Population variation in sexual dimorphism in the damselfly Megaloprepus caerulatus: evidence for genetic divergence?

Population differences in cues used in mate recognition are thought to play a role in speciation by limiting gene flow among diverging populations. Because such cues rarely vary within a single species, it is difficult to test their role in incipient divergence.  Across its range, the damselfly Megaloprepus caerulatus (a monospecific genus) exhibits both sexually monomorphic and sexually dimorphic wing patterns. In sexually dimorphic populations, the sex-specific white tips of female wings function in sexual recognition, whereas the white band of male wings functions in male-male interactions. In sexually monomorphic populations males lack the white wing band, and both males and females have relatively pale white wing tips. We quantified population density, body size, patch reflectance and wing shape, and genetic differences among five study populations ranging from Mexico to Ecuador. Males of all populations defended water-filled tree holes used by egg-laying females. Relative to sexually dimorphic populations, monomorphic populations had lower adult density with few male-male territorial interactions. In a sexually monomorphic population, we manipulated the wings of individuals to resemble sexually dimorphic individuals. Although males reacted to the individuals as conspecifics, their sexual reaction to manipulated females and males was ambiguous. This finding was consistent with molecular analyses indicating that the sexually monomorphic wing pattern was ancestral. The two monomorphic populations were genetically distinct from each other and from sexually-dimorphic populations, whereas the latter exhibited considerable gene flow. Although the wing patterns of Megaloprepus reflect genetic divergence, there is additional cryptic divergence within the monomorphic phenotypes.