Detection of sister-species in invasive populations of the fall armyworm Spodoptera frugiperda (Lepidoptera: Noctuidae) from Uganda

Abstract

The fall armyworm (FAW) Spodoptera frugiperda (J. E. Smith) is a species native to the Americas. This polyphagous lepidopteran pest was first reported in Nigeria and the Democratic Republic of São Tomé and Principe in 2016, but its presence in eastern Africa has not been confirmed via molecular characterisation. In this study, FAW specimens from western and central Uganda were identified based on the partial mtDNA COI gene sequences, with mtDNA COI haplotypes matching those identified in Nigeria and São Tomé. In this study, we sequence an additional partial mtDNA Cyt b gene and also the partial mtDNA COIII gene in Ugandan FAW samples. We detected identical mitochondrial DNA haplotypes for both the mtDNA Cyt b and COI partial genes, while combining the mtDNA COI/Cyt b haplotypes and mtDNA COIII haplotypes enabled a new maternal lineage in the Ugandan corn-preferred FAW samples to be identified. Our results suggested that the African incursions of S. frugiperda involved at least three maternal lineages. Recent full genome, phylogenetic and microsatellite analyses provided evidence to support S. frugiperda as likely consisted of two sympatric sister species known as the corn-preferred and rice-preferred strains. In our Ugandan FAW populations, we identified the presence of mtDNA haplotypes representative of both sister species. It is not known if both FAW sister species were originally introduced together or separately, and whether they have since spread as a single population. Further analyses of additional specimens originally collected from São Tomé, Nigeria and throughout Africa would be required to clarify this issue. Importantly, our finding showed that the genetic diversity of the African corn-preferred FAW species is higher than previously reported. This potentially contributed to the success of FAW establishment in Africa. Furthermore, with the additional maternal lineages detected, there is likely an increase in paternal lineages, thereby increasing the diversity of the African FAW population. Knowledge of the FAW genetic diversity will be needed to assess the risks of introducing Bt-resistance traits and to understand the FAW incursion pathways into the Old World and its potential onward spread. The agricultural implications of the presence of two evolutionary divergent FAW lineages (the corn and the rice lineage) in the African continent are further considered and discussed.

Initial detections and spread of invasive Spodoptera frugiperda in China and comparisons with other noctuid larvae in cornfields using molecular techniques

“…In Brazil, S. frugiperda attacks on corn reduced the yield up to 34%, causing annual losses of 400 million US dollars (Sena et al ., ; Figueiredo et al ., ; Lima et al ., ). It is native to tropical and sub‐tropical areas in America but has spread rapidly across almost all sub‐Saharan countries after it was first detected in western and central Africa in early 2016, and 2018 in Uganda (Goergen et al ., ; Otim et al ., ). Then it was confirmed in several Asian countries such as India (Mallapur et al ., ), Sri Lanka, Thailand, Yemen and Myanmar (CABI, ) as well as Bangladesh (Farmer, ) in 2018.…”

Evidence that a major subpopulation of fall armyworm found in the Western Hemisphere is rare or absent in Africa, which may limit the range of crops at risk of infestation

“…The 26 C-strain (historically designated the corn-strain) is typically found on corn, sorghum, and cotton, 27 while the R-strain (rice-strain) predominates on rice, alfalfa, pasture grasses, and millet, though 28 subsequent observations indicate that the preference to rice may be variable [4][5][6][7]. There is 29 genetic evidence that both strains are present in Africa suggesting that a variety of crop systems 30 are at risk [8][9][10][11]. However, reports of fall armyworm damage to date have been primarily 31 limited to C-strain preferred hosts, specifically corn and sorghum [12].…”

“…The presence of the R-strain in Africa is based on mitochondrial markers primarily in the 329 COI gene [8][9][10][11], but is made uncertain by three observations. First, there are widespread reports 330 throughout the sub-Sahara region of fall armyworm infestation in corn and sorghum, two hosts 331 preferred by the C-strain, but major infestations in host plants preferred by the R-strain have yet 332 to be documented.…”

Multiple incursion pathways forHelicoverpa armigerain Brazil show its genetic diversity spreading in a connected world

“…The rapid spread and establishment of H. armigera across much of the South American continent has generated a very large population with significant impacts on agricultural production. In contrast to usual invasions such as the African incursion of the New World fall armyworm S. frugiperda [16,34,[39][40][41][42], the invasive H. armigera population in the New World appears to be very diverse. Furthermore, the spatial distribution of this diversity strongly suggests that the population has spread from two different regions of introduction.…”