Geminivirus had probably arisen by interspecific recombination between


            Recombination is a process of
genetic exchange which allows for a rapid evolution of new pathogenic or
virulent strains, especially in geminiviruses. Recombination
plays a key role in driving evolution of many begomoviral DNA. Recombination,
especially inter-specific homologous recombination, is a key contributor to the
genomic diversification and evolution of begomoviruses (Harrison
& Robinson, 1999). The recent emergence of new begomovirus diseases
is due to the high rate of recombination. Evidence for recombination and
pseudorecombination events in the family Geminiviridae
is extensive (Stanley et al., 1986;
Sung and Coutts, 1995; Padidam et al.,
1999; Fondong et al., 2000; Unseld et al., 2000a,b). One of the earliest
pieces of evidence for recombination among geminiviruses was obtained from
studies of a severe mosaic disease of cassava in Uganda (Zhou et al. 1997).
Sequence analysis revealed that a virus responsible for the disease, East African cassava mosaic virus-Uganda
(EACMV-UG) had probably arisen by interspecific recombination between East African cassava mosaic virus
(EACMV) and African cassava mosaic virus (ACMV).
The important contribution of recombination to geminivirus evolution is well
established (Umaharan et al., 1998;
Padidam et al., 1999) and it is
suspected that it is directly responsible for the emergence of many of the most
agriculturally damaging begomovirus species complexes (Zhou et al., 1997; Monci et al., 2002; Garcia-Andres et
al., 2006, 2007). Recombination of geminiviruses is a very frequent and
widespread phenomenon and occurs between species as well as within and across
the genera, and is a significant contributor to begomovirus evolution. More
than 170 geminiviruses are known to infect plants (Briddon and Markham, 1995;
Padidam et al., 1999), and many more
remain to be characterized, particularly from weed species, which can act as
reservoirs of virus populations. Plant virus evolution is affected by various
factors including virus-vector and virus-host interactions. Three factors could
contribute significantly to recombination, namely mixed infections, high levels
of viral replication, and increased host range of the vector.

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            The cotton leaf curl epidemic in Pakistan is caused by several distinct
variants, with recombination events involving Okra yellow vein mosaic virus (OYVMV) and other unspecified
geminiviruses having probably been involved in their evolution (Zhou et al.,
1998). Evidence for recombination was found for the AC1 and CP ORFs and for the
non-coding intergenic region (IR). Origin of replication is a major recombination
point in the IR, but not the only one. Analyses of the IR suggest that
recombinants may be frequent in the population and that recombination may have
an important role in the generation of CLCuV variability (Sanz et al.,
1999).          Analysis of the genome of the natural
recombinant of Tomato yellow leaf curl Sardinia virus (TYLCSV) and Tomato
yellow leaf curl virus (TYLCV) showed that the recombination sites are
located in the intergenic region (Monci et al., 2002). Kirthi et al. (2002). Girish and Usha, (2005)
have analysed recombination events between legume-infecting begomoviruses from
South and Southeast Asia.    Mixed infection can contribute towards
disease severity as different viruses might display synergistic interactions.
As in the case of CMD, the mixed-infected samples always showed extremely
severe CMD symptoms, suggesting a synergistic interaction between virus
strains/species (Pita et al., 2001).

et al. (2014b) has revealed that the bipartite begomovirus BYVDV has sequences
derived from ToLCNDV and BYVMV viruses which were confirmed in all six
detection methods in RDP programme. They observed that the only DNA B component
associated with bhendi begomovirus is derived through recombination between
different DNA B components of ToLCNDV.  In case of BVYBhY, a very distinct bhendi
virus, two recombination events confirmed by six methods were detected in AC3, AC5
region with CYVMV-Rajasthan and RhYVMV as major and minor parents. Venkataravanappa
et al. (2014a, b), suggested that BYVHV has entire coat protein sequences derived
from one isolate of BYVMV (previously referred to as OYVMV) and the rest of the
genome from ToLCNDV. The BYVKnV is a recombinant between BYVMaV and OELCuV the
event detected was
in the right half of the genome extending upto AC3. The analysis of OELCuV
that most of its sequences originate from BYVMV, BYVBhV and MeYVMV. For all
the OELCuV isolates, the sequences containing the origin of replications
from BYVMV, including the iteron sequences. For only one isolate of OELCuV
(GU1119996) a small fragment is derived from ToLCNDV. Rishishwar et al. (2015)
recorded presence of BYVMV in infected samples in Kalyani (west Bengal) and
Aurangabad (Maharastra) but MeYVMV in samples from Varanasi (Uttar Pradesh)
and Jalgaon (Maharastra). Interestingly, they
found that the Jalgaon isolate of
MeVYMV showed a recombination event between MeYVMV as major parent and
Malvastrum yellow vein Yunnan virus (MaYVYV, AJ786711) as a minor parent. Vinoth-Kumar
et al. (2017a) has shown evidence for inter-specific and inter-strain
recombination events and suggested that cotton infecting and bhendi infecting
viruses may share a common ancestor.  Serfez et al. (2015) analyzed OELCuV isolates
from Pakistan and found that OELCuV is a recombinant between BYVMV and OELCuV.



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