SSS in Distichlis spicata

Background: Distichlis spicata (Poaceae) is a dioecious, wind-pollinated perennial grass that is common at high densities in estuaries on the east and west coasts of North America as well as in inland, saline habitats (Beetle 1943; Hitchcock 1971). This species exhibits spatial segregation of the sexes (SSS) within a population (Fig. 1; Bertness et al. 1987; Eppley et al. 1998; Fig. 1; Freeman et al. 1976); habitats in the marsh that are slightly higher in elevation and have lower nutrient concentrations have adult sex ratios near 100% male, and lower habitats with higher nutrient concentrations have adult sex ratios near 100% female (Eppley 2000; Eppley 2001). Levels of P, soluble K, Zn, Fe, Cu, exchangeable Ca, exchangeable Mg, exchangeable Na, and organic matter are significantly greater, and percent sand is significantly less, in low-elevation, majority-female sites within the marsh (n = 20; Eppley 2000). However, in asexually propagating species such as D. spicata and many species that exhibit SSS (Beetle 1943; Bierzychudek and Eckhart 1988; Hitchcock 1971), SSS may be indistinguishable from superficially similar patterns generated by (1) gender-specific differences in rates of clonal spread (Iglesias and Bell 1989) and (2) differential flowering. Working in three D. spicata populations that lie along the coast in north-central California, we identified a RAPD marker linked to the female phenotype (eliminating the possibility that sex is environmentally determined) and used it to show that patches exhibit significantly biased sex ratios for both ramets and genets, regardless of flowering status (Eppley et al. 1998). Eppley et al. (1998) also showed that in D. spicata, as in other clonal marine and estuarine plant species (e.g., Reusch et al. 2000; Richards et al. 2004), genetic diversity within populations is high, suggesting that sexual reproduction occurs frequently and that the spatial arrangements of the sexes has important consequences for the evolutionary trajectory of D. spicata populations.




Sexual Specialization: Using a RAPD marker to sex individuals, we found differences in seed germination, seedling survival, and seedling competitive effect between D. spicata males and females, and these differences were influenced by small variations in environmental conditions within populations (Eppley 2000; Eppley 2001; Eppley 2006). Differences in seed germination between the sexes occur in greenhouse treatments designed to mimic female-majority habitats; more male seeds germinated than did female. In majority-female habitats in the field, in which adult D. spicata plants were present, significantly more female seedlings survived than did male seedlings (25.17% of females versus 13.49% of males after the first high tide exceeding 162 cm). These results suggest that in majority-female habitats, sex-specific bias in seedling survivorship, rather than seed germination, is a causal factor in the underlying sex ratio bias. In majority-male habitats, sex-specific germination and seedling survival did not occur.
We evaluated the intraspecific competitive effects of male and female D. spicata seedlings in three soil types in the greenhouse (Eppley 2006). In soil from microsites where the majority of D. spicata plants were female, seedlings, regardless of sex, were six times larger when grown with male versus female conspecific seedlings; this sexual dimorphism of competitive effect was weaker or did not occur in other soil types, including sterile, nutrient-rich compost (Fig. 2). These results suggest that it is not just the higher costs of female versus male reproduction that cause SSS in D. spicata, but that inter-sexual differences in competitive abilities, which occur as early as the seedling stage, can generate sex ratio variation. Also, while there are many potential differences between soil from majority-female microsites and sterile greenhouse soil, a difference between biotic soil factors (bacterial or fungal) is one potential hypothesis for the variation in levels of inter-sexual competition documented in this experiment.


Effects of SSS on Mating Success: In 3 populations of D. spicata (Pt. Reyes, Tomales Bay, and, Bodega Bay), we examined patterns of pollen movement and effects of pollen load and nutrient availability on seed set to determine whether SSS reduces mating success (Eppley 2005). In two of the three populations, pollen dispersal was restricted, and pollen augmentation significantly increased seed set by 50-100%. However, in the third population, which had the lowest seed set, we found that although there were some indications of pollen limitation, pollen dispersal was not restricted, and seed production was limited primarily by nutrient availability. These results imply that, in some populations of D. spicata, nutrient-limited production of seeds by females may be sufficiently strong that SSS incurs little fitness cost. In other populations, pollen does limit mating success, and SSS in these populations reduces the fecundity of both sexes (Eppley 2005).

