Gametophytic competition as an ecological process

HARDER, L.D. Y M.A. AIZEN

Melbourne

Simposio; Simposio “Evolution and ecology of plant reproductive diversity”. XVIII International Botanical Congress.; 2011

International Association of Botanical and Mycological Societies

Seed production depends on the production of ovules, their fertilization, and subsequent embryo survival and development. Because of the serial dependence of these processes, limited performance during early stages necessarily diminishes options for later stages and poor performance during a late stage can restrict ultimate success, even though early stages functioned at capacity. Nevertheless, most ecological and evolutionary studies of plant reproduction focus on the early stage of plantpollinator interaction and/or the final stage of seed production. In contrast, less than 10% of studies of the influence of pollination of reproductive performance consider the intervening stages of pollen deposition on stigmas, pollen-tube growth and ovule fertilization, so that post-pollination (progamic) processes are typically either ignored or treated as a 'black box'. Although pollen germination and pollen-tube growth fundamentally involve biochemical and physiological processes, an ecological analogy may provide insights into their general consequences for siring success and seed production. Specifically, we consider progamic success as the outcome of competition governed by the availability of 'resources', including nutrients and/or space, and the density and activity of competitors. As independent haploid organisms, male gametophytes of angiosperms complete their lives in localized populations in the complex and dynamic environment provided by the pistils of conspecific sporophytes. Styles represent a competitive arena for such populations by providing resources that fuel pollen-tube growth. Therefore, if stylar resources are limiting, male gametophytes can compete in manners that depend on their characteristics and those of the resource supply (i.e., the pistil). 'Contest', or exploitative, competition could arise if pollen grains differ extensively in the timing of their arrival on a stigma and/or germination time, or in tube growth rate, whereas 'scramble', or interference, competition could occur when germination time differs little and pollen tubes grow at equal rates, so they compete as an advancing front. According to this ecological analogy, the nature and magnitude of resource competition in the style should influence the number and quality of the seeds set in individual fruits. Observed differences among species in the relation of the number of pollen-tubes entering ovaries or seed production to pollen receipt by stigmas support the contrasting expectations of contest and scramble competition. Furthermore, consideration of the implications of progamic resource competition reveals explanations for enigmatic aspects of plant reproduction and enriches understanding of the incidence and nature of sexual selection in plant populations.