Spatial analyses of two color polymorphisms in an alpine grasshopper reveal a role of small‐scale heterogeneity

Abstract

Discrete color polymorphisms represent a fascinating aspect of intraspecific diversity. Color morph ratios often vary clinally, but in some cases, there are no marked clines and mixes of different morphs occur at appreciable frequencies in most populations. This poses the questions of how polymorphisms are maintained. We here study the spatial and temporal distribution of a very conspicuous color polymorphism in the club‐legged grasshopper Gomphocerus sibiricus. The species occurs in a green and a nongreen (predominately brown) morph, a green–brown polymorphism that is common among Orthopteran insects. We sampled color morph ratios at 42 sites across the alpine range of the species and related color morph ratios to local habitat parameters and climatic conditions. Green morphs occurred in both sexes, and their morph ratios were highly correlated among sites, suggesting shared control of the polymorphism in females and males. We found that in at least 40 of 42 sites green and brown morphs co‐occurred with proportions of green ranging from 0% to 70% with significant spatial heterogeneity. The proportion of green individuals tended to increase with decreasing summer and winter precipitations. Nongreen individuals can be further distinguished into brown and pied individuals, and again, this polymorphism is shared with other grasshopper species. We found pied individuals at all sites with proportions ranging from 3% to 75%, with slight, but significant variation between years. Pied morphs show a clinal increase in frequency from east to west and decreased with altitude and lower temperatures and were more common on grazed sites. The results suggest that both small‐scale and large‐scale spatial heterogeneity affects color morph ratios. The almost universal co‐occurrence of all three color morphs argues against strong effects of genetic drift. Instead, the data suggest that small‐scale migration–selection balance and/or local balancing selection maintain populations polymorphic.

Phylogenetic, geographic and habitat distribution of the green-brown polymorphism in European orthopterans

“…However, there are pattern polymorphism in a number of species, such as among Tettrigidae, in which these polymorphisms have a genetic basis (Fisher 1930; Ahnesjö & Forsman 2003; Karlsson et al 2009). Lesser known pattern polymorphisms include pied variants (Dieker et al 2018) among many species of Gomphocerinae (about 41% in my survey) and Oedipodinae (32%) and in one species of Pamphaginae (3%). Pied morphs seem to be absent from Ensifera.…”

“…No attempt was made to identify all cases in which females and males showed different ratios of color morph. I also recorded the occurrence of pied morphs (sensu Dieker et al 2018). Pied morphs are characterized by a pale transverse stripe across the lateral sides of head and pronotum.…”

Green‐brown polymorphism in alpine grasshoppers affects body temperature

“…For P. parallelus, we pooled four variants of greenish morphs (green, greenwith-brown legs, dorsal stipe, and green-with brown sides, Köhler et al, 2017) into the green class and completely brown individuals into the brown class. For G. sibiricus, we pooled two variants of brown morphs (pied and brown, Dieker et al, 2018) into the brown class. In both these species, morph identities are (at least partly) heritable (Köhler, 2006, own unpublished data).…”

“…Green and brown morphs differ in the presence or absence of green pigments in the epidermis with the main candidate pigment being biliverdin (Fuzeau-Braesch, 1972;Shamim, Ranjan, Pandey, & Ramani, 2014). Since brown morphs usually appear darker to the human eye, it has been speculated that they might have a thermoregulatory advantage that potentially trades-off with other fitness-related functions (Dieker, Beckmann, Teckentrup, & Schielzeth, 2018;Köhler et al, 2017) such as crypsis and predator avoidance (Ahnesjö & Forsman, 2006;Bond & Kamil, 2006;Pitt, 1999). However, little is known about the thermal biology of green-brown polymorphic grasshoppers under field conditions (Chappell & Whitman, 1990;Dearn, 1990).…”

“…Specifically, we measured thorax temperature in males and females of 10 species, three of which are distinctly green-brown polymorphic (Figure 1). The polymorphic species included the club-legged grasshopper Gomphocerus sibiricus, in which green individuals typically represent about 25% of local populations with spatial heterogeneity in morph ratios across the range (Dieker et al, 2018). The club-legged grasshopper is a high-altitude species that typically lives above 1,500 m a.s.l.…”