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.

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 DIMBOAenriched 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.

Microbes have many mechanisms to exert their inhibitory activity against target pests. One such mechanism involves the production and secretion of hydrolytic enzymes, such as chitinases, which are produced naturally by plants in response to attack by insect herbivores and phytopathogens and have been sought as an additional factor to enhance pest management. Thus, our main aim was to screen the diverse actinomycete community associated with the integument of Acromyrmex subterraneus brunneus for a chitinase-producing strain and to characterize its chitinases. We identified isolate ENT-21-a Streptomyces sp.-as a chitinase-producer and our data indicate that this isolate produces a chitinolytic complex that contains a chitinase and a high-molecular-weight β-N-acetylglucosaminidase (>100 kDa) when cultured in Chitin-Czapek broth. The presence of chitinases in the genome of this isolate was checked by diagnostic PCR, and two chitinase genes belonging to family 18 group A and family 19 were verified. The chitinolytic activity of the crude extract was observed at pH values ranging from 3.8 to 11.0, with the highest chitinase activities recorded at pH 9.0 and 9.5, whereas optimum β-N-acetylglucosaminidase activity was observed over a narrow pH range, between pH 4.7 and 5.1. We describe some biochemical and molecular properties of the chitinase and β-N-acetylglucosaminidase produced by ENT-21, and discuss the potential for exploitation of these enzymes for pest control.

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