Cucurbit Genetics Cooperative Report 7:97-99 (article 44) 1984

Nuclear DNA Variation in Cucumis Species

Ramachandran, C.

Department of Agricultural Botany, University College of Wales, Aberystwyth, U.K.

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.
Species		Source	2n	Total DNA in picograms
Asiatic	C. trigonus (syn. C. callosus)	India	14	1.590
	C. sativus	India	14	1.777
	C. sativus var. hardwickii	IVT No. 1753	14	1.798
African	C. melo var. agrestis	IVT No. 1987	24	2.483
	C. melo var. utilissimus	India	24	2.358
	C. melo var. momordica	India	24	2.291
	C. metuliferus	IVT No. 1775	24	2.391
	C. anguria var. longipes	IVT No. 1735	24	1.587
	C. africanus	IVT No. 1984	24	1.782
	C. ficifolius	IVT No. 1801	24	1.373
	C. meeusi	IVT No. 1800	48	3.203
	C. dinteri	IVT No. 1794	24	2.167
	C. dipsaceus	IVT No. 1774	24	2.448
	C. figarei	IVT No. 1804	48	3.886
	C. zeyheri	IVT No. 0162	24	1.682
	C. zeyheri	IVT No. 1053	48	2.846
	C. sagitattus	IVT No. 2069	24	1.571
	C. prophetarum	India	24	1.656
	C. humifructus	South Africa	24	2.455
	C. heptadactylus	IVT No. 1798	48	2.225

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.

Literature Cited

1. 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- 256.
2. 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.
3. Ramachandran, C. 1983. Cytogenetical studies in Cucumis. Ph.D. Thesis, submitted to Indian Agricultural Research Institute, New Dehli, India (unpublished).
4. Trivedi, R.N. and R.P. Roy. 1970. Cytogenetical studies in Cucumis and Citrullus. Cytologia 35:561-569.