A mechanistic link between parasite resistance and expression of a sexually selected trait in a damselfly

This paper examines a held-based insect system in which a signal trait and an immune effector system responsible for parasite resistance rely on the s ame melanin-producing enzyme cascade (phenoloxidase, PO). Observations and experiments on males of the calopterygid damselfly Calopteryx splendens xan thostoma revealed that resistance to the prevalent parasite in the study sy stem (a eugregarine protozoan infecting the mid-gut) was correlated with qu antitative aspects of the sexually dimorphic melanized wingspot of males, a trait that is produced and fixed before the host comes into contact with t he sporozoites of the parasite. Regulation of PO during experimental immune challenge showed that males with dark, homogenous melanin distribution in their wings showed no change in PO levels 24 h after challenge. By contrast males with lighter and/or more heterogenous melanin distribution in their wings tended to show higher PO levels 24 h after immune challenge. The chan ges in PO levels occur despite the lack of a relationship between wing-pigm ent distribution and the cellular encapsulation response. These results sug gest a shared, limiting resource may form the mechanistic basis of the trad e-off between a condition-dependent signal trait and immune function in thi s system.