Research - The Eppley Lab

Spatial segregation of the sexes in Distichlis spicata: Understanding the evolution of ecological niches is a focal area of evolutionary ecology. This project examines the genetic, physiological, and ecological factors that allow male and female plants in the wetland grass species Distichlis spicata to have evolved and maintain different ecological niches. Despite the fact that sex ratio theory suggests that such a spatial structure should be highly disadvantageous in a sessile organism because it reduces mating success, patterns of intraspecific niche dimorphism, also known as spatial segregation of the sexes, are quite common in dioecious plants.

We have been working to understand the proximate and ultimate mechanisms responsible for spatial segregation of the sexes in three D. spicata populations that lie along the coast in north-central California: 1) the Limantour estuary in Pt. Reyes National Seashore, 2) Audubon Canyon Ranch’s Walker Creek estuary at Tomales Bay, and 3) an estuary at the UC Davis Bodega Marine Laboratory. We have identified a molecular marker linked to the female phenotype (eliminating the possibility that sex is environmentally determined) and used it to show that D. spicata patches exhibit significantly biased sex ratios for both ramets and genets, regardless of flowering status. We have also shown that male and female juveniles react differently to the environment in germination, survival, and competitive effects, and that these differences vary across environments.

In collaboration with Dr. Travis Glenn and Dr. Olga Tsyusko at the University of Georgia, Savannah River Ecology Laboratory, we have developed microsatellites markers for D. spicata in order to analyze pollen flow within populations to measure mating success. We are grateful to Dr. John Kelly at Audubon Canyon Ranch, Marshall, CA, Dr. Jim Morris, University of South Carolina, Columbia, SC, and Dr. Laura Murray at the University of Maryland’s Horn Point Laboratory, MD, for sending us D. spicata tissues samples for this project to increase the geographic utility of the microsatellites.

Currently, we are using greenhouse experiments and plan to set up long-term reciprocal transplant experiments in the field to test evolutionary hypotheses and physiological mechanisms that have been proposed to explain spatial segregation of the sexes in D. spicata. These data will allow the first comprehensive test of evolutionary and physiological mechanisms for intraspecific niche dimorphism in any species. This research will make important gains in our knowledge of sexual dimorphism, niche partitioning, and sex ratio evolution.

Stress and the evolution of sex: Producing offspring by sexual reproduction is potentially genetically and physically risky, yet most Eukaryotes reproduce primarily via sexual rather than asexual reproduction. Many studies have found positive correlations between environmental stress and sexual reproduction, suggesting that under stressful conditions sex has an adaptive function. However, the correlation between sex and stress has not been found to extend to extreme environments, although there has been only one published study of such a test (Kis-Papo et al. 2003. PNAS 100: 14970-14975). This result has striking implications for the evolution of sex, which may have evolved in organisms in extreme environments, and the maintenance of sex in organisms that inhabit extreme environments today. It has been suggested that when environmental stress is extremely high only a few genotypes are adaptive; if this is the case, then recombination would not be an adaptive strategy.

To test whether the positive correlation between genetic variation, sexual reproduction, and environmental stress occurs in the majority of environments except extreme environments, we are using bryophytes around geothermally heated hot springs as a model system. Preliminary data for this project were collected around hot springs on New Zealand’s North Island, and we are currently developing this project at Lassen Volcanic National Park, California. We plan to use a series of greenhouse experiments to determine the effects of environment and genetics in sexual versus asexual reproduction in extreme environments and to develop microsatellite markers for a handfull of bryophyte species to estimate the relationship between sexual reproduction and environmental stress.

In a side project, we are growing several bryophyte species in the greenhouse that were collected from around fumeroles at Boiling Spring Lake and Devil's Kitchen at Lassen Volcanic National Park to determine the heat tolerance of individual species. Moss plants were found green and growing (see photo below) at surface temperatures above 58ºC, and soil temperatures at 10 cm of 77ºC.

PSU undergraduate, Camille Graves, collecting data on moss communities at Lassen Volcanic National Park

Moss community around a fumarole, Boiling Spring Lake, Lassen

Sex ratio variation in Polytrichum juniperinum : We are using molecular markers and ecological correlates to understand the extreme female-biased sex ratio found in P. juniperinum. Most moss species (>55 %) have separate sexes, and female-biased sex ratios predominate in moss species with this type of breeding system. However, little information is available on whether sex-specific differences in clonal growth or mortality account for the sex bias and whether these differences correlate with sex-specific differences in allocation to reproduction.

Sex-ratio bias and environmental stress in mosses is particularly interesting in comparison to what we know about such interactions in angiosperms. In angiosperms with separate sexes, allocation to reproduction is generally higher in females than males, and females often occur in less stressful areas than males. In mosses, however, when conditions become stressful, male plants become less frequent and often disappear from populations altogether. Whether this indicates a differences in sex-specific stress-tolerance patterns in bryophytes versus angiosperms is unknown. For P. juniperinum, a cosmopolitan moss species with separate sexes, this project aims to determine the relative costs of reproduction in males and females, the relative clone sizes of males and females, population sex ratios, and ecological correlates (if any) with sex ratio variation. These data will allow us to assess the proximate mechanisms responsible for the extreme female-biased sex ratios found in this species.

Polytrichum juniperinum females with sporophytes (Photo: Phil Garnock-Jones)

Polytrichum juniperinum males with splash cups (Photo: Phil Garnock-Jones)