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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.
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