Cucurbit Genetics Cooperative Report 7:100-101 (article 45) 1984
Visser, D.L. and A.P.M. den Nijs
Institute for Horticultural Plant Breeding (IVT), P.O.B. 16, 6700 AA Wageningen, The Netherlands
Spontaneous fruit and seed set was observed in one out of ten plants of Cucumis ficifolius A. Rich (Genebanknumber 2061, IVT collection from Kenya), grown under isolation in an insect proof glasshouse in the summer of 1981. Under our conditions this monoecious wild species normally sets fruit only after artificial pollination. The single exceptional plant was selfed and used as pollen parent in crosses with several sibs of the same accession.
Thirty plants of a progeny obtained after selfing were examined in 1982, and all possessed perfect flowers in addition to staminate ones, so they were andromonoecious. Fruits set following inadvertent self pollinations contained on average 35 seeds. Not a single pistillate flower was observed on any of the 30 plants. All 27 plants of a progeny of a sib cross were monoecious, as was the selfed progeny of the monoecious female parent of this cross. The andromonoecious character thus appeared to be a recessive mutant character.
To investigate the inheritance of the trait, three monoecious plants of the sib cross were selfed and backcrossed with andromonoecious plants out of the self progeny of the putative mutant plant.
The resulting offspring was classified for sex expression, shown in Table 1. From the results it can be concluded that a single recessive gene governs andromonoecy in this species. We propose to designate the gene: andromonoecy, symbol a. This symbol is the same as in melon and watermelon, whereas in cucumber the symbol m has been given priority (2).
Table 1. Segregation of andromonoecious sextype in crosses in C. ficifolius.
| Population | Number of plants | Probability (%) | |
|---|---|---|---|
| Monoecious | Andromonoecious | ||
| F2 (monoecious x andromonoecious) | x2 (3:1) | ||
| 1 | 85 | 32 | 50-70 |
| 2 | 36 | 6 | 10-20 |
| 3 | 98 | 23 | 95 |
| Homogeneity | 20-30 | ||
| Total | 219 | 70 | 70-90 |
| (monoecious x andromonoecious) x andromonoecious | x2 (1:1) | ||
| 1 | 30 | 21 | 20-30 |
| 2 | 50 | 35 | 10-20 |
| 3 | 33 | 52 | 1-5 |
| Homogeneity | 1-5 | ||
| Total | 113 | 108 | 90-95 |
Counting of metaphase plates in root tips revealed 48 chromosomes in accession Gbn 2061, so it is a tetraploid like almost all accessions of C. ficifolius in our collection (Kroon, unpublished results). The apparently disomic inheritance of gene a testifies to the allotetraploid nature of the species. Meiotic chromosome studies of related tetraploid species revealed allotetraploidy (1), but no genetic segregation data have so far been presented to support this conclusion.
Monoecy is the prevalent sex expression in the genus Cucumis, C. melo excepted, and we have never before observed andromonoecy in any wild species of our collection. Rosa (3) considered monoecy as primitive in C. melo, from which the andromonoecious condition of the cultivated melons evolved. Our finding of one andromonoecious individual in one out of nine accessions of C. ficifolius presents another case of parallel evolution in the sex expression of the cucurbits.
Literature Cited
- Dane, F. and T. Tsuchiya. 1979. Meiotic chromosome and pollen morphological studies of polyploid Cucumis species. Euphytica 28:563-567.
- Robinson, R.W., H.M. Munger, T.W. Whitaker and G.W. Bohn. 1976. Genes of the Cucurbitaceae. HortScience 11:554-568.
- Rosa, J.T. 1928. The inheritance of flower types in Cucumis and Citrullus. Hilgardia 3:233-250.