Cucurbit Genetics Cooperative Report 19:67-69 (article 25) 1996
Jiannong Zhang
Melon Research Institute, Gansu University of Agriculture, Lanzhou, Gansu, 730070, P.R. China
Introduction: Watermelon (Citrullus lanatus (Thunb.) Matsum and Nakai) is divided into two types of crops in China: common watermelon for flesh; and watermelon grown for seeds for human consumption (edible seed). Considerable diversity for seed size is found among the available germplasm. The largest varieties have seed in excess of 11.5 x 17.0 mm (width x length, mean of 10 seed), and the smallest varieties have mean seed width and length of 2.6 x 4.2 mm, respectively. The expression of this character (seed size) is controlled by genetic differences between varieties, and is affected by environment and cultural conditions. In this experiment, two varieties, expressing extreme phenotypes for seed size, were crossed and the inheritance of the trait analyzed. These results will provide help to breeders working on seed size, especially for edible seed watermelons.
Materials and Methods: Parents and progeny: P1 : tomato seed watermelon (ts) (furnished by Dr. B. Rhodes, Clemson University, Clemson, SC, USA); mean of 10 seeds, width x length = 2.6 x 4.2 mm. P2 : ‘GN-1’ (GN); edible seed watermelon; self-pollinated; mean of 10 seeds, width x length = 11.5 x 17.6 mm. Progeny included: F1 = P1 x P2 , and the reciprocal cross, P2 x P1 ; F2 = F1 self-pollinated; BC1 (F1 x P1 ) and BC2 (F1 x P2 ). Field plots were grown from 1993 to 1995 at the Tiaoshan Farm in Gansu. Seed size (width and length) were recorded in September, 1995.
Results and Discussion: The results are presented in Table 1, and graphically in Figure 1. A t-test showed no statistical difference between the F1 progeny from the reciprocal crosses [P1 (ts) x P2 (GN) and P2 (GN) x P1 (ts)], negating cytoplasmic inheritance and allowing the progeny from the two crosses to be analyzed as one. The regression analysis showed that width and length are controlled by the same genes (r = 0.9898). The F1 seeds were all medium in size between the two parents; the F2 progeny segregated in a ratio of 1:2:1 (small:medium:large);; BC1 (ts x F1 ) segregated 1:1 (medium:large). These results fit a pattern of inheritance of a single gene with two alleles, and incomplete dominance between alleles. In addition, when the F2 , BC1 and BC2 progeny are compared in Figure 1, the group of small seeds in BC1 (F1 x ts) are smaller than the corresponding group in the F2 population, and the group of large seeds in BC2 (F1 x GN) are larger than the corresponding group among the F2 progeny. These comparisons suggest that several modifier or minor genes influence seed size, depending upon the parents involved in each cross.
Table 1. Segregation of seed size in the F2 and backcross populations of the cross ts x GN, from field plots at the Tiaoshan Farm, Gansu, P.R. China 1995.
Observed Number |
Ratio |
|||||||
Small |
Medium |
Large |
Small |
Medium |
Large |
X2z |
P |
|
| P1y | 20 | 0 | 0 | all | – | – | ||
| P1x | 0 | 0 | 20 | all | – | – | ||
| F1w | 0 | 40 | 0 | all | – | – | ||
| F2v | 24 | 48 | 15 | 1 | 2 | 1 | 2.7931 | 0.05 |
| BC1u | 32 | 42 | 0 | 1 | 1 | 0 | 1.3514 | 0.05 |
| BC2t | 0 | 57 | 60 | 0 | 1 | 1 | 0.0342 | 0.05 |
zX20.05 = 3.84 (v = l), = 5.99 (v = 2)
y Tomato seed watermelon = ts
x GN-1 = GN
w ts x GN and the reciprocal cross
v F1 self-pollinatedu F1 x ts
u F1 x ts
t F1 x GN
Literature cited
- Rhodes, B. and X. Zhang. 1995. Gene list for watermelon. cucurbit Genet. Coop. Rpt. 15:69-84.
- Suzuki, D.T., A.J.F. Griffiths, and R.C. Lewontin. 1981. Gene analysis, 2nd Edition, P. 123-162.