| Summary: | 碩士 === 國立中興大學 === 植物病理學系 === 84 === Papaya ringspot virus (PRSV), a member of the Potyvirus
genus, contains two major groups, type P ( PRSV P) and type W
(PRSV W) virus. The host range of PRSV W strains is limited
to Chenopodiaceae and Cucurbitaceae, whereas P type strains
infect Cariceae (papaya) in addition. In order to develop
transgenic cucurbits with resistance to PRSV W in Taiwan,
typical PRSV W isolates from the island were collected and the
variability in the nucleotide and amino acid sequences of
the CP genes were analyzed. Twelve isolates from the forty
ELISA positive samples, collected from six cucurbit crops
from different areas of Taiwan, were identified by host
reactions and serology tests as typical W type virus isolates.
They did not infect papaya but were serologically
indistinguishable from type P isolates when tested against the
antisera to PRSV P type or PRSV W type. In order to analyze the
polymorphisms of the CP genes of the 12 PRSV W type isolates
from Taiwan and other reported PRSV W and P type viruses,
digestion patterns of RT-PCR products,which were amplified
from the N-terminal half of the CP genes, with RsaI, AluI and
NlaIII were compared. The results showed that most of the 12
PRSV W isolates from Taiwan were closely related to PRSV P-
YK, a P type strain from Taiwan, and P-PD, a P type strain
from Thailand,and far apart from P-HA, a P type strain from
Hawaii, and W-FL, a W type strain from Florida. To further
analyze the variation of the CP genes of PRSV W isolates from
Taiwan, three isolates, PRSV W-CI (from Chiayi), W-TN (from
Tainan) and W-PT ( from Pingtung), were chosen for cDNA
cloning for determination of their CP gene sequences.
Comparison the three isolates of Taiwan with each other,
indicated that they shared 96.37-96.99% and 94.46-96.74%
nucleotide and amino acid identity, respectively, and the
nucleotide identity of the 3' non-coding regions were
98.56-99.04%, indicating that the three isolates were
closely related strains of the same virus. Comparison of
the three W isolates from Taiwan with other reported W isolates
of W-FL (from Florida) and W-AU (from Australia), and P isolates
of P- YK (from Yung Kang, Taiwan), P-HA (from Hawaii), and P-FL
(from Florida) revealed that the nucleotide identity of the
CP genes of the three W type isolates from Taiwan shared
higher percentages of 95.93-96.64% with P-YK, another P type
isolate from Taiwan, they shared lower percentages of
90.59-91.97% with other non-Taiwan PRSV W and P type isolates.
In terms of amino acid identity of the CP genes, the three PRSV
W type isolates from Taiwan, shared 95.21-96.16% with PRSV P-
YK. The comparison of non-Taiwan W and P type isolates showed
that they had 95.12-99.3% amino acid identity. The
comparison of the 3' non-coding regions of the eight PRSV
isolates, revealed that the three PRSV W isolates from Taiwan
shared 97.21-98.08% nucleotide identity with P-YK, whereas
they shared only 88.51-93.3% nucleotide identity with other
non-Taiwan PRSV P and W isolates. The sequences analyses
indicated that the relationships among these W and P type
isolates could be divided into two groups, one contained PRSV
W-CI, W-TN, W-PT and P-YK, the other one contained PRSV W-AU,
W-FL, P-HA and P-FL. These coupled with enzyme-digestion
polymorphisms indicated that the CP genes of the three W type
isolates from Taiwan were closely related to P-YK(a Taiwan
isolate) and far apart from W and P type isolates in other
different geographic areas. Our results also implicate that
degrees of variation of the CP genes do not follow major
differences in host specificity, such as papaya or non-papaya
infecting, but are more closely corresponding to geographic
distribution.
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