Alternative trait combinations and secondary resource partitioning in sexually selected color polymorphism

Abstract

Resource partitioning within a species, trophic polymorphism is hypothesized to evolve by disruptive selection when intraspecific competition for certain resources is severe. However, in this study, we reported the secondary partitioning of oviposition resources without resource competition in the damselfly Ischnura senegalensis. In this species, females show color polymorphism that has been evolved as counteradaptation against sexual conflict. One of the female morphs is a blue-green (andromorph, male-like morph), whereas the other morph is brown (gynomorph). These female morphs showed alternative preferences for oviposition resources (plant tissues); andromorphs used fresh (greenish) plant tissues, whereas gynomorphs used decaying (brownish) plants tissues, suggesting that they chose oviposition resources on which they are more cryptic. In addition, the two-color morphs had different egg morphologies. Andromorphs have smaller and more elongated eggs, which seemed to adapt to hard substrates compared with those of gynomorphs. The resource partitioning in this species is achieved by morphological and behavioral differences between the color morphs that allow them to effectively exploit different resources. Resource partitioning in this system may be a by-product of phenotypic integration with body color that has been sexually selected, suggesting an overlooked mechanism of the evolution of resource partitioning. Finally, we discuss the evolutionary and ecological consequences of such resource partitioning.

Keywords: Body color; camouflage; choice; damselfly; egg morphology; female polymorphism; oviposition; phenotypic integration; resource partitioning.

Figure 1

Female color polymorphism in common bluetail, Ischnura senegalensis. Andromorphs (top panel) have greenish thorax and gynomorphs (bottom panel) have brownish one.

Figure 2

Egg morphology of each color morph (mean ± SE) (A) The volume of mature eggs oviposited. (B) Egg shape, which was calculated as length/width. Populations: Tsukuba is denoted by open circles and Ozutsumi as filled circles. The length of mature eggs for andromorphs in population Ozutsumi and Tsukuba was 0.831 ± 0.004 mm (N = 39) and 0.791 ± 0.006 mm (N = 5), respectively, and that for gynomorphs was 0.835 ± 0.004 mm (N = 40) and 0.829 ± 0.005 mm (N = 9), respectively. Likewise, the width of mature eggs for andromorphs in these two populations was 0.161 ± 0.001 mm (N = 39) and 0.150 ± 0.006 mm (N = 5), respectively, and that for gynomorphs was 0.168 ± 0.001 mm (N = 40) and 0.168 ± 0.003 mm (N = 9), respectively.

Figure 3

Length of ovipositor of each color morph (mean ± SE).

Figure 4

Preference for oviposition substrates. Substrates were divided into two categories: decaying tissues (Typha latifolia, Phragmites australis) and fresh tissues (Trapa japonica, Egeria densa, Schoenoplectus triangulatus).

Figure 5

Oviposition efficiencies on each oviposition substrate. Substrates were divided into two categories: decaying tissues and fresh tissues (see Fig. 4 in detail).