A Test Of Several Hypotheses For The Determination Of Seed Number In Amelanchier Arborea, Using Simulated Probability Distributions To Evaluate Data

The number of seeds per fruit is variable within Amelanchier arborea trees. Because A. arborea flowers are five-carpellate and each carpel contains two ovules, we were able to use the pattern of seed maturation within fruits to test whether the failure of some ovules to develop into seeds is determined by mechanisms operating at the level of carpels, such as stigma-clogging, or by mechanisms operating at the level of ovules, such as ovule infertility. The presence of one-, two-, and zero-seeded carpels demonstrated that the number of ovules developing into seeds was not due entirely to carpel-level phenomena. In order to test the hypothesis of carpel independent seed development, without the assumption that all ovules have the same probability of developing into seeds, it was necessary to use simulation, since no conventional statistical models were appropriate. Analysis of this simulation allowed us to reject carpel independent processes as the only determinant of seed number. A mixed model of seed development, in which some carpels fail entirely and ovules in the remaining carpels develop equiprobably, was shown to be consistent with the data. MANY, IF NOT MOST, plants have a variable number of seeds per fruit. For these plants, seed number is one component of yield or fitness that the plant can adjust in response to environmental conditions (Harper, 1977). In some species seed number has a strong effect on fruit size (Wareing and Phillips, 1978, 123), pulp: seed ratio (Herrera, 1981), mass: volume ratio (Augspurger and Hogan, 1983), or fruit developmental time (Lee and Bazzaz, 1982), all of which can have consequences for seed dispersal. Pollination, resources, and seed predation have all been proposed as mechanisms for the determination of seed number per fruit, as well as other components of plant reproductive output (for reviews, see Bierzychudek, 1981; Stephenson, 1981; Rathcke, 1983; Bawa and Webb, 1984). Most studies to date have used pollen or resource augmentation to test whether these factors limit seed number. A recent review (Snow, 1986) showed that seed number is related to pollination intensity in only a minority of species investigated to date, and these are mostly species with a very large number of ovules per flower. Here we show that analysis of the distribution of seeds among the carpels I Received for publication 10 December 1986; revision accepted 26 March 1987. We thank two anonymous reviewers for helpful comments. Research support to DLG from the University of Michigan Museum of Zoology and Rackham School of Graduate Studies is gratefully acknowledged. of a fruit can be an additional tool for distinguishing among competing hypotheses. MATERIALS AND METHODS-A melanchier arborea (Michx. f.) Fern. (Downy Serviceberry) is a small tree of eastern North America that bears fleshy fruits containing one to ten filled seeds. Unlike some congeners, it is self-incompatible and not apomictic (Robinson, 1982; Gorchov, 1985). As in other members of the Rosaceous subfamily Maloideae, the gynoecium consists of five carpels each made up of a single stigma, a style, and two ovules (see Campbell et al., 1985, and Olson and Steeves, 1982, for detailed descriptions of other Amelanchier species). Thus, each flower should have the potential to produce fruits with ten seeds. Fruits usually do contain ten "seeds," but the number of these that are filled with an embryo ranges from one to ten and is always variable among fruits within a plant. Some of the remaining seeds are of nearly full length but contain no embryo ("empty seeds"), but most are very small. The latter ("undeveloped seeds") are not necessarily derived from fertilized ovules: they can develop from ovules in carpels that had their styles pinched off before pollination (Gorchov, 1987). Filled seed number in A. arborea is negatively correlated with the number of days from fruit initiation to ripening, resulting in a within-plant ripening phenology that is much more asynchronous than the flowering phenology