Background The western flower thrips, Frankliniella occidentalis (Pergande), is a globally invasive pest and plant virus vector on a wide array of food, fiber, and ornamental crops. The underlying genetic mechanisms of the processes governing thrips pest and vector biology, feeding behaviors, ecology, and insecticide resistance are largely unknown. To address this gap, we present the F. occidentalis draft genome assembly and official gene set. Results We report on the first genome sequence for any member of the insect order Thysanoptera. Benchmarking Universal Single-Copy Ortholog (BUSCO) assessments of the genome assembly (size = 415.8 Mb, scaffold N50 = 948.9 kb) revealed a relatively complete and well-annotated assembly in comparison to other insect genomes. The genome is unusually GC-rich (50%) compared to other insect genomes to date. The official gene set (OGS v1.0) contains 16,859 genes, of which ~ 10% were manually verified and corrected by our consortium. We focused on manual annotation, phylogenetic, and expression evidence analyses for gene sets centered on primary themes in the life histories and activities of plant-colonizing insects. Highlights include the following: (1) divergent clades and large expansions in genes associated with environmental sensing (chemosensory receptors) and detoxification (CYP4, CYP6, and CCE enzymes) of substances encountered in agricultural environments; (2) a comprehensive set of salivary gland genes supported by enriched expression; (3) apparent absence of members of the IMD innate immune defense pathway; and (4) developmental- and sex-specific expression analyses of genes associated with progression from larvae to adulthood through neometaboly, a distinct form of maturation differing from either incomplete or complete metamorphosis in the Insecta. Conclusions Analysis of the F. occidentalis genome offers insights into the polyphagous behavior of this insect pest that finds, colonizes, and survives on a widely diverse array of plants. The genomic resources presented here enable a more complete analysis of insect evolution and biology, providing a missing taxon for contemporary insect genomics-based analyses. Our study also offers a genomic benchmark for molecular and evolutionary investigations of other Thysanoptera species.

Citrus greening or Huanglongbing (HLB) is caused by the phloem-limited intracellular Gram-negative bacterium Candidatus Liberibacter asiaticus (CLas). HLB-infected citrus phloem cells undergo structural modifications that include cell wall thickening, callose and phloem protein induction, and cellular plugging. However, very little is known about the intracellular mechanisms that take place during CLas cell-to-cell movement. Here, we show that CLas movement through phloem pores of sweet orange (Citrus sinensis) and grapefruit (Citrus paradisi) is carried out by the elongated form of the bacteria. The round form of CLas is too large to move, but can change its morphology to enable its movement. CLas cells adhere to the plasma membrane of the phloem cells specifically adjacent to the sieve pores. Remarkably, CLas was present in both mature sieve element cells and nucleated nonsieve element cells. The sieve plate plugging structures of host plants were shown to have different composition in different citrus tissues. Callose deposition was the main plugging mechanism in the HLB-infected flush, where it reduced the open space of the pores. In the roots, pores were surrounded by dark extracellular material, with very little accumulation of callose. The expression of CALLOSE SYNTHASE7 and PHLOEM PROTEIN2 genes was upregulated in the shoots, but downregulated in root tissues. In seed coats, no phloem occlusion was observed, and CLas accumulated to high levels. Our results provide insight into the cellular mechanisms of Gram-negative bacterial cell-to-cell movement in plant phloem.

No abstract

Tomatoes (Solanum lycopersicum L.) have been bred to exude higher amounts or different types of the specialized plant metabolites, acylsugars, from type IV trichomes. Acylsugars are known to deter several herbivorous insect pests, including the western flower thrips (WFT), Frankliniella occidentalis (Pergande); however, all previous studies investigated the effect of acylsugars on leaves, or acylsugar extracts obtained from leaves. In spite of the WFT predilection for flowers, there is a gap in knowledge about flower defenses against thrips damage. This is especially important in light of their capacity to acquire and inoculate viruses in the genus Orthotospovirus, such as Tomato spotted wilt orthotospovirus (TSWV), in flowers. Therefore, we turned our attention to assessing thrips oviposition differences on flowers of 14 entries, including 8 interspecific hybrids, 5 tomato lines bred for specific acylsugar-related characteristics (type IV trichome densities, acylsugar amount, sugar moiety and fatty acid profile), and a fresh market tomato hybrid, Mt. Spring, which only produces trace amounts of acylsugars. Our results show that the density of the acylsugar droplet bearing type IV trichomes is greatest on sepals, relative to other flower structures, and accordingly, WFT avoids oviposition on sepals in favor of trichome-sparse petals. In concordance with past studies, acylsugar amount was the most important acylsugar-related characteristic suppressing WFT oviposition. Certain acylsugar fatty acids, specifically i-C5, i-C9 and i-C11, were also significantly associated with changes in WFT oviposition. These results support continued breeding efforts to increase acylsugar amounts and explore modifications of fatty acid profile and their roles in deterring thrips oviposition. The finding that acylsugar production occurs and reduces thrips oviposition in tomato flowers will be important in efforts to use acylsugar-mediated resistance to reduce incidence of orthotospoviruses such as TSWV in tomato by deterring virus transmission and development of thrips vector populations in the crop.