Mitochondrial DNA analysis of field populations of Helicoverpa armigera (Lepidoptera: Noctuidae) and of its relationship to H. zea

Abstract

Background: Helicoverpa armigera and H. zea are amongst the most significant polyphagous pest lepidopteran species in the Old and New Worlds respectively. Separation of H. armigera and H. zea is difficult and is usually only achieved through morphological differences in the genitalia. They are capable of interbreeding to produce fertile offspring. The single species status of H. armigera has been doubted, due to its wide distribution and plant host range across the Old World. This study explores the global genetic diversity of H. armigera and its evolutionary relationship to H zea.

Results: We obtained partial (511 bp) mitochondrial DNA (mtDNA) Cytochrome Oxidase-I (COI) sequences for 249 individuals of H. armigera sampled from Australia, Burkina Faso, Uganda, China, India and Pakistan which were associated with various host plants. Single nucleotide polymorphisms (SNPs) within the partial COI gene differentiated H. armigera populations into 33 mtDNA haplotypes. Shared haplotypes between continents, low F-statistic values and low nucleotide diversity between countries (0.0017-0.0038) suggests high mobility in this pest. Phylogenetic analysis of four major Helicoverpa pest species indicates that H. punctigera is basal to H. assulta, which is in turn basal to H. armigera and H. zea. Samples from North and South America suggest that H. zea is also a single species across its distribution. Our data reveal short genetic distances between H. armigera and H. zea which seem to have been established via a founder event from H. armigera stock at around 1.5 million years ago.

Conclusion: Our mitochondrial DNA sequence data supports the single species status of H. armigera across Africa, Asia and Australia. The evidence for inter-continental gene flow observed in this study is consistent with published evidence of the capacity of this species to migrate over long distances. The finding of high genetic similarity between Old World H. armigera and New World H. zea emphasises the need to consider work on both pests when building pest management strategies for either.

Figure 1

Haplotype network based on partial mtDNA COI (511 bp) of H. armigera, sampled from Australia, Burkina Faso, Uganda, China, India and Pakistan. Each haplotype is represented by a circle, and is identified by a number 1–33. Haplotype 1 included 156 individuals; haplotypes 2, 3, 5 and 10 have 17, 15, 5 and 10 individuals respectively. Haplotypes 6, 32 and 33 each have 4 individuals. Haplotypes 13 and 17 each has 3 individuals, and Haplotype 4, 11, 14, 15 and 19 each have 2 individuals. All remaining haplotypes have 1 individual each. Each base change involved in the differentiation between haplotypes is represented by a solid circle.

Figure 2

Maximum Likelihood (ML) tree of H. armigera (Harm-1 to Harm-31), H. zea (Hzea-1, Hzea-2), H. assulta and H. punctigera based on partial COI haplotypes sequences. Numbers above the nodes indicate bootstrap support. The outgroup used was Heliothis virescens. The inclusion of additional haplotypes Harm-32, Harm-33, and Hzea-3 to Hzea-11 did not alter the overall topology, and bootstrap values of the ML tree after 1,000 bootstrap replications remained high, with all H. zea haplotypes confidently clustered (bootstrap value = 96) within the H. armigera clade. H. punctigera remained basal to H. assulta (bootstrap value = 99), and the H. armigera/H. zea clade (bootstrap value = 78) shared a most common ancestor with H. assulta (bootstrap value = 97) (data not shown).