Analysis of strain distribution, migratory potential, and invasion history of fall armyworm populations in northern Sub-Saharan Africa

Abstract

Fall armyworm (Spodoptera frugiperda J.E. Smith) is a noctuid moth pest endemic throughout the Western Hemisphere that has recently become widespread in sub-Saharan Africa. There is a strong expectation of significant damage to African maize crop yield and a high likelihood of further dispersal, putting the rest of the Eastern Hemisphere at risk. Specimens from multiple locations in six countries spanning the northern portion of the infested region were analyzed for genetic markers. The similarity of haplotypes between the African collections was consistent with a common origin, but significant differences in the relative frequency of the haplotypes indicated limitations in migration. The mitochondrial marker frequently used to identify two host strains appears to be compromised, making uncertain previous reports that both strains are present in Africa. This more extensive study confirmed initial indications based on Togo populations that Florida and the Greater Antilles are the likely source of at least a subset of the African infestation and further suggest an entry point in western Africa. The origin of a second subgroup is less clear as it was rarely found in the collections and has a haplotype that has not yet been observed in the Western Hemisphere.

Figure 1

DNA segments from the COI and Tpi genes analyzed in this study are described showing sites with base substitutions with the observed alternatives indicated by IUPAC symbols. Sites in bold and underlined show strain-specific variation in Western Hemisphere populations. Three sites in the Tpi fragment showed additional variation in the African populations (underlined and italicized) and two sites had variations that were not strain-specific. Sites 165, 168, and 183 had been previously described as 352, 355, and 370, respectively^,. Primers used for PCR amplification and DNA sequencing are displayed at the ends of each DNA segment.

Figure 2

Strict consensus phylogenetic tree derived from neighbor- joining analysis comparing the two African COI haplotypes (COI) with those from fall armyworm host strains and related Spodoptera species. COI-CS01 and COI-RS09 are fall armyworm haplotypes common to the Western Hemisphere fall armyworm. The analogous COI barcode segment from Helicoverpa armigera was use as the outgroup. The tree is based on Kimura-2-Parameter distances. Numbers at branch points indicate 2000X bootstrap values. Scale bar represents substitutions per site. GenBank accession numbers are provided for each species.

Figure 3

Frequency distributions of the COIB haplotype that differentiates geographical populations in the Western Hemisphere are described for Africa. The Y-axis indicates proportion of the population sampled. Numbers in parenthesis are total samples tested. Asterisk indicates data from previous study.

Figure 4

Frequencies of COI and Tpi haplotype combinations for different regions in Africa are described and compared to those found in fall armyworm from the Americas. Haplotypes are configurations represent those found in all collections and can be categorized as concordant, COI-CS Tpi-C and COI-RS Tpi-R; discordant, COI-CS Tpi-R and COI-RS Tpi-C, and hybrid, COI-CS Tpi-h, and COI-RS Tpi-h. The Western Hemisphere (W.H.) graph includes data from South America (Argentina, Brazil, Bolivia) and North America (Florida, Texas). Statistical metrics are from Pearson correlation tests comparing the pooled W.H. profile with the pooled frequency profiles from each region. The value for Togo (*) combines new data with that previously published. Numbers after locations denote total samples analyzed.

Figure 5

Frequencies of COI-CS and Tpi-Ca2 haplotype frequencies for different regions are indicated on a map of Africa. Togo (B) data are from previous study, Togo (B) data from this study. Numbers above columns indicate total specimens for each collection. Means for groupings are indicated above horizontal line with Standard Error of the Mean. Statistical metrics are from two-tailed parametric t-test comparing the two means (West and Rest) for each haplotype. Circles on map approximate locations of sample collections. The map was created using Quantum Geographic Information System version 2.18.2 (QGIS Development Team (2016). QGIS Geographic Information System. Open Source Geospatial Foundation Project. http://qgis.osgeo.org).