Deborah L. Marr

Seed provisioning in gynodioeciousSilene acaulis (Caryophyllaceae)

In gynodioecious species, which contain females and hermaphrodites, the outcrossed seeds of females have been found to outperform the outcrossed seeds of hermaphrodites, in spite of the fact that their seeds are not larger in mass. Females do not make pollen. Hence the nutrients that hermaphrodites allocate to pollen, such as nitrogen, might be allocated to seeds by the females, such that individual seeds from females are better provisioned than those from hermaphrodites. Alternatively, females might make more seeds, rather than better provisioned seeds. We tested the hypothesis that seeds from females would be better provisioned for the gynodioecious species Silene acaulis, by comparing seed mass, embryo/endosperm mass, nitrogen and phosphorus content, and energy content for outcrossed seeds from females and hermaphrodites produced in a natural population. We also measured the proportion of flowers that set fruit in both morphs. Seeds from the two sexual morphs were not found to differ significantly for any of the measures of seed provisioning, with seeds from females containing either nonsignificantly less or equivalent amounts of each of the measures as compared to hermaphrodites. However, females set a significantly higher proportion of their flowers to fruit, as compared to hermaphrodites. These results indicate that females do not provision individual seeds more than hermaphrodites in S. acaulis, and alternative hypotheses will need to be examined to explain the difference in the performance of the seeds from the two sexual morphs.

Pollen dispersal in Yucca filamentosa (Agavaceae): the paradox of self-pollination behavior by Tegeticula yuccasella (Prodoxidae)

We investigated pollen dispersal in an obligate pollination mutualism between Yucca filamentosa and Tegeticula yuccasella. Yucca moths are the only documented pollinator of yuccas, and moth larvae feed solely on developing yucca seeds. The quality of pollination by a female moth affects larval survival because flowers receiving small amounts of pollen or self-pollen have a high abscission probability, and larvae die in abscised flowers. We tested the prediction that yucca moths primarily perform outcross pollinations by using fluorescent dye to track pollen dispersal in five populations of Y. filamentosa. Dye transfers within plants were common in all populations (mean ± 1 SE, 55 ± 3.0%), indicating that moths frequently deposit self-pollen. Distance of dye transfers ranged from 0 to 50 m, and the mean number of flowering plants between the pollen donor and recipient was 5 (median = 0), suggesting that most pollen was transferred among near neighbors. A multilocus genetic estimate of outcrossing based on seedlings matured from open-pollinated fruits at one site was 94 ± 6% (mean ± 1 SD). We discuss why moths frequently deposit self-pollen to the detriment of their offspring and compare the yucca-yucca moth interaction with other obligate pollinator mutualisms in which neither pollinator nor plant benefit from self-pollination.

IMPACT OF A POLLINATOR-TRANSMITTED DISEASE ON REPRODUCTION IN HEALTHY SILENE ACAULIS

Pathogens affect the evolution of their hosts only to the extent that the hosts ability to reproduce is altered. Microbotryum violaceum is a pollinator-transmitted fungal pathogen that causes infected individuals to produce sterile flowers with anthers full of fungal spores. In North American Rocky Mountain populations of Silene acaulis, a host to M. violaceum, reproduction in infected individuals is severely affected, but disease spread is slow (<1% new infections over a 4-yr period). This study addresses whether diseased plants can affect reproduction in healthy individuals even if the healthy plant does not become diseased. I studied Silene acaulis in the field to examine traits of diseased and healthy plants, pollen deposition, spore deposition, and seed production in healthy plants. Diseased plants were significantly larger than healthy plants, and in contrast to other studies of anther smut with different hosts, infected individuals did not produce more flowers than healthy individuals. Timing of flowering was similar for healthy and diseased plants, but peak deposition of pollen and fungal spores differed at some sites. Therefore, plants could potentially maximize their fitness by flowering at times of peak deposition of pollen while avoiding exposure to peak deposition of fungal spores. Finally, healthy flowers receiving additions of pollen and moderate to high loads of fungal spores produced significantly less seed compared to flowers receiving only pollen. Although the risk of becoming diseased may be low in this population, healthy individuals can be affected by anther smut because the majority of flowers receive fungal spores, which may be interfering with pollen grains and reducing seed production.

Nodulation restrictive genotypes of Glycine and Amphicarpaea: a comparative analysis

Several genes that restrict nodulation with specific Bradyrhizobiumstrains are known in Glycine max (soybean), and a similar system of nodulation restriction has recently been discovered in the related North American legume Amphicarpaea bracteata. We analyzed how nodulation-restrictive genotypes of each plant interacted with Bradyrhizobium strains sampled from the other host species. Ten bacterial isolates from A. bracteata that nodulated differentially with genotypes of their homologous host legume showed uniform responses to two soybean isogenic lines that differed at the Rj4 locus controlling nodulation restriction: all isolates formed nodules of normal size and morphology on both isolines. However, little or no nitrogen fixation occurred in any of these symbioses. A. bracteata genotypes that displayed broad vs. restricted symbiotic phenotypes toward naturally-associated bradyrhizobia were also tested with two bacterial isolates from soybean (USDA 76 and USDA 123). Both isolates formed nodules and fixed nitrogen in association with both A. bracteata genotypes. However, symbiotic effectiveness (as measured by acetylene reduction assays) was normal only for the combination of USDA 76 with the restrictive A. bracteata genotype. Overall, these results indicate that plant genes that restrict nodulation by certain naturally-associated bradyrhizobia do not confer comparable specificity when plants interact with bacteria from another related legume species.

The role of fungal pathogens in flower size and seed mass variation in three species of Hydrophyllum (Hydrophyllaceae).

Identifying ecological factors that affect seed number and seed size is key to understanding the persistence of large seed mass variation in some plant species. Pathogens may increase seed mass variation by increasing resource demand over the growing season such that late fruits experience higher resource competition than early fruits. We tested whether Fusarium sp. and Rhizoctonia sp., soil fungi that cause wilt, contributed to seasonal decline in flower size, seed number, or seed mass in Hydrophyllum appendiculatum and H. canadense. A third species not infected by these soil fungi, H. virginianum, was studied to determine how seasonal decline in floral traits and seed mass variation varies within this genus. Flower size declined seasonally for all species, but was greatest for H. appendiculatum, a monocarpic biennial with indeterminate inflorescences. Seed number decreased between first and last inflorescences in H. appendiculatum, but not in H. canadense or H. virginianum, perennials with determinate inflorescences. Seed mass varied most in H. appendiculatum and H. canadense (4-20-fold in 50% of individuals) and least in H. virginianum (4-8-fold in >30% of individuals). Fungal infection increased seed mass variation among diseased plants in H. canadense and H. appendiculatum. However, within plants fungal infection only increased seasonal decline in flower size, seed number, and seed mass in H. appendiculatum when flowers received supplemental pollination.