Evidence of Recombinant Citrus tristeza virus Isolate Occurring in Acid Lime cv. Pant Lemon Orchard in Uttarakhand Terai Region of Northern Himalaya in India

Codon Usage Bias Analysis of Citrus tristeza Virus: Higher Codon Adaptation to Citrus reticulata Host

Citrus tristeza virus (CTV), a member of the aphid-transmitted closterovirus group, is the causal agent of the notorious tristeza disease in several citrus species worldwide. The codon usage patterns of viruses reflect the evolutionary changes for optimization of their survival and adaptation in their fitness to the external environment and the hosts. The codon usage adaptation of CTV to specific citrus hosts remains to be studied; thus, its role in CTV evolution is not clearly comprehended. Therefore, to better explain the host–virus interaction and evolutionary history of CTV, the codon usage patterns of the coat protein (CP) genes of 122 CTV isolates originating from three economically important citrus hosts (55 isolate from Citrus sinensis, 38 from C. reticulata, and 29 from C. aurantifolia) were studied using several codon usage indices and multivariate statistical methods. The present study shows that CTV displays low codon usage bias (CUB) and higher genomic stability. Neutrality plot and relative synonymous codon usage analyses revealed that the overall influence of natural selection was more profound than that of mutation pressure in shaping the CUB of CTV. The contribution of high-frequency codon analysis and codon adaptation index value show that CTV has host-specific codon usage patterns, resulting in higheradaptability of CTV isolates originating from C. reticulata (Cr-CTV), and low adaptability in the isolates originating from C. aurantifolia (Ca-CTV) and C. sinensis (Cs-CTV). The combination of codon analysis of CTV with citrus genealogy suggests that CTV evolved in C. reticulata or other Citrus progenitors. The outcome of the study enhances the understanding of the factors involved in viral adaptation, evolution, and fitness toward their hosts. This information will definitely help devise better management strategies of CTV.

Analyses of 3' half genome of citrus tristeza virus reveal existence of distinct virus genotypes in citrus growing regions of India

Citrus tristeza virus (CTV, genus Closterovirus) is one of the most serious pathogens responsible for huge loss of citrus trees worldwide. Four Indian CTV isolates, Kat1 (C. reticulata/Central India), D1 (C. sinensis/North India), B5 (Citrus limettoides/South India) and G28 (C. lemon/Northeast India) collected from different regions of India were characterized based on sequencing of 3' half genome (~ 8.4 kb) comprising 10 open reading frames (ORFs2-11) and 3' UTR and the sequences were submitted to NCBI database as Acc. No KJ914662, HQ912022, HQ912023 and KJ914661, respectively. The present and previously reported Indian isolates Kpg3 and B165 were analyzed and compared with other Asian and international CTV isolates. The Indian CTV isolates had 92-99% nt identities among them. The phylogenetic analysis generated overall ten genogroups/lineages. Of them, all the Asian isolates fell into seven genogroups, whereas the Indian isolates into four. Indian isolates Kat1, D1 and Kpg3 grouped together, termed "Kpg3Gr", along with Florida severe isolate T3. The Indian isolates B5, and G28 were found to be two distinct and separate lineages, indicating that these isolates are two new CTV entities. Based on phylogenetic analysis, Kpg3Gr was identified as "Indian VT" subtype which is distinct from the Asian and the Western VT subtype within diversified VT genotype. The recombination detecting-program, RDP4 detected Indian isolates Kat1, B5, B165 and G28 as recombinants, where G28 as strong recombinant. The present study determined the occurrence of at least four CTV genotypes, B5 (distinct), B165 (T68 type) G28 (distinct) and Kpg3Gr in citrus growing regions of India.

High incidence of citrus tristeza virus in mandarin (Citrus reticulata) in North-East states of India

Khasi mandarin is one of the most remunerative crop in North Eastern Hills region of India playing a very critical role in the socio-economic upliftment of the people. The orchards of this region were found to express the typical symptoms of citrus tristeza virus (CTV). An extensive survey of the Khasi mandarin orchard for six North Eastern states of India namely Arunachal Pradesh, Meghalaya, Assam, Nagaland, Sikkim and Tripura, was carried out to establish the identity of these viruses using ELISA and PCR/RT-PCR techniques. Out of 300 Khasi mandarin tree samples collected from the six states, 172 were found to be positive for CTV infection by DAS-ELISA indicating 57.33% overall CTV disease incidence. Results revealed presence of CTV in all the surveyed states showing a maximum incidence of 66.00% in Arunachal Pradesh followed by 62.00% in Assam, 60.00% in Meghalaya and Nagaland, 54.00% in Sikkim and 42.33% in Tripura. Higher CTV concentration was recorded in the age group > 15 years (69.09%) followed by 10-15 (57%) and 5-10 years (43.33%). However, this study, to the best of our knowledge, is the first report for the detection of CTV in Khasi mandarin from Arunachal Pradesh, Nagaland and Tripura and also the first authentic survey of overall disease incidence of CTV Khasi mandarins from the six major mandarin growing North Eastern states of India.