Melanic individuals in color polymorphic Enallagma damselflies result from phenotypic, not genetic, variation


Genetically determined color polymorphisms have a long history in the study of evolutionary change acting on populations. The Odonata exhibit relatively high levels of sex-specific color polymorphisms in mature adults. In Ischnura and Coenagrion, female-specific polymorphisms are known to be controlled by Mendelian genes. Nearly half of Enallagma species have polymorphic females, but the inheritance of any has yet to be determined. Our aims here were to determine: (1) the inheritance of the color polymorphism in E. hageni; and (2) inherent reproductive characteristics of blue female andromorphs and green heteromorphs reared under controlled conditions as tenerals. Maternal morphs, which developed normal coloration in field enclosures within a week, did not differ in copulation time or clutch size, and their offspring did not differ in sex ratio or survivorship to emergence. Surprisingly, no laboratory-reared offspring developed normal mature coloration. Rather, the initially pale parts of the thorax and abdomen, that normally would turn either blue or green, became melanized. Black novel phenotypes also developed in adults of E. civile, E. anna, E. carunculatum, and E. annexum that as larvae or teneral adults were reared to sexual maturity under greenhouse conditions that differed from the laboratory conditions used to rear E. hageni. We hypothesize that the phenotypic plasticity in body coloration documented in Enallagma results from the quality of UV radiation experienced as a sexually immature adult, which is known to affect melanization in other insects. These examples in Enallagma offer insights into the origin of color novelty in Odonata.

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