Cucurbit Genetics Cooperative Report 4:26-28 (article 14) 1981
N. N. Shinde* and V. S. Seshadri
Division of Vegetable Crops and Floriculture, Indian Agricultural Research Institute, New Delhi-11012, India
An attempt was made to verify the observations of Rosa (2), Kubicki (1), and Wall (3) regarding the extent of association between monoecious sex expression and oblong fruit shape and of andromonoecious sex expression with round fruit shape. Further, it was possible to study the existence of linkage, if any, of hermaphroditic sex expression and the extent of association of flat fruit shape with any of these sex forms. In these studies, the fruit shape was characterized as flat, round and oblong on the basic of shape index (polar diameter/equatorial diameter) of < 1.0, around 1.0 and > 1.0 respectively.
In F2 populations derived from monoecious x andromonoecious crosses involving plants with oblong and round fruit, linkage was detected and estimated in monoecious-1 x andromonoecious-2 in coupling phase and monoecious-4 x andromonoecious-1 in repulsion phase. The recombination fraction for the former was 0.310 ± 0.072 in straight combination and 0.247 ± 0.068 in reciprocal combination, whereas in monoecious-4 x andromonoecious-1, it was 0.699 ± 0.084 indicating absence of linkage in the reciprocal combination. In andromonoecious-2 x monoecious-3 in coupling phase, no linkage was detected.
In the cross monoecious flat x andromonoecious round, linkage was detected in the reciprocal cross with a recombination fraction of 0.643 ± 0.080, while no linkage was detected in monoecious flat x andromonoecious oblong. The recombination obtained in both coupling and repulsion phase was quite high, 24 to 31% and 63 to 69%, respectively, indicating thereby very loose or no linkage between sex of the plant and fruit shape. The p-value obtained was more than 0.50 viz. 0.699 ± 0.084 and 0.643 ± 0.080 and they were within the range of 5% standard error. This is indicative that there was no linkage.
These observation indicate only loose linkage of oblong and round shapes with monoecious and andromonoecious sex types. Two possibilities are indicated. The genes for monoecious and andromonoecious sex forms appear to be located on different chromosomes or a second possibility is that genes for fruit shape are located in the same chromosome where genes for sex are present. The spatial distance between these two loci could not be estimated. Data clearly indicate that the chances of recombinations through crossover are comparatively high and linkage in coupling phase can at best be termed as a loose one. However, the flat fruit shape did not seem to have any particular kind of association with any of the two sex forms as did those of oblong and round fruit shapes.
No linkage was detected in the round x oblong crosses involving andromonoecious (Ga) and hermaphrodite (ga) parents differing at the G locus.
In a monoecious (GA) x hermaphrodite (ga) cross, the segregation of sex is assumed to be mainly on two major genes (A and G) and fruit shape on one gene basis. In a hermaphorodite-2 and monoecious-3 cross representing parents in coupling phase, linkage with A gene was detected with a recombination fraction of 0.358 ± 0.057 and no linkage was detected between G gene and fruit shape. In the repulsion phase, the recombination fraction was 0.649 ± 0.067 for A gene and no linkage was detected with G gene in a cross between monoecious-4 x hermaphrodite-1. Similarly, no linkage was noticed in monoecious-2 x hermaphrodite-1 involving parents with flat x oblong fruit shape, either with the A or G loci.
On the basis of these results obtained in three different sets of crosses, it can be concluded that there appears to be very loose linkage between A gene with oblong fruit shape and recessive a gene with round fruit shape. However, there is no linkage with G gene as has been noted from the nonsignificant X2 values for linkage in monoecious x hermaphrodite and andromonoecious x hermaphrodite crosses. Flat fruit shape was also not found to be associated with any of the sex forms.
Wall’s observation (3) that muskmelon fruit shape was determined by single gene with incomplete dominance and it was linked in coupling phase with genes for sex forms were not corroborated by these results. High recombinant fraction obtained in the present studies clearly ruled out the possibility of pleiotropy suggested by Rosa (2). Results of Kubicki (1) and Wall (3) were related to the cultivated varieties of New World Origin and hence an impression was gained of the possibility of sex linked inheritance. The present studies gave credit to some kind of loose, and probably chance, association of fruit shape in monoecious and andromonoecious sex forms and it might have been due to unconscious selection during domestication and evolution of present day cultivars. There is clear evidence against the hypothesis of sex linked inheritance with fruit shape.
Literature Cited
- Kubicki, B. 1962. Inheritance of some characters in muskmelon (C. melo L.) Genet. Polon. 3: 265-274. (Plant Breeding Abs. 1964, no. 1162).
- Rosa, J. T. 1928. Inheritance of flower types in Cucumis and Citrullus. Hilgardia 3: 235-250.
- Wall, J. R. 1967. Correlated inheritance of sex expression and fruit shape in Cucumis. Euphytica 16: 199-208.
* Presently at the Department of Horticulture, Marathwada Agricultural University, PARBHANI-431402, India.