The evolution of polyphenic male dimorphism in arthropods
Discrete morphological variation within conspecific males puzzled Darwin, and still fascinates evolutionary biologists today. Known as male dimorphism, this phenomenon reflects alternative reproductive tactics among males: the large male morphs typically guard females or reproductive territories and have more elaborate weaponry; the small male morphs sneak copulations and have reduced weaponry. Male dimorphism is particularly common among arthropods, and usually results from a polyphenism: the differential expression of alternative phenotypes from a single genotype depending upon environmental conditions. With a series of experiments using mites, harvestmen, and dung beetles, my research will investigate the following questions about polyphenic male dimorphism:
1. Is genetic compatibility between parental genotypes important for male phenotype determination in the offspring?
2. How does population density affect the fitness of alternative male phenotypes?
3. Can selection act independently on alternative male phenotypes?
4. Is the co-option from sexual dimorphism mechanisms important for male dimorphism?
Although polyphenic male dimorphism has already been studied in several different taxa, there are still many unanswered questions about the mechanisms underlying this phenomenon. Answering the first two questions of my research will enhance our comprehension of the different mechanisms affecting the expression of polyphenic male dimorphism, from genetic and ecological perspectives.
From an evolutionary perspective, it has been suggested that polyphenic male dimorphism results from a developmental reprogramming experienced by one of the male morphs, and that the uncoupled development of alternative male phenotypes allows them to respond to selection independently. There is also recent evidence for a correlated evolution of male dimorphism and sexual dimorphism mechanisms, indicating that male dimorphism mechanisms are evolutionarily co-opted from previously existing mechanisms of sexual dimorphism. In order to answer the third and fourth questions of my research, I will test both of these hypotheses experimentally.