Genetic basis of allochronic differentiation in the fall armyworm.

BACKGROUND: Very little is known on how changes in circadian rhythms evolve. The noctuid moth Spodoptera frugiperda (Lepidoptera: Noctuidae) consists of two strains that exhibit allochronic differentiation in their mating time, which acts as a premating isolation barrier between the strains. We investigated the genetic basis of the strain-specific timing differences to identify the molecular mechanisms of differentiation in circadian rhythms. RESULTS: Through QTL analyses we identified one major Quantitative trait chromosome (QTC) underlying differentiation in circadian timing of mating activity. Using RADtags, we identified this QTC to be homologous to Bombyx mori C27, on which the clock gene vrille is located, which thus became the major candidate gene. In S. frugiperda, vrille showed strain-specific polymorphisms. Also, vrille expression differed significantly between the strains, with the rice-strain showing higher expression levels than the corn-strain. In addition, RT-qPCR experiments with the other main clock genes showed that pdp1, antagonist of vrille in the modulatory feedback loop of the circadian clock, showed higher expression levels in the rice-strain than in the corn-strain. CONCLUSIONS: Together, our results indicate that the allochronic differentiation in the two strains of S. frugiperda is associated with differential transcription of vrille or a cis-acting gene close to vrille, which contributes to the evolution of prezygotic isolation in S. frugiperda.

Pheromonal divergence between two strains of Spodoptera frugiperda.

Spodoptera frugiperda consists of two genetically and behaviorally different strains, the corn- and the rice-strain, which seem to be in the process of sympatric speciation. We investigated the role of strain-specific sexual communication as a prezygotic mating barrier between both strains by analyzing strain-specific variation in female pheromone composition of laboratory and field strains, and also male attraction in wind tunnel and field experiments. Laboratory-reared and field-collected females from Florida exhibited strain-specific differences in their relative amount of (Z)-7-dodecenyl acetate (Z7-12:OAc) and (Z)-9-dodecenyl acetate (Z9-12:OAc). In wind tunnel assays, we did not find strain-specific attraction of males to females. However, in field experiments in Florida, we observed some differential attraction to synthetic pheromone blends. In a corn field, the corn-strain blend attracted more males of both strains than the rice-strain blend, but both blends were equally attractive in a grass field. Thus, habitat-specific volatiles seemed to influence male attraction to pheromones. In dose-response experiments, corn-strain males were more attracted to 2 % Z7-12:OAc than other doses tested, while rice-strain males were attracted to a broader range of Z7-12:OAc (2-10 %). The attraction of corn-strain males to the lowest dose of Z7-12:OAc corresponds to the production of this compound by females; corn-strain females produced significantly smaller amounts of Z7-12:OAc than rice-strain females. Although corn-strain individuals are more restricted in their production of and response to pheromones than rice-strain individuals, it seems that differences in sexual communication between corn- and rice-strain individuals are not strong enough to cause assortative mating.

bric à brac controls sex pheromone choice by male European corn borer moths.

The sex pheromone system of ~160,000 moth species acts as a powerful form of assortative mating whereby females attract conspecific males with a species-specific blend of volatile compounds. Understanding how female pheromone production and male preference coevolve to produce this diversity requires knowledge of the genes underlying change in both traits. In the European corn borer moth, pheromone blend variation is controlled by two alleles of an autosomal fatty-acyl reductase gene expressed in the female pheromone gland (pgFAR). Here we show that asymmetric male preference is controlled by cis-acting variation in a sex-linked transcription factor expressed in the developing male antenna, bric à brac (bab). A genome-wide association study of preference using pheromone-trapped males implicates variation in the 293 kb bab intron 1, rather than the coding sequence. Linkage disequilibrium between bab intron 1 and pgFAR further validates bab as the preference locus, and demonstrates that the two genes interact to contribute to assortative mating. Thus, lack of physical linkage is not a constraint for coevolutionary divergence of female pheromone production and male behavioral response genes, in contrast to what is often predicted by evolutionary theory.

Development of a simple and accurate molecular tool for Spodoptera frugiperda species identification using LAMP.

