Testing inter-sexual competition and the influence of mycorrhizal fungi on fitness of a dioecious grass

Background/Question/Methods

Spatial Segregation of the Sexes (SSS), in which males and females show a high degree of separation along an environmental gradient, is a strategy exhibited by some dioecious plants. If SSS results in a reproductive cost, this cost must be offset by another benefit such as reduced competition for the pattern to be adaptive. The niche differentiation hypothesis states that segregation occurs in response to the harsh effects of inter-sexual competition. SSS has evolved independently numerous times within angiosperms, but selective forces responsible for sexual segregation remain unclear. Distichlis spicata is a dioecious perennial saltgrass that exhibits extreme sex ratios within microhabitats making it ideal to study competition as a selective force favoring SSS. Previous research has shown that female plants have higher mycorrhizal infection than males even though sites contain the same spore densities. Research has also shown that male and female sites differ in soil phosphorus, the nutrient that mycorrhizal fungi make more accessible to the plant in exchange for carbons. We used a multi-factorial greenhouse experiment varying treatments including mycorrhizal fungi, phosphorus and competition applied to both male and female D. spicata to determine how these factors affect fitness and ultimately how fitness affects SSS. We measured chlorophyll content, chlorophyll fluorescence, and dry weight of plants in these treatments.

Results/Conclusions

We found significant differences in chlorophyll content, dark adapted fv/fm fluorescence values, and dry weights among plants among treatments. The results obtained from this study indicate that D. spicata exhibits sex-specific competitive effects.  These results suggest that competition is an important factor influencing the sexual segregation occurring within D. spicata populations, and  these data suggest that mycorrhizal fungi plays an important role in competitive interactions.