PhD.

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.