Shape Changes During Fruit Development in Cucurbita and their Importance in the Study of Shape Inheritance

Abstract

(1) During fruit development from early ovary primordium to maturity, in disk fruited and sphere fruited plants of Cucurbita pepo, there is a slight progressive increase in degree of flattening, or ratio of equatorial diameter to polar diameter. (2) Fruits will stop growth and become mature at widely different sizes if environmental conditions vary or if there is segregation for size factors. The mature fruit retains the shape index characteristic of the developmental stage which it had attained when growth ceased. Thus the segregation of genetic factors controlling shape is rendered much less distinct if there are size differences in the population. (3) By determining shape indices for small ovary primordia instead of for mature fruit, a population segregating for shape may be studied at a uniform size and the variability due to size differences may be eliminated. This method has the added advantages of making possible a much larger number of determinations for each plant, and of eliminating minor shape differences which often appear during development. (4) In three F2 pedigrees showing monofactorial segregation for disk and sphere fruit shape, a comparison of the plotted indices of young primordia, 1 to 8 cc in volume, with those of mature fruits from the same plant showed in every case a much sharper segregation for shape in the primordia than in the fruits. (5) It is believed that this method of studying earlier stages in development will make possible a more accurate analysis of shape inheritance in cases where the genetic situation is more complex.