Transcriptome differential co-expression reveals distinct molecular response of fall-armyworm strains to DIMBOA.

BACKGROUND: 2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one (DIMBOA), the main benzoxazinoid found in corn, elicits variable larval responses from different pest moths. For the widespread and highly polyphagous Spodoptera frugiperda (Lepidoptera: Noctuidae), the fall-armyworm (FAW), DIMBOA acts as a feeding stimulant and improves larval growth at low concentrations. The FAW present two host plant-related strains, corn and rice strains, related to host preference on corn and other Graminae or rice. Based on both host preference and strain divergence of the FAW on corn, a cereal containing DIMBOA, and rice, lacking this compound, we question if corn and rice strains larvae respond equally toward DIMBOA. We evaluated differential expression in the transcriptome of both midgut and fat body larval tissues of the two strains reared on either DIMBOA-enriched artificial diet or control diet and inferred Bayesian networks. RESULTS: We found differences in performance between corn and rice strain larvae reared on DIMBOA, as well as several differentially regulated contigs annotated as esterases, peptidases, transferases and reductases, all of them known for being related to responses of lepidopterans and other insects to DIMBOA. We also found a UDP-glucuronosyltransferase very similar to others found in many lepidopterans occupying a central hub within a transferase Bayesian network, suggesting that it is essential to an effective response to DIMBOA in FAW. CONCLUSION: Our results suggest that there is an intrinsic cost for FAW rice strain larvae to metabolize corn-originated hydroxamic acids, which could have resulted in the partial host-associated genetic isolation found at FAW field populations.

Loci under selection and markers associated with host plant and host-related strains shape the genetic structure of Brazilian populations of Spodoptera frugiperda (Lepidoptera, Noctuidae).

We applied the ddRAD genotyping-by-sequencing technique to investigate the genetic distinctiveness of Brazilian populations of the noctuid moth Spodoptera frugiperda, the fall armyworm (FAW), and the role of host-plant association as a source of genetic diversification. By strain-genotyping all field-collected individuals we found that populations collected from corn were composed primarily of corn-strain individuals, while the population collected from rice was composed almost entirely of rice-strain individuals. Outlier analyses indicated 1,184 loci putatively under selection (ca. 15% of the total) related to 194 different Gene Ontologies (GOs); the most numerous GOs were nucleotide binding, ATP binding, metal-ion binding and nucleic-acid binding. The association analyses indicated 326 loci associated with the host plant, and 216 loci associated with the individual strain, including functions related to Bacillus thuringiensis and insecticide resistance. The genetic-structure analyses indicated a moderate level of differentiation among all populations, and lower genetic structure among populations collected exclusively from corn, which suggests that the population collected from rice has a strong influence on the overall genetic structure. Populations of S. frugiperda are structured partially due to the host plant, and pairs of populations using the same host plant are more genetically similar than pairs using different hosts. Loci putatively under selection are the main factors responsible for the genetic structure of these populations, which indicates that adaptive selection on important traits, including the response to control tactics, is acting in the genetic differentiation of FAW populations in Brazil.