Southeastern Asia fall armyworms are closely related to populations in Africa and India, consistent with common origin and recent migration

Abstract

The discovery of fall armyworm, a native of the Western Hemisphere, in western Africa in 2016 was rapidly followed by detections throughout sub-Saharan Africa, India, and most recently southeastern Asia. This moth pest has a broad host range that threatens such important crops as corn, rice, millet, and sorghum, creating concern for its potential impact on agriculture in the Eastern Hemisphere. Although genetic data suggest populations sampled in Africa and India originate from a recent common source, it is not known whether this is the case for populations in southeastern Asia, nor whether the subgroup with a preference for rice and millet is present in the region. This study found through comparisons of genetic markers that the fall armyworm from Myanmar and southern China are closely related to those from Africa and India, suggesting a common origin for these geographically distant populations. The results are consistent with a single recent introduction into the Eastern Hemisphere followed by rapid dispersion. The molecular similarities include discrepancies between the genetic markers that brings into question whether the subpopulation most likely to be a threat to rice and millet is present in significant numbers in Asia.

Figure 1

Map and coordinates of collection sites in Myanmar and China combined with CLIMEX modeling of area suitability for fall armyworm. Collection (i) describes pheromone trapping of adult males. All others represent larval collections from corn host plants. (a) locations of sites overlaid on CLIMEX projection of fall armyworm suitability based on calculations of the Ecoclimatic Index (EI), with higher values indicating greater likelihood of persistent fall armyworm populations. (b) CLIMEX projections for the southeastern United States with the same parameters used in Asia. Circles indicate approximate regions where fall armyworm populations are localized during the winter in the United States based on monitoring studies.

Figure 2

Diagrams of relevant regions in the COI and Tpi genes used for molecular analysis. (a) the COI gene segments with locations of polymorphic sites used for categorizing the COI-CS h1-h4 variants. (b) Map of the Tpi gene segment consisting of the fourth exon of the presumptive open reading frame and adjacent intron. Site gTpi183Y defines the Tpi-based strain identity. Below are chromatographs for the exon segment containing gTpi183Y and two other strain-specific polymorphic sites. Mya1907B40 is a TpiC allele found in Myanmar while AfrRS1 is the TpiR allele identified in Africa. Combining the two produces the overlapping chromatograph pattern found with TpiH.

Figure 3

Phylogenetic tree calculated for the COIB259 sequence using the UPGMA method and Tamura-Nei modeling together with comparisons of haplotype frequencies from different regions. (a) Comparison of COIB259 haplotypes from Myanmar (MyaXXXXXXX) compared to 13 Spodoptera species (from GenBank) and haplotypes observed in Africa (AfrXXXX). (b) COI-CS variants that differ from those observed in Africa.

Figure 4

Comparisons of C-strain marker frequencies of larval collections from different locations and host plants. Myanmar sites were grouped by region with letter designations defined in Fig. 1. Eastern Africa is represented by pooled data from Burundi, Kenya, and Tanzania.

Figure 5

Frequencies of the TpiR haplotype from different regions and host plants calculated on the basis of specimens expressing the haplotype or estimation of chromosome numbers that include contributions from the presumed TpiH interstrain hybrid.

Figure 6

Phylogenetic tree calculated for the TpI4 intron segment using the UPGMA method and Tamura-Nei modeling. Relative frequencies of the different haplotypes are in parentheses.