Sawant, A. C.
With the advanced knowledge of hybrid vigor and its application to a wide variety of plants, particularly those of food values more and more scientists are searching for inexpensive and easy means of exploiting this phenomenon. The objective is to find methods for mass production of hybrid seeds.
In a dioecious species like asparagus or papaya the objective is obtained merely by interplanting the two sex forms of complimentary varieties, while in corn an additional operataion of detasseling has to be performed unless the female parent line is male-sterile. In the case of hermaphroditic plants like tomato, geric or cytoplasmic male sterility has been utilized to emasculate the female parent. But outside its native range the tomato has no natural pollinating agent. Logically, hybrid tomato seeds are produced where labor is cheap for hand-pollination. Recently a new idea has come up to produce seeds in a region where there is a natural insect vector of tomato pollen and then distribute the seeds to other regions where there is no such vector activity. And here may lie the real crux of the matter.
The formation and fusion of any two gametes are acts dependent on a set of micro and macro environmental factors. These factors operate from the time of early meiosis and determine the amount of pairing, the number and localization of chiasma, amount and type of pollen abortion and amount and type of spem reaching the eggs. Another set of factors probably determines which fertilized eggs will develop into mature seeds. As a result the seeds produced in a given region may not perform the same as those in the region of hybrid crop production.
The opportunity for the differential action of such factors is doubtless greater in naturally cross-pollinated, highly heterozygous crops, but some may also operate in self-pollinated species. In the summer of 1954 I grew at Davis, California two groups of F1 hybrids between L. esculentum var. San Marzano and L. hirsutum f. glabratum from Banos, Ecuador, the latter being used as the male parent. One group was from seeds obtained by crossing plants in the field, the other from those growing in the greenhouse. Both parents are strictly self-pollinating, but may possess some residual heterozygosity. The two groups of hybrids, however, showed small differences with respect to the habit of growth, leaf and stipule size, the intensity of anthocyanin, and the angle of leaf inclination -- most of which are quantitative characters. In a visit to England and the Netherlands in early 1956 I came across similar but more extensive and detailed observations made by workers there.
Apart from factual observations, it is a foregone conclusion based on principles of cytology and natural selection that such differences could occur, and there is no reason to believe that these principles cannot be extended to plant breeding in general.