Spatial sorting may explain evolutionary dynamics of wing polymorphism in pygmy grasshoppers

Abstract

Wing polymorphism in insects provides a good model system for investigating evolutionary dynamics and population divergence in dispersal-enhancing traits. This study investigates the contribution of divergent selection, trade-offs, behaviour and spatial sorting to the evolutionary dynamics of wing polymorphism in the pygmy grasshopper Tetrix subulata (Tetrigidae: Orthoptera). We use data for > 2800 wild-caught individuals from 13 populations and demonstrate that the incidence of the long-winged (macropterous) morph is higher and changes faster between years in disturbed habitats characterized by succession than in stable habitats. Common garden and mother-offspring resemblance studies indicate that variation among populations and families is genetically determined and not influenced to any important degree by developmental plasticity in response to maternal condition, rearing density or individual growth rate. Performance trials show that only the macropterous morph is capable of flight and that propensity to fly differs according to environment. Mark-recapture data reveal no difference in the distance moved between free-ranging long-and short-winged individuals. There is no consistent difference across populations and years in number of hatchlings produced by long-and shorter-winged females. Our findings suggest that the variable frequency of the long-winged morph among and within pygmy grasshopper populations may reflect evolutionary modifications driven by spatial sorting due to phenotype-and habitat type-dependent emigration and immigration.

Morphological variation and sex-biased frequency of wing dimorphism in the pygmy grasshopper Tetrix subulata (Orthoptera: Tetrigidae)

“…Furthermore, morph frequencies remained stable during the course of a year in two repeatedly sampled populations, with little variation between the individuals in the spring and autumn cohorts. Interestingly, the morph frequencies showed a previously unreported sex-biased difference (Lock et al, 2006;Berggren et al, 2012), with a shift towards more LW males in populations in habitats of medium quality. Females with their assumed greater investment in reproduction might switch their developmental program from LW to SW in habitats deteriorating in quality before the males.…”

“…However, the greater mobility of LW individuals might allow them to find better sources of food, as free ranging individuals of the different morphs differ in their stable isotope signatures (Karpestam & Forsman, 2013). Phenotypic variation in Swedish T. subulata has a clear genetic background, as there is a strong association between wing length of the parental generation collected from the field and the wing-morph frequencies of their captive-reared offspring (Berggren et al, 2012). However, a maternal affect on wing morph frequencies cannot be ruled out, as only the first generation of offspring was studied (Berggren et al, 2012).…”

“…They were placed in small jars and transferred to the laboratory. In order to avoid the female-bias associated with hand collecting the specimens were collected using a sweep net (Berggren et al, 2012;unpubl. data).…”

“…Individuals described as long-winged, develop hind wings and pronota that overhang the knees by several millimetres. However, previous studies have classified individuals visually and only provide measurements of the extended pronotum (Lock et al, 2006;Berggren et al, 2012). In order test for the existence of distinct wing morphs we measured wing size in this study.…”

“…We studied populations of Tetrix subulata (Linnaeus, 1758), because in this species two wing morphs occur within the same population (Devriese, 1996;Lock et al, 2006;Berggren et al, 2012). In the SW morph the pronotum reaches the knees of the hind femurs and the folded hind wings the tip of the pronotum.…”