Ontogeny of color development in two green-brown polymorphic grasshopper species.

Many insects, including several orthopterans, undergo dramatic changes in body coloration during ontogeny. This variation is particularly intriguing in gomphocerine grasshoppers, where the green and brown morphs appear to be genetically determined (Schielzeth & Dieker, 2020, BMC Evolutionary Biology, 20, 63; Winter et al., 2021, Heredity, 127, 66). A better understanding of how these color morphs develop during ontogeny can provide valuable insights into the evolution and ecology of such a widespread color polymorphism. Here, we focus on the color development of two green-brown polymorphic species, the club-legged grasshopper Gomphocerus sibiricus and the steppe grasshopper Chorthippus dorsatus. By following the color development of individuals from hatching to adulthood, we found that color morph differences begin to develop during the second nymphal stage, are clearly defined by the third nymphal stage, and remain stable throughout the life of an individual. Interestingly, we also observed that shed skins of late nymphal stages are identifiable by color morphs based on their yellowish coloration, rather than the green that marks green body parts. Furthermore, by assessing how these colors are perceived by different visual systems, we found that certain potential predators can chromatically discriminate between morphs, while others may not. These results suggest that the putative genes controlling color morph are active during the early stages of ontogeny, and that green color is likely composed of two components, one present in the cuticle and one not. In addition, the effectiveness of camouflage appears to vary depending on the specific predator involved.

Condition dependence of (un)predictability in escape behavior of a grasshopper species.

(Un)predictability has only recently been recognized as an important dimension of animal behavior. Currently, we neither know if (un)predictability encompasses one or multiple traits nor how (un)predictability is dependent on individual conditions. Knowledge about condition dependence, in particular, could inform us about whether predictability or unpredictability is costly in a specific context. Here, we study the condition dependence of (un)predictability in the escape behavior of the steppe grasshopper Chorthippus dorsatus. Predator-prey interactions represent a behavioral context in which we expect unpredictability to be particularly beneficial. By exposing grasshoppers to an immune challenge, we explore if individuals in poor condition become more or less predictable. We quantified three aspects of escape behavior (flight initiation distance, jump distance, and jump angle) in a standardized setup and analyzed the data using a multivariate double-hierarchical generalized linear model. The immune challenge did not affect (un)predictability in flight initiation distance and jump angle, but decreased unpredictability in jump distances, suggesting that unpredictability can be costly. Variance decomposition shows that 3-7% of the total phenotypic variance was explained by individual differences in (un)predictability. Covariation between traits was found both among averages and among unpredictabilities for one of the three trait pairs. The latter might suggest an (un)predictability syndrome, but the lack of (un)predictability correlation in the third trait suggests modularity. Our results indicated condition dependence of (un)predictability in grasshopper escape behavior in one of the traits, and illustrate the value of mean and residual variance decomposition for analyzing animal behavior.

Simple inheritance of color and pattern polymorphism in the steppe grasshopper Chorthippus dorsatus.

The green-brown polymorphism of grasshoppers and bush-crickets represents one of the most penetrant polymorphisms in any group of organisms. This poses the question of why the polymorphism is shared across species and how it is maintained. There is mixed evidence for whether and in which species it is environmentally or genetically determined in Orthoptera. We report breeding experiments with the steppe grasshopper Chorthippus dorsatus, a polymorphic species for the presence and distribution of green body parts. Morph ratios did not differ between sexes, and we find no evidence that the rearing environment (crowding and habitat complexity) affected the polymorphism. However, we find strong evidence for genetic determination for the presence/absence of green and its distribution. Results are most parsimoniously explained by three autosomal loci with two alleles each and simple dominance effects: one locus influencing the ability to show green color, with a dominant allele for green; a locus with a recessive allele suppressing green on the dorsal side; and a locus with a recessive allele suppressing green on the lateral side. Our results contribute to the emerging contrast between the simple genetic inheritance of green-brown polymorphisms in the subfamily Gomphocerinae and environmental determination in other subfamilies of grasshoppers. In three out of four species of Gomphocerinae studied so far, the results suggest one or a few loci with a dominance of alleles allowing the occurrence of green. This supports the idea that brown individuals differ from green individuals by homozygosity for loss-of-function alleles preventing green pigment production or deposition.