BACKGROUND: The fall armyworm, Spodoptera frugiperda is a native species of the Americas. First detected in western and central Africa in early 2016, it has become one of the most serious invasive lepidopteran pests in many African and Asian countries. S. frugiperda has spread very quickly; however, there are no molecular-based, simple and accurate diagnostic tools for identification of this species in the field. Methods to identify invasive S. frugiperda are urgently needed because farmers and agricultural managers have no prior experience with this pest. RESULTS: Based on mitochondrial genome sequence alignment, a S. frugiperda-specific sequence region was identified in the transfer RNA-coding region between NADH dehydrogenase, ND3, and ND5. Using this unique region, species-diagnostic primers were designed and applied in a loop-mediated isothermal amplification (LAMP) assay and a conventional polymerase chain reaction to identify field-collected samples of S. frugiperda. The optimal incubation conditions for the LAMP assay were 61°C for 90 min with four LAMP primers; an additional loop primer increased the amplification efficiency. A response was obtained for a wide range of DNA concentrations in the LAMP assay and the minimum detectable DNA concentration was 10 pg. CONCLUSIONS: We developed a new LAMP-based molecular diagnostic method that it is easy to use and accurate. The LAMP assay was used with a DNA-releasing technique for larval and adult samples, without a DNA extraction step, by incubating the tissue sample at 95°C for 5 min. This method can be applied in intensive field monitoring of S. frugiperda and its ecological studies. © 2021 Society of Chemical Industry.

Sexual communication of Spodoptera frugiperda from West Africa: Adaptation of an invasive species and implications for pest management.

The pest species Spodoptera frugiperda, which is native to North and South America, has invaded Africa in 2016. The species consists of two strains, the corn-strain and rice-strain, which differ in their sexual communication. When we investigated populations from Benin and Nigeria, consisting of corn-strain and rice-corn-hybrid descendants, we found no strain-specific sexual communication differences. Both genotypes exhibited the same pheromone composition, consisting of around 97% (Z)-9-tetradecenyl acetate (Z9-14:Ac), 2% (Z)-7-dodecenyl acetate (Z7-12:Ac), and 1% (Z)-9-dodecenyl acetate (Z9-12:Ac), they had similar electrophysiological responses, and all mated around three hours into scotophase. However, we found geographic variation between African and American populations. The sex pheromone of African corn-strain and hybrid descendant females was similar to American rice-strain females and showed higher percentages of the male-attracting minor component Z7-12:Ac. In addition, African males exhibited the highest antennal sensitivity towards Z7-12:Ac, while American males showed highest sensitivity towards the major pheromone component Z9-14:Ac. Increasing the production of and response to the critical minor component Z7-12:Ac may reduce communication interference with other African Spodoptera species that share the same major pheromone component. The implications of our results on pheromone-based pest management strategies are discussed.

Geographic variation in sexual attraction of Spodoptera frugiperda corn- and rice-strain males to pheromone lures.

The corn- and rice-strains of Spodoptera frugiperda exhibit several genetic and behavioral differences and appear to be undergoing ecological speciation in sympatry. Previous studies reported conflicting results when investigating male attraction to pheromone lures in different regions, but this could have been due to inter-strain and/or geographic differences. Therefore, we investigated whether corn- and rice-strain males differed in their response to different synthetic pheromone blends in different regions in North America, the Caribbean and South America. All trapped males were strain-typed by two strain-specific mitochondrial DNA markers. In the first experiment, we found a nearly similar response of corn- and rice-strain males to two different 4-component blends, resembling the corn- and rice-strain female blend we previously described from females in Florida. This response showed some geographic variation in fields in Canada, North Carolina, Florida, Puerto Rico, and South America (Peru, Argentina). In dose-response experiments with the critical secondary sex pheromone component (Z)-7-dodecenyl acetate (Z7-12:OAc), we found some strain-specific differences in male attraction. While the response to Z7-12:OAc varied geographically in the corn-strain, rice-strain males showed almost no variation. We also found that the minor compound (Z)-11-hexadecenyl acetate (Z11-16:OAc) did not increase attraction of both strains in Florida and of corn-strain males in Peru. In a fourth experiment, where we added the stereo-isomer of the critical sex pheromone component, (E)-7-dodecenyl acetate, to the major pheromone component (Z)-9-tetradecenyl acetate (Z9-14:OAc), we found that this compound was attractive to males in North Carolina, but not to males in Peru. Overall, our results suggest that both strains show rather geographic than strain-specific differences in their response to pheromone lures, and that regional sexual communication differences might cause geographic differentiation between populations.