Diversity and Phylogenetic Relationships of Full Genome Sequences of Cassava Brown Streak Viruses in Kenya

Abstract

Cassava brown streak disease is caused by cassava brown streak virus (CBSV) and Uganda cassava brown streak virus (UCBSV). Many of the CBSV and UCBSV diversity studies utilize partial coat protein sequences due to the unavailability of representative full genome sequences. Hence, there is little information on the diversity of cassava brown streak viruses in the rest of the genomes of the two species that are present in the farmers’ fields. The aim of this study was to determine Kenyan full CBSV and UCBSV genomes, and their sequence diversity and phylogenetic relationships within various genome and genome segments. Twenty four CBSVs positive samples tested by RT PCR from major cassava producing regions in Kenya were sequenced using Illumina MiSeq. Quality assessment of the output reads was done using the CLC Genomics 5.5.1 software programs. Genome assembly was done by de novo and reference guided assembly. Nucleotide sequence similarity of CBSV and UCBSV was determined. Phylogenetic relationships between CBSV and UCBSV were determined by performing the neighbour-joining analysis using MEGA 6.0 software. Six CBSV and 9 UCBSV genomes were generated from this study. The coat protein of the CBSV sequences had nucleotide sequence similarity of 92-100% while P1 and P3 gene had 75-100% and 76-100%, respectively. The coat protein of the UCBSV sequences had nucleotide sequence similarity of 91-99%. P1 and P3 gene had 83-100% and 86-99%, respectively. The phylogenetic analysis of full genomes revealed two distinct clusters one for UCBSV and another cluster for CBSV. Individual gene segments phylogenetic tree resembled that of the whole genome by clustering the nucleotide sequences into two clusters, one belonging to UCBSV and the other CBSV. The study revealed an average genome nucleotide diversity of 21.5% and 15.8% in CBSV and UCBSV respectively. A vast genetic diversity observed in CBSV and UCBSV in this study portends a lot of challenges in developing molecular diagnostic techniques as well as control strategies against the viruses.