In sub-Saharan Africa, maize is a staple food and key determinant of food security for smallholder farming communities. Pest and disease outbreaks are key constraints to maize productivity. In September 2011, a serious disease outbreak, later diagnosed as maize lethal necrosis (MLN), was reported on maize in Kenya. The disease has since been confirmed in Rwanda and the Democratic Republic of Congo, and similar symptoms have been reported in Tanzania, Uganda, South Sudan, and Ethiopia. In 2012, yield losses of up to 90% resulted in an estimated grain loss of 126,000 metric tons valued at $52 million in Kenya alone. In eastern Africa, MLN was found to result from coinfection of maize with Maize chlorotic mottle virus (MCMV) and Sugarcane mosaic virus (SCMV), although MCMV alone appears to cause significant crop losses. We summarize here the results of collaborative research undertaken to understand the biology and epidemiology of MLN in East Africa and to develop disease management strategies, including identification of MLN-tolerant maize germplasm. We discuss recent progress, identify major issues requiring further research, and discuss the possible next steps for effective management of MLN.
Aphids are emerging as model organisms for both basic and applied research. Of the 5,000 estimated species, only three aphids have published whole genome sequences: the pea aphid Acyrthosiphon pisum, the Russian wheat aphid, Diuraphis noxia, and the green peach aphid, Myzus persicae. We present the whole genome sequence of a fourth aphid, the soybean aphid (Aphis glycines), which is an extreme specialist and an important invasive pest of soybean (Glycine max). The availability of genomic resources is important to establish effective and sustainable pest control, as well as to expand our understanding of aphid evolution. We generated a 302.9 Mbp draft genome assembly for Ap. glycines using a hybrid sequencing approach. This assembly shows high completeness with 19,182 predicted genes, 92% of known Ap. glycines transcripts mapping to contigs, and substantial continuity with a scaffold N of 174,505 bp. The assembly represents 95.5% of the predicted genome size of 317.1 Mbp based on flow cytometry. Ap. glycines contains the smallest known aphid genome to date, based on updated genome sizes for 19 aphid species. The repetitive DNA content of the Ap. glycines genome assembly (81.6 Mbp or 26.94% of the 302.9 Mbp assembly) shows a reduction in the number of classified transposable elements compared to Ac. pisum, and likely contributes to the small estimated genome size. We include comparative analyses of gene families related to host-specificity (cytochrome P450's and effectors), which may be important in Ap. glycines evolution. This Ap. glycines draft genome sequence will provide a resource for the study of aphid genome evolution, their interaction with host plants, and candidate genes for novel insect control methods.
Chromosomal inversion polymorphisms are thought to play a role in adaptive divergence, but the genes conferring adaptive benefits remain elusive. Here we study 2La, a common polymorphic inversion in the African malaria vector Anopheles gambiae. The frequency of 2La varies clinally and seasonally in a pattern suggesting response to selection for aridity tolerance. By hybridizing genomic DNA from individual mosquitoes to oligonucleotide microarrays, we obtained a complete map of differentiation across the A. gambiae genome. Comparing mosquitoes homozygous for the 2La gene arrangement or its alternative (2L+a), divergence was highest at loci within the rearranged region. In the 22 Mb included within alternative arrangements, two ∼1.5 Mb regions near but not adjacent to the breakpoints were identified as being significantly diverged, a conclusion validated by targeted sequencing. The persistent association of both regions with the 2La arrangement is highly unlikely given known recombination rates across the inversion in 2La heterozygotes, thus implicating selection on genes underlying these regions as factors responsible for the maintenance of 2La. Polymorphism and divergence data are consistent with a model in which the inversion is maintained by migration-selection balance between multiple alleles inside these regions, but further experiments will be needed to fully distinguish between the epistasis (coadaptation) and local adaptation models for the maintenance of 2La.
Recently, a few insects, including the caterpillar larva of the greater wax moth Galleria mellonella , have been identified as avid ‘plastivores’. These caterpillars are able to ingest and metabolize polyethylene at unprecedented rates. While it appears that G. mellonella plays an important role in the biodegradation process, the contribution of its intestinal microbiome remains poorly understood and contested. In a series of experiments, we present strong evidence of an intricate relationship between an intact microbiome, low-density polyethylene (LDPE) biodegradation and the production of glycol as a metabolic by-product. First, we biochemically confirmed that G. mellonella larvae consume and metabolize LDPE, as individual caterpillars fed on polyethylene excreted glycol, but those excretions were reduced by antibiotic treatment. Further, while the gut bacterial communities remained relatively stable regardless of diet, we showed that during the early phases of feeding on LDPE (24–72 h), caterpillars exhibited increased microbial abundance relative to those starved or fed on their natural honeycomb diet. Finally, by isolating and growing gut bacteria with polyethylene as their exclusive carbon source for over 1 year, we identified microorganisms in the genus Acinetobacter that appeared to be involved in this biodegradation process. Taken collectively, our study indicates that during short-term exposure, the intestinal microbiome of G. mellonella is intricately associated with polyethylene biodegradation in vivo .
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