No abstract

Because inbreeding, herbivory, and disease are common in plants and their separate impacts on fitness are well documented, investigators have begun to examine the consequences of inbreeding on plant-herbivore and plant-pathogen interactions. In this study, we examined the interrelationships among inbreeding, herbivory, and disease on reproductive output through both male (pollen production and pollen performance) and female (fruit and seed production and seed germination) functions. Inbred (selfed) and outbred plants from five families of a wild gourd (Cucurbita pepo ssp. texana) were grown in five 0.2-ha randomized blocks in an experimental garden. Half of the inbred and half of the outbred plants were sprayed weekly with a contact pesticide to control herbivores. The other half of the plants served as the controls (no-spray treatment). The spray treatment significantly reduced damage by cucumber beetles and aphid infestation levels, the two primary herbivores of the wild gourd. Moreover, because these herbivores are the sole vectors for the five most common viral diseases of cucurbits (Watermelon mosaic-2, Cucumber mosaic, Zucchini yellow mosaic, Papaya ringspot, and Squash mosaic), the spray treatment also significantly reduced the amount and severity of the symptoms of viral diseases. Our analyses reveal that (1) there is variation for resistance to herbivores and pathogens among the five families used in the study, indicating that there is broad-sense heritability for resistance; (2) inbred plants suffer greater herbivore damage and have more symptoms of viral infection than outbred plants, i.e., there is inbreeding depression for herbivore/pathogen resistance; (3) the impacts of insecticide applications and inbreeding on reproduction are mostly additive; (4) both male and female functions are significantly affected by the spray treatment and inbreeding; and (5) the adverse effects of both inbreeding and the increased exposure to herbivores/pathogens in the no-spray treatment are greater for female function than for male function, indicating that the impact of genetic and environmental stress can differentially affect the two sexual functions. Together, these findings have important implications for the evolution of selfing, the conservation of small populations, and the establishment and transmission of diseases within populations.

For species with bicellular pollen, the attrition of pollen tubes is often greatest where the style narrows at the transition between stigmatic tissue and the transmitting tissue of the style. In this region, the tubes switch from predominantly autotrophic to predominantly heterotrophic growth, the generative cell divides, the first callose plugs are produced, and, in species with RNase-type self-incompatibility (SI), incompatible tubes are arrested. We review the literature and present new findings concerning the genetic, environmental and stylar influences on the performance of pollen before and during the autotrophic-heterotrophic transition of pollen tube growth. We found that the ability of the paternal sporophyte to provision its pollen during development significantly influences pollen performance during the autotrophic growth phase. Consequently, under conditions of pollen competition, pollen selection during the autotrophic phase is acting on the phenotype of the paternal sporophyte. In a field experiment, using Cucurbita pepo, we found broad-sense heritable variation for herbivore-pathogen resistance, and that the most resistant families produced larger and better performing pollen when the paternal sporophytes were not protected by insecticides, indicating that selection during the autotrophic phase can act on traits that are not expressed by the microgametophyte. In a study of a weedy SI species, Solanum carolinense, we found that the ability of the styles to arrest self-pollen tubes at the autotrophic-heterotrophic transition changes with floral age and the presence of developing fruits. These findings have important implications for selection at the level of the microgametophyte and the evolution of mating systems of plants.

Each inflorescence on Lotus corniculatus commonly aborts about half of its immature fruits. Compared to random patterns of fruit abortion, natural patterns of fruit abortion produce mature fruits that contain significantly more seeds. Moreover, these progeny are more likely to germinate, are more vigorous as seedlings, and have greater reproductive output as adults. These results indicate that L. corniculatus selectively aborts those fruits with the fewest seeds and, by doing so, increases the average quality of its offspring.

The pollen competition hypothesis predicts that when the number of pollen grains deposited onto stigmas exceeds the number of ovules, selection can operate in the time frame between deposition and fertilization. Moreover, because of the overlap in gene expression between the two phases of the life cycle, selection on microgametophytes may alter the resulting sporophytic generation. The extent to which pollen competition occurs in nature has been unclear, because tests of the predictions of the pollen competition hypothesis have used cultivars and/or artificial growth conditions and hand‐pollination techniques. In this study we used a wild species, Cucurbita foetidissima, in its natural habitat (southern New Mexico) to determine the amount and timing of the arrival of pollen onto stigmas, the relationship between pollen deposition and seed number, and the effects of the intensity of pollen competition on progeny vigor. We found that ∼900 pollen grains are necessary for full seed set and that a single visit by a pollinator results in the deposition of 653.0 ± 101.8 pollen grains. About 29% of the flowers receiving a single pollinator visit had 900 or more pollen grains on its stigma. Moreover, within 2 h of anthesis, >4000 pollen grains were deposited onto a typical stigma, indicating that multiple pollinator visits must have occurred. Fruits produced by multiple visits had greater seed numbers (206 vs. 147) than fruits produced by a single visit. Finally, the progeny produced by multiple pollinator visits were more vigorous than those produced by single visits with respect to five measures of vegetative growth (MANCOVA, Wilks’ lambda = 0.96, F6,370 = 2.54, P < 0.02. These data demonstrate that conditions for pollen competition exist in nature and support the prediction that pollen competition enhances offspring vigor.