Cucurbit Genetics Cooperative Report 12:18-19 (article
Electrophoretic Examination of Cucumis sativus
L. and Cucumis melo L.
V.S. Sujatha and V.S. Seshadri
Division of Vegetable
Crops, Indian Agricultural Research Institute, New Delhi
The genus Cucumis contains 30 species of which only two
species, C. sativus (cucumber) and C. melo
(muskmelon), are extensively cultivated. While C. melo
has chromosome number of x=12, and C. sativus has
x=7, the attempts to produce interspecific hybrids between
the two have not succeeded (2,3). An isozymic analysis was
designed to compare the two taxa and determine whether justification
exists to classify them as a single genus. Taxa were compared
using peroxidase ( PRX), glutamate oxaloacetate transaminase
(GOT) and esterase (EST).
Methods. Polyacrylamide gel electrophoresis was performed
using varietal slab gels (ADCO, India) at 5°C under
40 MV in the following manner. Peroxidase (PRX) was sampled
from the root and hypocotyl region of 4 to 5 week-old seedlings.
Gels consisted of 7% acrylamide, and electrophoresis was
carried out using a Tris-chloride gel buffer (pH 8.3).
Gels were stained according to Conklin and Smith (1). Glutamate
oxaloacetate transaminase was sampled from 3 to 4 day-old
seedling, and electrophoresis was performed using 9.5% gels.
Gel and electrode buffers were the same as those used for
peroxidase, and staining procedures were those of Shaw and
Koen (6). Esterase was sampled from 3 to 4 day old seedling
and extracts electrophoresed on a 7.0% gel. Gel and electrode
buffers were the same as above, and staining was performed
according to Shaw and Koen (6).
Results. In the peroxidase system, C. sativus
was lacking in the fastest moving PRX which was present
in all the Cucumis species (x=12; data not shown).
The allozymes of C. sativus were observed at PRX2,
PRX3 and PRX4, corresponding to the three loci of C.
melo. However, allozymes of C. sativus were
not similar in mobility to those of C. melon (Fig.1).
The two taxa shared a common band at GOT4. This allozyme
was common to the 13 Cucumis species studied and
absent in the other general in the Cucurbitaceae
like Citrullus, Luffa, Momordica, Praecitrullus, Lagenaria
(data not presented). The allozyme at GOT1 present in C.
melo was absent in C. sativus. The allozyme
at GOT2 of C. sativus had identical mobility with
the hybrid isozyme at GOT2 in C. melo. However,
since the banding pattern at GOT2 in C. melo was
identified to be a hybrid type, the allozyme at GOT2 of
C. sativus was treated as having a different subunit
constitution than that of the hybrid allozyme of C.
melo. The allozymes at GOT3 also differed in mobility.
There were no similarities between the two taxa in esterase
Data suggest that there is little similarity between C.
melo and C. sativus for the 3 enzymes studied.
However, isozyme constitution at GOT4 in both species was
characteristic of the genus Cucumis, and justifies
their classification under the genus Cucumis. This
conclusion contrasts to that of Pangalo (4), who suggested
that the two Cucumis species should be elevated
to generic status because of their wide variability, non-crossability,
and chromosome number differences. Also, Ramachandran and
Seshadri (5) consider C. sativus cytogenetically
very different from C. melo. Our data (common
band at GOT4) of C. melo and C. sativus
lends support to the proposition that these widely divergent
taxa remain under one genus.
Fig. 1. Comparison of zymograms of C. sativus (S) and C. melo (M) for peroxidase (a), esterase (b), and glutamate oxaloacetate transaminase (c).
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