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.

In order to investigate the inheritance of color polymorphism in the steppe grasshopper Chorthippus dorsatus, we applied a half-sib-full-sib breeding design to produce offspring from known parental morph combinations. Our breeding design was implemented with field-caught individuals from Jena, Germany (50.94°N, 11.61°E). A total of 511 individuals (226 males, 285 females) were sampled as third or fourth instar nymphae, ensuring that all individuals were virgin at the time of capture. After their final molt, individuals had their color morph scored (MorphF and MorphM) and were transferred to mating cages. 51 males (MaleID) were mated with five females each (249 females in total, FemaleID). Each female was kept in a separate mating cage, and males were rotated between "their" five mating cages every third day. We aimed to equalize morph ratios, but since morph ratios were severely skewed in the field, we mated each male to two brown females, two dorsal green females, and either a uniform green female, a lateral green female, or another brown female. Egg cases were collected every seven days and were transferred to Petri dishes lined with moist filter paper, and were kept at room temperature for several weeks. Eggs were then transferred to refrigerators and kept until spring at about 4°C. Diapause was ended in seven cohorts between March and September 2019, with one cohort per month (Cohort). Offspring hatched after approximately two weeks at room temperature. All offspring from a single egg case were transferred to family cages (CageID). We assigned full-sib families from the same egg case to one of two treatments (Treatment). Half of the families were raised in standard plain white housing cages. The other half of the families were raised in a more complex, enriched environment, in which the floor was lined with patterned green and brown foam rubber, a part of a gray or green egg box for cover and climbing, and a piece of pipe cleaner for climbing. Multiple egg cases from the same mating pair were assigned in equal numbers to both treatments. Offspring were scored for color morphs when all individuals of a family had molted into the third nymphal stage. Scoring was done by counting the number of individuals of each morph per cage (MorphOff and nOffsp). 

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