Effect of crossing distance and male parent on in vivo pollen tube growth in Chamaecrista fasciculata

Gametophytic competition among pollen grains has been proposed as an important mechanism of sexual selection in plants. The purpose of this paper is to examine the contribution of pollen source on in vivo pollen tube growth in Chamaecristafasciculata. We addressed two questions: 1) Is pollen tube growth affected by the genetic relatedness between the pollen source and the pollen recipient? 2) Is there significant phenotypic variation among pollen donors for pollen tube growth? We compared pollen tube growth by measuring the number of pollen tubes which germinated, reached quarter length of style, and reached the ovary resulting from selfand outcross-pollinations. The outcross pollinations included three interplant distance classes: near (within genetic neighborhood, ca. 1 m), far (between far neighborhoods and within subpopulation, ca. 20 m), and distant (between neighborhoods and adjacent subpopulations, ca. 50-100 m). Our results show that pollen tube growth was not affected by genetic relatedness, by differences between self and outcross, nor by differences due to phenotypic variation among pollen donors. In contrast, maternal environment had a strong impact on pollen tube growth. These results suggest a lack of gametophytic competition and indicate little opportunity for sexual selection on pollen tube growth in C. fasciculata. GAMETOPHYTIC COMPETITION among pollen grains has been proposed as an important mechanism of sexual selection in plants (Stephenson and Bertin, 1983). Three necessary conditions for sexual selection to operate on pollen tube growth are 1) phenotypic variation among male pollen donors for the selected trait (Crow, 1958; Arnold and Wade, 1984), 2) underlying genetic basis for variation of the trait (Haldane, 1954; Lande and Arnold, 1983), and 3) an absence of pollen limitation (Stephenson and Bertin, 1983). In vitro (Schemske and Fenster, 1983; Bookman, 1984; Mazer, 1987) 'Received for publication 15 December 1987; revision accepted 31 March 1988. CF was supported by funds from a NIH Genetics Training Grant, Sigma XI, the University of Chicago Hinds Fund, and NSF Grant BSR-8501229. VS was supported by NSF Grant RII-85035 12 and the University of Missouri Weldon Springs Fund. We thank the Illinois Dept. of Conservation and the Illinois Nature Preserve Commission, J. Nyhoffand S. Villalobos of Gooselake Prairie State Park, and Sue Yamins, Betty McCarthy, and Wilbur Schroeder from the University of Chicago greenhouse. We also thank D. Schemske for advice and encouragement, P. Feinsinger for pollen tube staining techniques, and S. Weller for use of an epifluorescent microscope. The following individuals commented on earlier versions of this manuscript: D. Schemske, M. DeMauro, M. Dudash, D. Buckley, L. Mule, L. Dom, and N. Pavlovic. 2 Current address: Department of Botany, University of Toronto, Toronto, Ontario M5S lAl, Canada. 3Current and reprint request address: Department of Biology, University of Missouri-St. Louis, St. Louis, MO 63121. and in vivo studies (Hill and Lord, 1986; Marshall and Ellstrand, 1986) have found significant differences among donors in pollen tube growth. However, the significance of in vitro studies is limited because pollen performance is not measured in the maternal environment where all ecologically significant interactions for pollen tube growth occur (Charlesworth, Schemske, and Sork, 1987). An additional source of variation in pollen tube growth among males is the effect of the interaction between the paternal and maternal genotypes. The genetic relatedness among parents may influence pollen performance. Some studies have shown that outcross pollen are more likely to fertilize than self-pollen (e.g., Bateman, 1956; Smith and Clarkson, 1956; Bowman, 1987). When comparing pollen tube growth among males, it is important to control for the genetic relatedness among parents. Because gene flow often is limited in plants (Ehrlich and Raven, 1969; Levin, 1981), populations may be subdivided so that interparent distance reflects the genetic similarity between parents (Wright, 1969; Schaal, 1974; Levin, 1984; Epperson and Clegg, 1986). Therefore, variation among pollen donors for pollen tube growth may not be due to the individual male genotype nor reflect opportunities for sexual selection, but may simply reflect male/female interactions. In this paper we examine the contribution