competition among pollen grains may be important in the ecology and evolution of plant reproduction.

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An ultrastructural study of soybean embryo sac development was performed. Prior to the final mitotic division and cellularization a nuclear rearrangement occurs that involves the chalazal movement of one of the two micropylar nuclei. During cellularization this nucleus divides to form the egg and micropylar polar nuclei and produces the wall that separates the central ceil from title space occupied by the egg apparatus. Within this space the other nucleus divides to form the two synergid nuclei and one of the two walls that separate the egg and synergid cells from one another. Egg apparatus cells are initially densely cytoplasmic, each is enclosed by thick, highly dissected walls, and they are all similar with respect to distribution of organelles except that synergid nuclei are micropylar to the egg nucleus. There is a progressive thinning and segmentation of egg apparatus walls during cellular expansion until they resemble the beaded chain structure seen in the mature egg and synergid cell walls. Taken as a whole these observations suggest that the chalazal movement of one of the two micropylar nuclei during the 4-nucleate stage is pivotal in determining future patterns of egg apparatus development.

Development of the soybean central cell and events associated with fertilization are described. Central cell development involves the simultaneous formation of numerous multigrain amyloplasts, development of wall ingrowths, reduction of the cell's major vacuole, and nuclear migration. As these processes reach completion, a phase of lytic activity begins. This activity ultimately results in both the breakdown of most multigrain amyloplasts in the central cell and the reestablishment of the cell's large vacuole. While these processes occur, the egg and synergid cells continue development. Examination of the egg apparatus after plasmogamy but prior to karyogamy reveals several changes. Whereas the synergid containing the pollen tube is degenerate and the distribution of organelles in the persistent synergid has changed, the egg cell with a free sperm nucleus exhibits higher levels of metabolite storage and changes in its wall structure. The walls of the newly formed sporophyte are similar to those of egg apparatus cells (Folsom and Cass, Canadian Journal ofBotany 68: 2135-2147) in that they seem to lack a middle lamella. This layer becomes apparent only in the two-celled proembryo. While these changes occur in the egg, zygote, and proembryo, synergids undergo different degradative pathways.