Characterization of Bemisia tabaci (Gennadius) (hemiptera: aleyrodidae), for genetic variability, endosymbionts and vector-virus interactions in cassava

Abstract

Cassava is one of the important tuber crops cultivated all over the World. Cassava Mosaic Disease (CMD) is the most important limiting factor in its production. Silverleaf whitefly, Bemisia tabaci (Gennadius) is the vector responsible for the transmission of Cassava mosaic virus in cassava, which causes CMD. Genetic variation among the members of B. tabaci, makes them very difficult to manage. Endosymbionts present in the whitefly system could be a factor responsible for making them a successful sucking pest. There are various kinds of interactions existing between whitefly and the CMV. Studying these interactions precisely will help to understand the behavioural and physiological variations in whiteflies. In this background the present study, “Characterization of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae), for genetic variability, endosymbionts and vector-virus interactions in cassava” was proposed and carried out at the Department of Agricultural Entomology, College of Horticulture, Vellanikkara, during Mrch 2014 to April 2016, with the objectives to analyse the genetic variability in cassava whitefly, characterization of its endosymbionts and elucidation of cassava whitefly - cassava mosaic virus interactions. Various life stages of B. tabaci were collected from different cassava growing agro ecological zones of Kerala and reared in laboratory as well as in polyhouse at optimum conditions. Genetic variability study was conducted with 10 selected ISSR primers which had shown polymorphism in their banding pattern; with amplicon size ranged between 200bp to 2900bp. Phylogenetic analysis using NTsys software revealed the presence of two major clusters with Sultan Bathery population as out group. Similarity matrix had shown up to 49 per cent variation between the samples. Polymerase chain reaction using mitochondrial cytochrome oxidase1 primers, C1-J2195 and L2-N-3014 had given amplicon of 850bp. Nucleotide sequences had shown variation up to 16.5 per cent and dendrogram generated out of the sequences using MEGA-6 (Neighbor Joining Method) gave two clusters and one out group. Sequence similarity check using reference sequences from NCBI data base indicated the presence of two biotypes, AsiaI and AsiaII5 in cassava plants of Kerala. Morphometric studies were conducted to assess the variations in different pupal and adult characters of thirteen whitefly populations. Significant variations were found in pupal length and pupal width of the biotypes. Pupal length varied between 0.746 mm to 0.668 mm and pupal width varied between 0.539 mm to 0.468 mm in female pupa. Out of 14 characters of pupa studied, variations in length and width were found to be significant. Among seven characters of adults studied, variations in wing, antennal length, body length and width were significant. AsiaI biotype was found to have lesser body length, but more width compared to AsiaII5. AsiaII5 was found to be an important biotype of B. tabaci infesting cassava in 12 out of the 13 locations surveyed. Endosymbiont characterization from whitefly using Next Generation Sequencing (NGS) - Illumina platform revealed the variations in microbiota. At phylum level, Proteobacteria was found at 87.57 per cent in whitefly populations collected from plains. The populations from high ranges contained Firmicutes at 82.67 per cent. Arsenophonus, an ‘indirect helper’ for virus spread by protecting viral coat protein from degradation in insect system with their GroEL chaperones were found at 24. 69 per cent in B. tabaci populations collected from plains. Behavioural and life cycle variation study of B. tabaci using six cassava genotypes had shown that virus infection in B. tabaci altered the dispersal and settling. Speed of movement observed to be maximum at 16.25 cm/s in non- virulent female whiteflies on the genotype CMR-9. Life cycle of virulent and non-virulent whiteflies was found to vary between 19.57 days to 30.77 days. A thorough understanding of genetic variations, endosymbiont diversity and behavioural response to virus could help the researchers in planning proper management strategies for B. tabaci. In future, information generated of such kinds could also help the researchers and policy makers to foresee and manage any possible outbreak of the pest and avoid any havoc caused by them.