Frequency of hybridization between Ostrinia nubilalis E‐and Z‐pheromone races in regions of sympatry within the United States

Abstract

Female European corn borer, Ostrinia nubilalis, produce and males respond to sex pheromone blends with either E-or Z-D11-tetradecenyl acetate as the major component. E-and Z-race populations are sympatric in the Eastern United States, Southeastern Canada, and the Mediterranean region of Europe. The E-and Z-pheromone races of O. nubilalis are models for incipient species formation, but hybridization frequencies within natural populations remain obscure due to lack of a high-throughput phenotyping method. Lassance et al. previously identified a pheromone gland-expressed fatty-acyl reductase gene (pgfar) that controls the ratio of D11-tetradecenyl acetate stereoisomers. We identified three single nucleotide polymorphism (SNP) markers within pgfar that are differentially fixed between E-and Z-race females, and that are ≥98.2% correlated with female pheromone ratios measured by gas chromatography. Genotypic data from locations in the United States demonstrated that pgfar-z alleles were fixed within historically allopatric Z-pheromone race populations in the Midwest, and that hybrid frequency ranged from 0.00 to 0.42 within 11 sympatric sites where the two races co-occur in the Eastern United States (mean hybridization frequency or heterozygosity (H O ) = 0.226 AE 0.279). Estimates of hybridization between the E-and Z-races are important for understanding the dynamics involved in maintaining race integrity, and are consistent with previous estimates of low levels of genetic divergence between E-and Z-races and the presence of weak prezygotic mating barriers.

A combination of sexual and ecological divergence contributes to rearrangement spread during initial stages of speciation

“…E and Z strain individuals in all populations were identified by genotyping a polymorphic Taq1α restriction enzyme site in the pgFAR gene (Coates et al . ). Voltinism of population samples were determined using published evidence from seasonal trapping data and experimentally determined phenotypes of post‐winter diapause termination (PDD) time (Glover et al .…”

“…; Coates et al . ). By comparing the presence of this rearrangement among ECB populations that are divergent in sexual traits, ecological traits or both, we can provide insight into what type of divergence facilitates rearrangement spread during the initial stages of speciation.…”

Influence of host plant, geography and pheromone strain on genomic differentiation in sympatric populations of Ostrinia nubilalis

“…In addition, genetic assignment of pheromone strain was determined for each male based on SNP genotype of a marker at the pgfar locus. This assay detects alleles that are >98.7% correlated with E‐ or Z‐pheromone strains (Coates, Johnson, et al, ). Specifically, the Taq I PCR‐RFLP assay was used to assign homozygous pgfar‐e / pgfar‐e and pgfar‐z / pgfar‐z genotypes to respective strains, and pgfar‐e / pgfar‐z heterozygotes to a hybrid class.…”

A recombination suppressor contributes to ecological speciation in OSTRINIA moths

“…Lack of genetic evidence for differential introgression runs counter to observations in nature. An appreciable number of hybrid offsprings occur at sampled sympatric localities in New York (5-15%; Dopman et al, 2010;Coates et al, 2013) and yet differentiation between Z and E strains at Tpi has been maintained at these sites for more than 40 generations (Glover et al, 1991;Dopman et al, 2005;Dopman, 2011). Disruptive selection following introgression of the nonrecombining region that includes Tpi might be important in maintaining divergence at New York localities, where sweeps and incomplete lineage sorting alone cannot easily account for patterns of nucleotide variation across loci.…”

“…The sex (Z) chromosome harbors several known genetic factors for adaptation and/or barrier traits, including those underlying behavioral isolation (Roelofs et al, 1987;Dopman et al, 2004) and temporal isolation (Glover et al, 1992;Dopman et al, 2005). Current and historical gene exchange between ECB strains is likely, as indicated by hybridization in Europe and North America (Dopman et al, 2010;Coates et al, 2013), geographic variation in strength of reproductive isolation (ranging from 0.91 to 0.99; Dopman et al, 2010) and molecular evidence for an isolationwith-migration model of divergence (2 Nm~10; Malausa et al, 2007). Whereas most loci show extensive shared polymorphism (for example, F ST o0.05), a possible island of speciation along an~1-cM swath of the Z chromosome consists of four genes (Tpi and three olfactory receptors, ORs) and reveals ECB strains as nearly reciprocally monophyletic (for example, F ST 40.7; Dopman et al, 2005;Dopman, 2011;Lassance et al, 2011).…”