Cucurbit Genetics Cooperative Report 7:84-85 (article 37) 1984

Isozyme Studies Indicate that the Genus Cucurbita is an Ancient Tetraploid

Weeden, N.F.

New York State Agricultural Experiment Station, Cornell University, Geneva, NY 14456

The high chromosome number (2n=40) of Cucurbita suggests that this genus may be of polyploid origin. Evidence for allotetraploidy in several Cucurbita species was provided in cytogenetic studies of Weiling (3). However, remarkably few examples of characters controlled by paired loci have been reported. Genetic analysis of isozyme phenotypes provides an excellent method for identifying gene duplications, for in many isozyme systems the number of loci expressed in diploid plants appears to be conserved (1). For example, 4 genes are normally involved in the expression of the aspartate aminotransferases in the plant cell, subunits of the cytosolic-, plastid-, mitochondrial- and microbody-specific isozymes each being coded by a separate locus (Weeden, unpublished). Twice this number of loci would be expected in tetraploid plants. The additive expression of diploid genomes in a tetraploid has been most clearly demonstrated in the recent allotetraploid Tragopogon miscellus (2).

Eight isozyme systems (aspartate aminotransferase, glucose phosphate isomerase, phosphoglucomutase, 6-phosphogluconate dehydrogenase, NAD-malate dehydrogenase, triose phosphate isomerase, NADP-isocitrate dehydrogenase and shikimic dehydrogenase) were selected for a study of gene expression in Cucurbita. In diploid plants each system is specified by a predictable number of genes (Table 1), usually one for each subcellular compartment in which the enzyme is found. The total number of gene products predicted in a diploid plant for the 8 systems is 16. Preliminary analysis of Cucumis sativus (2n=14) indicated that 17 loci were being expressed. In contrast, genetic studies on the isozyme phenotypes of Cucurbita maxima, C. ecuadorensis, C. pepo, C. moschata and C. palmata demonstrated that at least 28 loci were contributing isozymic forms. Similar studies on isozyme expression in C. pepo and C. texana have produced additional evidence for gene duplication in these species (T.C. Andrus and H.D. Wilson, unpublished).

Table 1. Comparison of isozyme loci expressed in known diploids and Cucurbita.
Isozyme system	# loci predicted in a diploid	# loci found in Cucumis	# loci found in Cucurbita
AAT	4	4	7
GPI	2	2	4
PGM	2	2	3
6PGD	2	2	3
MDH*	2	2	4
TPI	2	3	4
IDH	1	1	2
SKDH	1	1	1
Totals	16	17	28
*Cytosolic and mitochondrial forms only, the microbody-specific form was not resolved.

Although duplication of specific genes or chromosomal segments has been described in a considerable number of species (1), it is unlikely that a series of such events generated the extensive gene duplication observed in Cucurbita. Many of the loci involved in the isozyme systems investigated assort independently (Weeden, unpublished), thus indicating that a large portion of the genome would have had to have been duplicated. The results of the present study in conjunction with the high chromosome number in this genus and the cytogenetic findings of Weiling provide compelling evidence for the allotetraploid origin of Cucurbita.

Literature Cited

1. Gottlieb, L.D. 1982. Conservation and duplication of isozymes in plants. Science 216:373-380.
2. Roose, M.L. and L.D. Gottleib. 1976. Genetic and biochemical consequences of polyploidy in Tragopogon. Evolution 30:818:830.
3. Weiling, F. 1959. Genomanalytische Untersuchungen bei Kuerbis (Cucurbita L.). Der Zuechter 29:161-179.