What triggers colour change? Effects of background colour and temperature on the development of an alpine grasshopper

Abstract

BackgroundColour polymorphisms are a fascinating facet of many natural populations of plants and animals, and the selective processes that maintain such variation are as relevant as the processes which promote their development. Orthoptera, the insect group that encompasses grasshoppers and bush crickets, includes a particularly large number of species that are colour polymorphic with a marked green-brown polymorphism being particularly widespread. Colour polymorphism has been associated with the need for crypsis and background matching and background-dependent homochromy has been described in a few species. However, when and how different environmental conditions influence variation in colour remains poorly understood. Here we test for effects of background colour and ambient temperature on the occurrence of colour morph switches (green to brown or brown to green) and developmental darkening in the alpine dwelling club-legged grasshopper Gomphocerus sibiricus.ResultsWe monitored individually housed nymphae across three of their four developmental stages and into the first week after final ecdysis. Our data show an absence of colour morph switches in G. sibiricus, without a single switch observed in our sample. Furthermore, we test for an effect of temperature on colouration by manipulating radiant heat, a limiting factor in alpine habitats. Radiant heat had a significant effect on developmental darkening: individuals under low radiant heat tended to darken, while individuals under high radiant heat tended to lighten within nymphal stages. Young imagoes darkened under either condition.ConclusionsOur results indicate a plastic response to a variable temperature and indicate that melanin, a multipurpose pigment responsible for dark colouration and presumed to be costly, seems to be strategically allocated according to the current environmental conditions. Unlike other orthopterans, the species is apparently unable to switch colour morphs (green/brown) during development, suggesting that colour morphs are determined genetically (or very early during development) and that other processes have to contribute to crypsis and homochromy in this species.

Host plants of the non‐swarming edible bush cricket Ruspolia differens

“…Overall, a larger proportion of brown individuals were observed on B. ruziziensis . Difference in use of host plants between colour morphs could be an adaptation of R. differens to optimise camouflage (Bailey & McCrae, ; Valverde & Schielzeth, ). However, for the strictly nocturnal nymphs, the role of colour morph in improving their survival in dim‐light conditions should be explored in further studies (see e.g., Meyer‐Rochow & Teh, ).…”

“…Overall, a larger proportion of brown individuals were observed on B. ruziziensis. Difference in use of host plants between colour morphs could be an adaptation of R. differens to optimise camouflage (Bailey & McCrae, 1978;Valverde & Schielzeth, 2015).…”

High‐throughput sequencing and graph‐based cluster analysis facilitate microsatellite development from a highly complex genome

“…This species is a valuable model system for studying the evolution sexual ornamentation and the long‐term maintenance of color polymorphisms in natural populations (Valverde and Schielzeth. 2015). Fitness assays under competitive conditions in the field and in the laboratory, quantitative genetic studies and inbreeding studies in relation to sexual ornamentation all require genetic markers, yet none are currently available.…”

“…The club-legged grasshopper, Gomphocerus sibiricus, that we study here is an Acridid grasshopper with a sizable genome of around 8.7 Gb (Gregory 2015) and a high prevalence of supernumerary chromosomes (L opez-Fern andez et al 1986). This species is a valuable model system for studying the evolution sexual ornamentation and the long-term maintenance of color polymorphisms in natural populations (Valverde and Schielzeth. 2015).…”

Condition‐dependence and sexual ornamentation: Effects of immune challenges on a highly sexually dimorphic grasshopper

“…We collected egg pods from field‐caught individuals (near Sierre, Valais, Switzerland, same population as in R1) that were temporarily housed in individual cages (dimensions 22 × 16 × 16 cm 3 ) in July 2012 with a cup of sand‐vermiculite mixture as substrate for egg laying. Eggs were collected once per week, kept for approximately 6 weeks at room temperature and subsequently stored at +4 °C for diapause (see Valverde & Schielzeth, ). We used the laboratory‐reared male offspring from a total of 42 haphazardly chosen egg pods after overwintering in refrigerators for about 7 months.…”