A test of the size-constraint hypothesis for a limit to sexual dimorphism in plants
- 331 Downloads
In flowering plants, many dioecious species display a certain degree of sexual dimorphism in non-reproductive traits, but this dimorphism tends to be much less striking than that found in animals. Sexual size dimorphism in plants may be limited because competition for light in crowded environments so strongly penalises small plants. The idea that competition for light constrains the evolution of strong sexual size dimorphism in plants (the size-constraint hypothesis) implies a strong dependency of the expression of sexual size dimorphism on the neighbouring density as a result of the capacity of plants to adjust their reproductive effort and investment in growth in response to their local environment. Here, we tested this hypothesis by experimentally altering the context of competition for light among male–female pairs of the light-demanding dioecious annual plant Mercurialis annua. We found that males were smaller than females across all treatments, but sexual size dimorphism was diminished for pairs grown at higher densities. This result is consistent with the size-constraint hypothesis. We discuss our results in terms of the tension between selection on size acting in opposite directions on males and females, which have different optima under sexual selection, and stabilizing selection for similar sizes in males and females, which have similar optima under viability selection for plasticity in size expression under different density conditions.
KeywordsDioecy Light competition Plasticity Mercurialis annua Ecological selection
We thank T. Martignier, M. Voillemot, I. El M’Ghari and K. Gullotta for field assistance and data collection, R. Piault for statistical advice, and the Swiss National Science Foundation for funding (grant no. 31003A_141052, to J. R. P.). We thank Anne Worley and the anonymous reviewers for helpful comments on the manuscript.
A. M. L. and J. R. P. conceived and designed the experiment; A. M. L. performed the experiment and analysed the data. A. M. L. and J. R. P. wrote the manuscript.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no conflict of interest.
- Ågren J, Danell K, Elmqvist T, Ericson L, Hjalten J (1999) Sexual dimorphism and biotic interactions. In: Geber MA, Dawson TE, Delph LF (eds) Gender and sexual dimorphism in flowering plants. Springer, BerlinGoogle Scholar
- Andersson M (1994) Sexual selection. Princeton University Press, Princeton, NJGoogle Scholar
- Dawson TE, Geber MA (1999) Dimorphism in physiology and morphology. In: Geber MA, Dawson TE, Delph LF (eds) Gender and sexual dimorphism in flowering plants. Springer, BerlinGoogle Scholar
- Delph LF (1999) Sexual dimorphism in live history. In: Geber MA, Dawson TE, Delph LF (eds) Gender and sexual dimorphism in flowering plants. Springer, BerlinGoogle Scholar
- Delph LF, Bell DL (2008) A test of the differential-plasticity hypothesis for variation in the degree of sexual dimorphism in Silene latifolia. Evol Ecol Res 10:61–75Google Scholar
- Durand B (1963) Le complexe Mercurialis annua L. s.i.: une étude biosystématique. Ann Sci Nat Bot Paris 12:579–736Google Scholar
- Harper JL (1977) Population biology of plants. Academic Press, LondonGoogle Scholar
- Lankinen A, Madjidian JA (2011) Enhancing pollen competition by delaying stigma receptivity: pollen deposition schedules affect siring ability, paternal diversity, and seed production in Collinsia heterophylla (Plantaginaceae). Am J Bot 98:1191–1200. doi: 10.3732/ajb.1000510 CrossRefPubMedGoogle Scholar
- R Core Team (2013) R: a language and environment for statistical computing, 3.0.2. R Foundation for Statistical Computing, ViennaGoogle Scholar
- Shuster SM, Wade MJ (2003) Mating systems and strategies. In: Monographs in behavior and ecology. Princeton University Press, NJGoogle Scholar
- Tutin TG et al (1968) Flora Europea. Cambridge University Press, CambridgeGoogle Scholar