MATERNAL INVESTMENT AND FRUIT ABORTION IN PHASEOLUS VULGARIS

Plants lose some of their parental investment through fruit abortion. Resource conservation theory predicts that the maternal plant should abort those fruits furthest from maturity in order to conserve scarce resources. Embryo-quality theory predicts that the maternal plant should favor offspring of particular genotypes. I recorded the patterns of maternal investment in floral cohorts of domesticated and wild beans, Phaseolus vulgaris L. In accord with the resource conservation theory, late pod cohorts have the highest fruit abortion rates. When domesticated beans are grown in smaller pots, late cohort survivorship declines. When domesticated beans are crossed with pollen donors of different degrees of relatedness, the fruit abortion rate is the same even though seeds from the outbred crosses weigh more. The challenge for ecologists is to model and test the combined and possibly conflicting effects of selection for resource conservation and high offspring quality. IN RECENT YEARS the levels of maternal investment in reproductive organs and their mortality have gained increased attention from plant ecologists (Lloyd, 1980; Stephenson, 1981; Lovett Doust and Lovett Doust, 1983; Stephenson and Bertin, 1983; Willson and Burley, 1 983). The general questions ecologists seek to answer are, first, why do so many plants initiate many more flowers than ever develop into mature fruits, and, second, which fruits and seeds should a plant abort to maximize its fitness. This paper addresses the second question. Given that many plants produce an excess of flowers, it is reasonable to ask which flowers and fruits should a plant abort when they cannot all be brought to maturity. Current theories provide two types of adaptive answers. Plants should abort fruits in a pattern that conserves maternal resources or one that discriminates in embryo genetic quality. The null hypothesis would be a random pattern of abortion. A maternal plant conserves resources if it invests in those reproductive organs that require the least amount of future parental in' Received for publication 19 April 1985; revision accepted 19 February 1986. I thank J. Rodman, S. Handel, R. Harrison, N. Knowlton, J. Powell, I. Sussex and B. Tiffney for their guidance. M. Stanton, T. Lee, S. Mazer and A. Snow improved earlier drafts of this paper. M. Boylan, A. Litt, S. Rachootin and L. Roth aided in the data collection. L. Sablitz and crew cared for the greenhouse plants. P. Rundel provided facilities for writing. This paper is drawn from a doctoral dissertation submitted to Yale University. Financial support came from Yale fellowships and NIH Genetics Training Grant GM07499-04. 2 Present address: Botany Dept., Univ. of California, Davis, CA 95616. vestment to complete development (Adams, 1967; Dawkins and Carlisle, 1976; Lloyd, 1980; Stephenson, 1981). An organism should end expenditures for inviable organs, be they unvisited and unpollinated flowers or insectdamaged fruits. When fruits contain viable offspring but resources are limited, parental investment should go first to the earliest fruits since they are usually the ones closest to maturity. Brood reduction in birds provides an animal analogy (Howe, 1976), as the oldest nestlings in asychronous broods have the highest survival rates. Among fruits at the same stage of development, theory predicts that fruits with the most seeds have an advantage. The pattern of fruit abortion should maximize the number of mature seeds at optimal sizes for each unit of future investment. The embryo-quality metric for parental choice would favor fruits containing offspring ofthe highest quality genotypes (Janzen, 1977), not simply the earliest fruits. Several different measuring sticks have been used to assess embryo quality. One measure of embryo quality is the degree of genetic similarity to the parent (Stephenson and Bertin, 1983; Willson and Burley, 1983). The progeny of outbred plants often lack homozygous recessive genes for lethal or deleterious traits. The heterotic effects of outbreeding give a vigor and flexibility to genets. Members of an outbred progeny also are more diverse and more likely to find suitable microsites for growth and reproduction (Williams, 1975; Maynard Smith, 1978). On the other hand a physiological mismatch of genetically dissimilar genes or the creation of genotypes maladapted to the maternal plant's environment may cause outbreeding depres-