Population genetic structure, differentiation, and diversity in Tetrix subulata pygmy grasshoppers: roles of population size and immigration

Abstract

Genetic diversity within and among populations and species is influenced by complex demographic and evolutionary processes. Despite extensive research, there is no consensus regarding how landscape structure, spatial distribution, gene flow, and population dynamics impact genetic composition of natural populations. Here, we used amplified fragment length polymorphisms (AFLPs) to investigate effects of population size, geographic isolation, immigration, and gene flow on genetic structure, divergence, and diversity in populations of Tetrix subulata pygmy grasshoppers (Orthoptera: Tetrigidae) from 20 sampling locations in southern Sweden. Analyses of 1564 AFLP markers revealed low to moderate levels of genetic diversity (PPL = 59.5–90.1; Hj = 0.23–0.32) within and significant divergence among sampling localities. This suggests that evolution of functional traits in response to divergent selection is possible and that gene flow is restricted. Genetic diversity increased with population size and with increasing proportion of long‐winged phenotypes (a proxy of recent immigration) across populations on the island of Öland, but not on the mainland. Our data further suggested that the open water separating Öland from the mainland acts as a dispersal barrier that restricts migration and leads to genetic divergence among regions. Isolation by distance was evident for short interpopulation distances on the mainland, but gradually disappeared as populations separated by longer distances were included. Results illustrate that integrating ecological and molecular data is key to identifying drivers of population genetic structure in natural populations. Our findings also underscore the importance of landscape structure and spatial sampling scheme for conclusions regarding the role of gene flow and isolation by distance.

Contrasting patterns of neutral and functional genetic diversity in stable and disturbed environments

“…Grasshoppers were collected in spring and early summer (for details, see Forsman, Karlsson et al., , ; Tinnert, Hellgren, Lindberg, Koch‐Schmidt, & Forsman, ; Tinnert & Forsman, ). Individuals were identified to species according to Holst (), classified according to their sex, wing morph (long‐winged with functional wings or short‐winged and flightless) (Berggren et al., ; Tinnert & Forsman, ), and color morph.…”

“…Census population size (Table ) was estimated for each locality based on information on number of collected individuals adjusted for capture probability (Tinnert & Forsman, ), which was set to 0.4 based on previous extensive capture–mark–recapture studies (Berggren et al., ; Forsman & Appelqvist, ; Tinnert & Forsman, ; Tinnert, Hellgren et al., ). We searched for grasshoppers while walking slowly through the area during days with weather conditions suitable for grasshopper activity, that is, clear or overcast days with a temperature of at least 15°C (Forsman, Karlsson et al., , ).…”

“…We searched for grasshoppers while walking slowly through the area during days with weather conditions suitable for grasshopper activity, that is, clear or overcast days with a temperature of at least 15°C (Forsman, Karlsson et al., , ). The number of individuals collected at each site underestimates actual population size, but it is a reliable relative measure which is robust to factors such as differences in the area covered for sampling, time invested in sampling, number of people involved in each sampling event, habitat type, and weather conditions that could potentially influence total catch (Tinnert & Forsman, ; Tinnert, Hellgren et al., ).…”

“…Furthermore, only the macropterous phenotype is able to fly (Berggren et al, 2012). Collectively, this suggests that a high incidence of the long-winged flight-capable morph may be used as a proxy to identify T. undulata populations that have been recently established or that represent older populations that have been much influenced by recent immigration (and that might show signatures associated with admixture) (Tinnert, Berggren, & Forsman, 2016;Tinnert & Forsman, 2017;Tinnert, Hellgren et al, 2016). Available evidence indicates that wing morph in pygmy grasshoppers is heritable and not influenced to any important degree by developmental plasticity (Berggren et al, 2012;Forsman, 2018).…”

“…Analysis of AFLP was carried out as described previously (Bensch & Åkesson, 2005;Vos et al, 1995), using the restriction enzymes EcoRI and Tru1 pre-amplification primers M A X E A and combinations of four selective primers (pair 1-E TAG X M CGA , pair 2-E TAG X M CAG , pair 3-E TCG X M CAC , and pair 4-E TAG X M CAC ) (Tinnert & Forsman, 2017;Tinnert, Hellgren et al, 2016). Three negative and nine positive controls were included on each plate.…”