Determination of the bacterial flora as a microbial control agent of Toxoptera aurantii (Homoptera: Aphididae)

The genetic adaptations of Toxoptera aurantii facilitated its rapid multiple plant hosts dispersal and invasion

Toxoptera aurantii Boyer de Fonscolombe (Hemiptera: Aphididae) can attack many plant hosts, including tea (Camellia sinensis L.), citrus (Citrus spp.), lychee (Litchi chinensis Sonn.), banana (Musa spp.), and pineapple (Ananas comasus L.) among others. It is a widely distributed hexapod and one of the most destructive pests in tea plantations, causing enormous economic losses in tea production each year. A high-quality reference genome is important to study the phylogenetics and evolution of T. aurantii because its genome is highly heterozygous and repetitive. We obtained a de novo genome assembly of T. aurantii at the chromosome level using a combination of long Nanopore reads from sequencing with high-throughput chromosome conformation capture technology. When finally assembled, the genome was 318.95 Mb on four chromosomes with a 15.19 Mb scaffold N50. A total of 12,162 genes encoded proteins, while there were 22.01% repetitive sequences that totaled 67.73 Mb. Phylogenetic analyses revealed that T. aurantii and Aphis gossypii parted ways approximately 7.6 million years ago (Mya). We used a combination of long-read single-molecule sequencing with Hi-C-based chromatin interaction maps that resulted in a reference chromosomal level reference genome of T. aurantii that was high quality. Our results will enable the exploration of the genetics behind the special biological features of T. aurantii and also provide a source of data that should be useful to compare the compare genome among the Hemiptera.

Influence of reduced N-fertilizer application on foliar chemicals and functional qualities of tea plants under Toxoptera aurantii infestation

BACKGROUND

The tea aphid, Toxoptera aurantii (Boyer de Fonscolombe) is a polyphagous pest predominant in tea orchards and has become the most pernicious pest deteriorating tea quality. Nitrogen (N) is essential to plant growth improvement, and it can significantly impact plant defensive ability against aphid infestation. This study was designed to quantify the influence of reduced N-fertilizer application on foliar chemicals and functional quality parameters of tea plants against the infestation of T. aurantii. In this study, the tea seedlings (cv. Longjing43) were applied with normal level (NL) of N-fertilizer (240 kg N ha^-1) along with reduced N-fertilizer levels (70%NL and 50%NL), and with and without T. aurantii infestation.

RESULTS

The results showed that N-fertilizer application significantly affected plant biomass and photosynthetic indexes, foliar soluble nutrients and polyphenols, tea catechins, caffeine, essential amino acids, volatile organic compounds of tea seedlings, and the population dynamics of T. aurantii. Compared with the normal N-fertilizer level, the reduced N-fertilizer application (70%NL and 50%NL) significantly decreased all the foliar functional quality components of tea seedlings without aphid infestation, while these components were increased in tea seedlings with aphid infestation. Moreover, the transcript expression levels of foliar functional genes (including CsTCS, CsTs1, and CsGT1) were significantly higher in the NL, and significantly lower in the 50%NL for tea seedlings without aphid infestation, while the transcript expression levels were significantly higher in 50%NL in aphid inoculated tea seedlings.

CONCLUSION

The results demonstrated that the reduced N-fertilizer application could enhance foliar chemicals and functional quality parameters of tea plants especially with T. aurantii infestation, which can relieve soil nitrogen pressure and reduce pesticide use for control of tea aphid infestation in tea plantations.

Specialization on Ficus Supported by Genetic Divergence and Morphometrics in Sympatric Host-Populations of the Camellia Aphid, Aphis aurantii

Adaptation to different host plants is considered to be an important driver of the divergence and speciation of herbivorous insects. The application of molecular data and integrated taxonomic practices in recent years may contribute to our understanding of population divergence and speciation, especially for herbivorous insects considered to be polyphagous. Aphis aurantii is an important agricultural and forestry pest with a broad range of host plants. In this study, samples of A. aurantii feeding on different host plants in the same geographical area were collected, and their population genetic divergence and morphological difference were analyzed. Phylogenetic analysis and haplotype network analysis based on five genes revealed that the population on Ficus exhibited significantly genetic divergence from populations on other host plants, which was also supported by the statistical analysis based on measurements of 38 morphological characters. Our results suggest that A. aurantii has undergone specialized evolution on Ficus, and the Ficus population may represent a lineage that is experiencing ongoing sympatric speciation.

Phylogenetic relationship and characterization of the complete mitochondrial genome of the black citrus aphid, Aphis aurantii (Hemiptera: Aphididae)

The black citrus aphid Aphis aurantii is a major pest of citrus and tea plants. In this study, we determined the complete mitochondrial genome (mitogenome) of A. aurantii. The mitogenome was 15,469 bp in length, consisting of 13 protein-coding genes (PCGs), 22 transfer RNA (tRNA) genes, 2 ribosomal RNA (rRNA) genes, and a non-coding control region. Gene order of A. aurantii was highly conserved and identical to most other previously sequenced Aphididae. Initiation codons ATT, ATA, and ATG were identified in eight, three, and two PCGs, respectively, while stop codon TAA was found in 11 genes except for 2 genes (cox1 and nad4) which use incomplete stop codon T-. Phylogenetic analysis showed the topology (Pemphigidae + (Hormaphididae + (Greenideidae + Aphididae))) within Aphidoidea, and A. aurantii was more closely related to Aphis craccivora than to Aphis gossypii.