References:

Beetle A (1943) The North American variations of Distichlis spicata. Bulletin of the Torrey Botanical Club 70:638

Bertness MD, Wise C, Ellison AM (1987) Consumer pressure and seed set in a salt marsh perennial plant community. Oecologia 71:190-200

Bierzychudek P, Eckhart V (1988) Spatial segregation of the sexes in dioecious plants. American Naturalist 132:34-43

Eppley SM (2000) Intrapopulation sex ratio variation and sexual specialization in the dioecious grass Distichlis spicata. In: Department of Evolution and Ecology. University of California, Davis

Eppley SM (2001) Gender-specific selection during early life-history stages in the dioecious grass Distichlis spicata. Ecology 82:2022-2031

Eppley SM (2005) Spatial segregation of the sexes and nutrients affect reproductive success in a dioecious, wind-pollinated grass. Plant Ecology 181:179-190

Eppley SM (2006) Females make tough neighbors: sex-specific competitive effects in seedlings of a dioecious grass. Oecologia 146:549-554

Eppley SM, Mercer CA, Haaning, C, Graves CB. (2009) A sex-specific mutualistic interaction in a dioecious grass. American Journal of Botany 96: 1967-1973.

Eppley SM, Stanton ML, Grosberg RK (1998) Intrapopulation sex ratio variation in the salt grass Distichlis spicata. American Naturalist 152:659-670

Freeman DC, Klikoff LG, Harper KT (1976) Differential resource utilization by the sexes of dioecious plants. Science 193:597-599

Hitchcock AS (1971) Manual of the grasses of the United States. Dover, New York, New York USA

Iglesias MC, Bell G (1989) The small-scale spatial distribution of male and female plants. Oecologia 80:229-235

Mercer, C and Eppley SM (2010) Intra- verssus inter-sexual competitin in the dioecious grass, Distichlis spicata. Oecologia 164: 657-664.

Reusch TB, Stam WT, Olsen JL (2000) A microsatellite-based estimation of clonal diversity and population subdivision in Zostera marina, a marine flowering plant. Molecular Ecology 9:127-140

Richards CL, Hamrick JL, Donovan LA, Mauricio R (2004) The unexpectedly high clonal diversity of two salt marsh perennials across a severe environmental gradient. Ecology Letters 7:1155-1162

Current Research: We are using greenhouse experiments and 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. To aid in these experiments, we have developed sex-specific STS markers and microsatellite markers (Tsyusko et al. 2007; Eppley et al. 2009). The data from these greenhouse and field experiments 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.

Based on initial greenhouse studies, we conduced a greehouse study looking at mycorrhizal colonization in Distichlis spicata and compared the results to colonization rates in the field. Our results demonstrate that D. spicata males and females interact with mycorrhizal fungi differently and that this difference occurs before the sexes incur the costs associated with sexual reproduction (Eppley at al. 2009). These results have implications for the role of this sex-specific mutualistic interaction in the coevolution of the mutualism and in the within-population sex ratio bias exhibited by D. spicata.

Reciprocal transplant experiments were put in place in spring 2007 and 2008, Whalen Island, Oregon. D. spicata seeds were planted in cone-tainers in the greenhouse at PSU. Seedlings were transplanted into female-dominated and male-dominated sites in D. spicata marsh in late spring. In 2009, the first set of transplants were removed after 15 months. Survival, dry weight, and root/shoot ratio was assessed for plants in majority-male and majority-female sites with and without intra-sexual competition. Plants grown without conspecific competitors grew equally well in both male- or female-majority habitats, suggesting that male and female plants do not have differential resource needs, at least at this life-history stage. However, plants subject to intra-sexual competition were significantly larger than plants subject to inter-sexual competition, suggesting that niche partitioning to reduce inter-sexual competition may occur in D. spicata.

D. spicata seedlings grown in the greenhouse at PSU prior to transplant in the marsh.