Peripheral genetic structure of Helicoverpa zea indicates asymmetrical panmixia

Authors: Seymour, Mathew; Perera, Omaththage; FESCEMYER, HOWARD - Pennsylvania State University; JACKSON, RYAN - Syngenta Crop Protection; FLEISCHER, SHELBY - Pennsylvania State University; Abel, Craig

Submitted to: Ecology and Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/9/2016
Publication Date: 5/1/2016
Citation: Seymour, M., Perera, O.P., Fescemyer, H.W., Jackson, R.E., Fleischer, S.J., Abel, C.A. 2016. Peripheral genetic structure of Helicoverpa zea indicates asymmetrical panmixia. Ecology and Evolution. 6(10):3198-3207. doi: 10.1002/ece3.2106.

Interpretive Summary: Seasonal changes create temporary habitat during parts of the year, such as summer in North America, which is preferred by many migratory species. However, during other parts of the year, such as winter, these temporary habitats lead to localized extinction or return migration of several species that are unable to cope with the change in environmental conditions. Such habitat, where temporary habitat does not allow permanent residence, could allow migratory species to evolve high genetic diversity, resulting from repeated, seasonal mixing of migratory populations from multiple regionally distant areas. Migrant populations of Helicoverpa zea (Boddie), commonly known as corn earworm, cotton bollworm or tomato fruitworm, captured during two different seasons were assessed for genetic structure using microsatellite markers and for host plant type using stable carbon isotope analysis. Individuals (N = 568) were genotyped and divided into 13 putative populations based on collection site and time. Individuals showed high genetic similarity within populations and little to no genetic differences among populations, suggesting captured migrants originated from breeding populations with different genetic makeups. Across all individuals we found high genetic diversity, which combined with the finding of low level of genetic differences across the populations, suggest migrating populations of Helicoverpa zea are the result of asymmetrical migration. This migration pattern is likely due to the high dispersal and reproductive behavior of Helicoverpa zea, which may hinder the adaptation and establishment of Helicoverpa zea to temporary seasonal habitat and its inability to overwintering in colder, northern habitats.

Technical Abstract: Seasonal climatic shifts create peripheral habitats that alternate between habitable and uninhabitable for migratory species. Such dynamic peripheral habitats are potential sites where migratory species could evolve high genetic diversity resulting from convergence of immigrants from multiple regionally distant areas. Migrant populations of Helicoverpa zea (Boddie) captured during two different seasons were assessed for genetic structure using microsatellite markers and for host plant type using stable carbon isotope analysis. Individuals (N = 568) were genotyped and divided into 13 putative populations based on collection site and time. Fixation indices (F-statistics), analysis of molecular variance (AMOVA), and discriminant analysis of principal components (DAPC) were used to examine within and among population genetic variation. Mean number of alleles per locus was 10.25 (± 3.2 SD) and allelic richness ranged from 2.38 to 5.13 (± 3.2 SD). The observed and expected heterozygosity ranged from 0.07 to 0.48 and 0.08 to 0.62, respectively. Low FST (0.01 – 0.02) and high FIS (0.08 – 0.33) values suggest captured migrants originated from breeding populations with different allele frequencies. We postulate that high genetic diversity within migrant populations and low genetic differentiation among migrant populations of H. zea are the result of asymmetrical immigration due to the high dispersal and reproductive behavior of H. zea, which may hinder the adaptation and establishment of H. zea to peripheral habitat. These findings highlight the importance of assessing peripheral population structure in relation to ecological and evolutionary dynamics of this and other highly reproductive and dispersive species.