Comparative Genetic Analysis of Invasive Fall Armyworm Populations Provides Evidence for Population Expansion in India

Abstract

Fall Armyworm (FAW), Spodoptera frugiperda, is a polyphagous pest capable of feeding over 80 plant species and was indigenous to Western Hemisphere until recently. Within a span of four years, FAW has established itself throughout most of the regions in Africa and Asia incurring significant losses in maize production. Owing to its revamped distribution range, it would be prudent to analyse the ensuing genetic changes and study the emerging phylogeographic patterns across the world. In this regard, we would like to provide a current snapshot of genetic diversity of FAW in India after two years of the initial introduction and compare it with the worldwide diversity in order to trace the origins and evolutionary trajectories of FAW in India.We have investigated around 145 FAW samples from different regions in India for strain identity and polymorphism analysis on the basis of partial mitochondrial cytochrome oxidase I (COI) gene sequences. Apart from the ancestral rice and corn strain haplotype, our study demonstrates the presence of 19 more haplotypes unique to India at a haplotype diversity of 0.498. We were also able to record the occurrence of inter-strain hybrid haplotypes of rice and corn strain in India. Regional heterogeneity within Indian populations seems to be quite low representative of extensive migration of FAW within India. Distribution analysis of pairwise differences and rejection of neutrality tests suggest that the FAW population in India is undergoing expansion. However, the scenario is much different for rest of the invaded regions like Africa and other Asian countries where FAW still appear to evolve neutrally. Our data is consistent with the findings suggesting a recent and common origin for invasive FAW populations in Asia and Africa, with significant population structure observed in Indian populations.This study reports the highest genetic diversity for Indian FAW populations till date and identifies India as the emerging hotspot for expansion of COIA rice haplotypes across the world. The findings will be useful to track the subsequent evolution of FAW and would have important ramifications for FAW behaviour and composition throughout the world.