A large-scale field study of bacterial communities in cereal aphid populations across Morocco

Insects are frequently associated with bacteria that can have significant ecological and evolutionary impacts on their hosts. To date, few studies have examined the influence of environmental factors to microbiome composition of aphids. The current work assessed the diversity of bacterial communities of five cereal aphid species (Sitobion avenae, Rhopalosiphum padi, R. maidis, Sipha maydis and Diuraphis noxia) collected across Morocco, covering a wide range of environmental conditions. We aimed to test whether symbiont combinations are host or environment specific. Deep 16S rRNA sequencing enabled us to identify 17 bacterial operational taxonomic units (OTUs). The obligate symbiont Buchnera aphidicola was represented by five OTUs with multiple haplotypes in many single samples. Facultative endosymbionts were presented by a high prevalence of Regiella insecticola and Serratia symbiotica in S. avenae and Si. maydis, respectively. In addition to these symbiotic partners, Pseudomonas, Acinetobacter, Pantoea, Erwinia and Staphyloccocus were also identified in aphids, suggesting that the aphid microbiome is not limited to the presence of endosymbiotic bacteria. Beside a significant association between host species and bacterial communities, an inverse correlation was also found between altitude and α-diversity. Overall, our results support that symbiont combinations are mainly host specific.

Symbiont Diversity of Aphis (Toxoptera) citricidus (Hemiptera: Aphididae) as Influenced by Host Plants

Aphids are well known for their association with endosymbiont bacteria. Almost all aphids harbor Buchnera aphidicola as an obligate symbiont and several other bacteria as facultative symbionts. Associations of facultative symbionts and aphids are quite variable in terms of diversity and prevalence across aphid species. Facultative symbionts can have a major impact on aphid bioecological traits. A number of factors shape the outcome of the facultative symbiont-aphid association, including aphid clone, bacterial genotype, geography, and host plant association. The effects of host plant on aphid-facultative symbiont associations are the least understood. We performed deep sequencing of the bacterial community associated with field populations of the oligophagous aphid Aphis (Toxoptera) citricidus collected from different host plants. We demonstrate that (i) A. citricidus has low symbiont diversity, (ii) symbiont diversity is affected by host plant, and (iii) host plants affect the relative abundance of the obligate symbiont Buchnera and an unknown genus of Enterobacteriaceae.

Highly effective bacterial agents against Cimbex quadrimaculatus (Hymenoptera: Cimbicidae): isolation of bacteria and their insecticidal activities

Cimbex quadrimaculatus (Hymenoptera: Cimbicidae) is one of the serious pests of almonds in Turkey and worldwide. Since there is no effective control application against this pest, it has been a serious problem up to now. Therefore, we aimed to find an effective bacterium that can be utilized as a biocontrol agent against C. quadrimaculatus in pest management. We isolated seven bacteria from dead and live C. quadrimaculatus larvae, and evaluated the larvicidal potency of all isolates on the respective pest. Based on the morphological, physiological, biochemical and molecular properties (partial sequence of 16S rRNA gene), the isolates were identified to be Bacillus safensis (CQ1), Bacillus subtilis (CQ2), Bacillus tequilensis (CQ3), Enterobacter sp. (CQ4), Kurthia gibsonii (CQ5), Staphylococcus sp. (CQ6) and Staphylococcus sciuri (CQ7). The results of the larvicidal activities of these isolates indicated that the mortality value obtained from all treatments changed from 58 to 100 %, and reached 100 % with B. safensis (CQ1) and B. subtilis (CQ2) on the 3rd instar larvae within 10 days of application of 1.89 × 10(9) cfu/mL bacterial concentration at 25 °C under laboratory conditions. Findings from this study indicate that these isolates appear to be a promising biocontrol agent for C. quadrimaculatus.

Culturable bacterial microbiota of Plagiodera versicolora (L.) (Coleoptera: Chrysomelidae) and virulence of the isolated strains

Plagiodera versicolora (Laicharting, 1781) (Coleoptera: Chrysomelidae) is an important forest pest which damages many trees such as willow, poplar, and hazelnut. In order to find new microbes that can be utilized as a possible microbial control agent against this pest, we investigated the culturable bacterial flora of it and tested the isolated bacteria against P. versicolora larvae and adults. We were able to isolate nine bacteria from larvae and adults. The isolates were characterized using a combination of morphological, biochemical, and physiological methods. Additionally, we sequenced the partial sequence of the 16S rRNA gene to verify conventional identification results. Based on characterization studies, the isolates were identified as Staphylococcus sp. Pv1, Rahnella sp. Pv2, Rahnella sp. Pv3, Rahnella sp. Pv4, Rahnella sp. Pv5, Pantoea agglomerans Pv6, Staphylococcus sp. Pv7, Micrococcus luteus Pv8, and Rahnella sp. Pv9. The highest insecticidal activity against larvae and adults was obtained from M. luteus Pv8 with 50 and 40 % mortalities within 10 days after treatment, respectively. Extracellular enzyme activity of the bacterial isolates such as amylase, proteinase, lipase, cellulose, and chitinase was also determined. Consequently, our results show that M. luteus Pv8 might be a good candidate as a possible microbial control agent against P. versicolora and were discussed with respect to biocontrol potential of the bacterial isolates.