EFFECTS OF PARENTAL IDENTITIES AND ENVIRONMENT ON COMPONENTS OF CROSSING SUCCESS IN PHLOX DRUMMONDII

A controlled pollination study, involving a diallel crossing scheme among 15 Phlox drummondii individuals grown under either control, low water or low nutrient conditions, was performed. The results of this research demonstrate that, although plant size and flower number are responsive to the treatments, pollen germination percentage, the survival probability of developing seeds and the weight of mature seeds are buffered against water and nutrient deprivation. However, the identities of male and female parents did have significant effects on pollen germination and seed survival, and female identity had a significant effect on seed weight. UNTIL RECENTLY, interest in the components of crossing success, i.e., pollen germination, ovule fertilization and seed survival to maturity, has been confined to studies of agricultural plants. Recent interest in sexual selection in plants has caused a shift in the focus of such research to natural populations. In both domesticated and naturally occurring plants, there is evidence that crossing success may be influenced by parental identity (Schaaf and Hill, 1979; Bertin, 1982, 1985; Bookman, 1984; Marshall and Ellstrand, 1986). Crossing success may also be influenced by the environment in which a plant grows (Stephenson, 1981; Lee and Bazzaz, 1986; Schlichting, 1986; SmithHuerta and Vasek, 1987). Seed provisioning as measured by seed weight may vary between pollen donors (Marshall and Ellstrand, 1986) and between pollen recipients (Marshall and Ellstrand, 1986; Mazer, 1987), and may also be subject to environmental control (Parrish and Bazzaz, 1985; Marshall and Ellstrand, 1986; Smith-Huerta and Vasek, 1987). This study was undertaken to determine the extent to which water or nutrient deprivation affects the success of crosses in Phlox drummondii through reductions in pollen germination, seed survival to maturity and seed provisioning. It was also undertaken to determine the degree to which these components of crossing success and seed weight are influenced by Received for publication 1 June 1987; revision accepted 9 September 1988. This work was supported by National Science Foundation grant DEB 8119484. We would like to thank Norma Fowler for comments on the manuscript, and Peter W. John, Hwa Chi Liang, and Pat Firey for statistical advice. This research benefited from discussion with Carl Schlichting and Jerry Coleman. 2 Current address: Microelectronics and Computer Technology Corp., 3500 W. Balcones Center Dr., Austin, TX 78759. the identity of maternal and paternal plants, and whether the male and female crossing success components of individuals are related to each other or to plant size. MATERIALS AND METHODS-Phlox drummondii Hook. is an outbreeding, hermaphroditic, spring-flowering, lepidopteran-pollinated winter annual, common along roadsides and in sandy fields in Central Texas. Seeds germinate in the fall, and plants overwinter as rosettes. The self-incompatibility system of P. drummondii shows gametophytic control (Levin, unpublished data), but is unusual in that inhibition of incompatible pollen occurs on the stigmatic surface. Incompatible grains either fail to germinate or produce tubes which do not penetrate the stigma. Fruits are threeseeded capsules which dehisce explosively when mature. Seeds are oblong, hard, brown or black, and wedge-shaped in cross section. Bulk seed harvested from a P. drummondii population 2 miles south of Columbus, Texas, were grown in a greenhouse in late fall of 1983 in sand-filled 6-inch clay pots. Each of the following treatments was applied to one-third of the plants: 1) Control plants were watered daily and fertilized every two weeks with 20:20:20 N:P:K solution. 2) Low water plants were fertilized as above, but were watered only upon wilting. 3) Low nutrient plants were watered daily, but were only fertilized once, soon after being transplanted into pots as seedlings. These environmental treatments are referred to later in the text and in the tables as "female treatment" and "male treatment" when discussing their effe6t on female and male reproductive traits. Fifteen plants, 5 per treatment, were randomly chosen for use in a complete diallel cross. Crosses were begun by January 1984, when all