RNA-Seq analysis and transcriptome assembly for Blackberry (Rubus sp. Var. Lochness) fruit

There is an increasing interest in berries, especially blackberries in the diet, because of recent reports of their health benefits due to their high content of flavonoids. A broad range of genomic tools are available for other Rosaceae species but these tools are still lacking in the Rubus genus, thus limiting gene discovery and the breeding of improved varieties. De novo RNA-seq of blackberry grown under field conditions was performed using Illumina Hiseq 2000. Almost 9 billion nucleotide bases were sequenced in total. Following assembly, 42,062 unigenes were detected. For functional annotation, 33,040 (NR), 32,762 (NT), 21,932 (Swiss-Prot), 20,134 (KEGG), 13,676 (COG), 24,168 (GO) unigenes were annotated using different databases; in total 34,552 annotated unigenes were identified. For protein prediction analysis, the number of coding DNA sequences (CDS) that mapped to the protein database was 32,540. Non redundant (NR), annotation showed that 25,418 genes (77.5%) has the highest similarity with Fragaria vesca subespecies vesca. A reanalysis was undertaken by aligning the reads with this reference genome for a deeper analysis of the transcriptome. In this reanalysis, 67,845 SNP were found, 462 alternative splicing events were analyzed, and the distribution of the reads on the reference chromosomes was established. We demonstrated that de novo assembly, using Trinity and later annotation with Blast using different databases, were complementary to alignment to the reference sequence using SOAPaligner/SOAP2. The Fragaria reference genome belongs to a species of the same family as blackberry (Rosaceae) but to a different genus. Since blackberries are tetraploids, the possibility of artefactual gene chimeras resulting from mis-assembly was tested with one of the genes sequenced by RNAseq, Chalcone Synthase (CHS). cDNAs encoding this protein gene were cloned and sequenced. Primers designed on the assembled sequences accurately distinguished different contigs, at least for the chalcone synthase genes. We prepared and analysed transcriptome data from ripe blackberries, for which prior genomic information was limited. This sequence information will improve the knowledge of this important and healthy fruit, providing an invaluable new tool for biological research.