Differential gene expression between viruliferous and non-viruliferous Schizaphis graminum (Rondani)

Abstract

An experiment was performed to measure the effect of Cereal Yellow-Dwarf Virus (CYDV), strain CYDV-RPV, on gene expression in its insect vector, greenbug aphid (Schizaphis graminum (Rondani)). RNA was sampled in three replicates from four treatments (biotypes B and H with or without carried CYDV), at 0, 1, 2, 3, 5, 10, 15 and 20 days from the introduction of carrier and virus-free greenbugs to uninfected wheat cv. ‘Newton’. Illumina paired-end sequencing produced 1,840,820,000,000 raw reads that yielded 1,089,950,000 clean reads, which were aligned to two greenbug, Trinity transcriptome assemblies with bowtie2. Read counts to contigs were analyzed with principal components and with DESeq2 after removing contaminating contigs of wheat or microbial origin. Likelihood ratio tests with one transcriptome showed that CYDV influenced gene expression about seven-fold less than time or biotype, which were approximately equal. With the other transcriptome, virus, time, and biotype were about equally important. Pairwise comparisons of virus to no virus for each timepoint yielded estimates of fold-change that comprised expression profiles for each contig when ordered by timepoint. Hierarchical clustering separated expression profiles into 20 groups of contigs that were significantly differentially expressed for at least one timepoint. Contigs were also sorted by timepoint of maximally differential expression between virus and no virus. All contigs that were significantly differentially expressed at FDR = 0.05 were annotated by blast searches against NCBI nr and nt databases. Interesting examples of up-regulation with virus included a lysosomal-trafficking regulator, peptidylprolylisomerase, RNA helicase, and two secreted effector proteins. However, carried virus did not consistently change aphid gene expression overall. Instead there was complex interaction of time, biotype, host response, and virus.