Cucurbit Genetics Cooperative Report 7:97-99 (article
Nuclear DNA Variation in Cucumis Species
Department of Agricultural Botany, University College of Wales,
The genus Cucumis includes two sets of species with basic
chromosome numbers x = 7 and x = 12, respectively. While the
former group is indigenous to Asia, the latter group is believed
to be native of Africa. The evolution and speciation within the
genus and in particular the relationship between the species with
different chromosome numbers is not clear.
Divergence and evolution of species of higher plants is
accompanied by large variation in nuclear DNA amounts. A survey
of 2C DNA amounts in 20 Cucumis species was done and the
results are reported in this communication.
The author expresses his gratitude to the institute of
Horticultural Plant Breeding, Wageningen, Netherlands for the
supply of seeds of Cucumis species. The seeds were
germinated in an incubator at 24 C and primary roots were fixed
in 4 per cent formaldehyde for 2 hr at room temperature. The
roots were washed in distilled water (several changes) for 24 hr
and then fixed in acetoalcohol (1:3) for another 24 hr. Fixed
roots can be stored in the same fixative in a refrigerator at 4 C
until required. Roots were washed in distilled water and
hydrolyzed in 5N HCl at room temperature for one hr. After
hydrolysis the roots were stained in Feulgen stain (pH 2.2) for
one hour and washed with three changes of SO2 water,
10 minutes for each change. The root tips were transferred to
distilled water and the meristems squashed in a drop of glycerol.
The DNA measurements were made on a Vickers M85
microdensitometer. Fifteen 2C nuclei were measured in each of
the three replicates in each species. The estimated DNA values
were corrected to picograms using Allium cepa (2C = 33.05
pg) as a standard (2).
In Table 1, the total nuclear DNA amounts in 16 diploid and four
tetraploid species of Cucumis are presented. The
estimates vary from 1.373 to 3.886 pg. The African (2n = 24)
group has species with both lower and higher DNA amounts than the
Asiatic species. The botanical varieties within a particular
species do not differ significantly for DNA content.
Table 1. Nuclear DNA content of Cucumis species.
Total DNA in picograms
C. trigonus (syn. C. callosus)
C. sativus var. hardwickii
IVT No. 1753
C. melo var. agrestis
IVT No. 1987
C. melo var. utilissimus
C. melo var. momordica
IVT No. 1775
C. anguria var. longipes
IVT No. 1735
IVT No. 1984
IVT No. 1801
IVT No. 1800
IVT No. 1794
IVT No. 1774
IVT No. 1804
IVT No. 0162
IVT No. 1053
IVT No. 2069
IVT No. 1798
The two economically important vegetables in this genus, viz.
cucumber (C. sativus) and muskmelon (C. melo),
differ for their nuclear DNA contents. Hence, neither the
fragmentation hypothesis of origin of muskmelon from cucumber (1)
nor the fusion hypothesis of origin of cucumber from muskmelon
(4) is supported. The detailed cytological studies at mitotic
metaphase, pachytene and meiotic metaphase stages and also the
Giemsa C-banding of somatic chromosomes in cucumber and muskmelon
have revealed the untenability of the above two hypotheses (3).
Another possibility of having any relationship between these two
species is amplification or deletion of DNA segments within the
chromosomes followed by species divergence. A more detailed
study which would include quantitative estimation of different
nuclear components (satellite sequence, middle repetitive DNA and
nonrepetitive DNA) would be useful to assess the taxonomical
relationship between these two species. The distribution of
these sequences as revealed by in situ hybridization
experiments would give valuable information regarding the
chromosome evolution in these species.
- Bhaduri, P.N. and P.C. Bose. 1947. Cytogenetical investigations
in some cucurbits with special reference to fragmentation of
chromosomes as a physical basis of speciation. J. Genet. 48:237-
- McLeish, J. and N. Sunderland. 1961. Measurements of
deoxyribonucleic acid in higher plants by Feulgen photometry and
chemical methods. Exp. Cell Res. 24:527-540.
- Ramachandran, C. 1983. Cytogenetical studies in Cucumis. Ph.D.
Thesis, submitted to Indian Agricultural Research Institute, New
Dehli, India (unpublished).
- Trivedi, R.N. and R.P. Roy. 1970. Cytogenetical studies in Cucumis and Citrullus. Cytologia 35:561-569.