Hybrid cultivars
There are eight types of hybrid cultivars that have been described by Pickett (1998). The most commonly used types used for vegetables are F1, double- cross, triple-cross and synthetic hybrids. It is vital that the advice and instruc- tions of the plant breeder responsible for maintaining individual cultivars be adhered to during seed production of the various types of hybrids.
F1 hybrids
This type of hybrid is now available in a wide range of vegetable species; it is by far the most widely used hybrid technique used by vegetable breeders. An F1 hybrid is produced by crossing two distinct lines. In practice each of the two parent lines are the result of inbreeding. As far as commercial seed production is concerned, F1 hybrid cultivars are the result of crossing two inbred lines that have been main- tained under the control or supervision of plant breeders, and which are known to produce a desirable hybrid. Since the late 1940s or so, we have seen the increasing development and use of F1 hybrid vegetable cultivars. Riggs (1987, 1988) has reviewed the breeding of F1 vegetable cultivars. Choudhury (1966) discussed the exploitation of F1 vegetable hybrids developed for India and Takahashi (1987) has described the utilization of hybrid vegetable cultivars in Japan.
The advantages of F1 hybrid cultivars include uniformity, increased vigour, earliness, higher yield and resistance to specific pests and pathogens, although these factors are not always all present in any one cultivar of a vegetable spe- cies. Heat tolerance has been an important character in the development of F1 Chinese cabbage for tropical areas.
Theoretically, all the plants in an F1 hybrid cultivar resemble each other exactly, but because some self-pollination of the female parent used in the cross may take place, some plants that are not F1 hybrids may occur and are usually morphologically different. These off-types in an F1 hybrid are usually the result of accidental self-pollination of the female parent and are generally known as
‘sibs’. These are so-called because they are the result of ‘sister’ or ‘brother’
plants crossing or selfing with the female line. The incidence of sibs in seed production of Brussels sprouts and the influence of pollinating insects on the percentage of a seed lot have been investigated by Faulkner (1978), who dem- onstrated that blowflies pollinate in a more random manner than honeybees;
the flies drastically reduced the percentage of sibs under glass and polythene structures and to a lesser extent in cages.
In addition to the problems associated with sibs in an F1 hybrid seed lot, there are increased production costs compared with open-pollinated cultivars due to the following factors. The development of the initial breeding pro- gramme; subsequent maintenance of the inbred parents; extra land required to allow for male parents; care with sowing, isolation and harvesting; high labour input when flowers of the female parent have to be emasculated (especially when hand emasculation is necessary) and the lower seed yield per unit of land sometimes experienced with production of F1 hybrid cultivars.
BREEDING SYSTEMS UTILIZED IN F1 VEGETABLE CULTIVAR PRODUCTION The different sys- tems utilized in the seed production of F1 vegetable cultivars have been reviewed by Wills and North (1978); details of the genetic principles involved are described in leading plant-breeding texts such as Frankel and Galun (1977). The follow- ing are the main methods applied for the production of F1 hybrids with crop examples to which the principle is applied:
● Self-incompatibility (SI), e.g. Brussels sprouts. This results from the pres- ence of the so-called S-allele that prevents self-fertilization.
● Cytoplasmic male sterility (CMS), e.g. carrot, onion, parsnip. This type of male sterility is controlled by the cytoplasm and has been described by Innes (1983).
● Gynoecious lines, e.g. cucumber. The gynoecious parent has only (or pre- dominantly) female flowers, thus crossing only takes place with the male parent.
● Dioecy, e.g. spinach (Spinacea oleracea), use one line of male and another of female, but rogue out any plants showing male flower characteristics in the designated female line.
● Monoecy, e.g. sweetcorn. In male and female inflorescences borne on dif- ferent parts of the same plant, the male inflorescence is either manually removed or chemically suppressed before anthesis.
● Manual control, e.g. tomato and aubergine. It involves emasculation of individual flowers on the female parent before dehiscence of the sta- mens, followed by manual addition of pollen from the designated male parent.
● Chemical suppressants, e.g. some cucurbits such as squash (including cour- gette and vegetable marrow). A chemical suppressant, such as ethephon, is applied to stimulate the production of pistillate (male) flowers.
● Chemical hybridizing agent (CHA) involves application of a chemical to cause pollen abortion that results in male sterility. It is essential that the chemical does not result in ovule abortion. While this technique has been adopted for some agricultural crop species, including cereals and maize, it has so far only been of research and development application for vegetable crop hybrids.
SYNCHRONIZATION OF FLOWERING OF MALE AND FEMALE LINES Where the pollen donor (male) and pollen receptor (female) parts are on separate plants as in the pro- duction of hybrid seeds, it is essential that the shedding of pollen and receptive- ness of the stigma occur simultaneously. Seed producers refer to this synchronization of flowering as ‘nicking’. The matching of parent lines for hybrid seed production therefore depends upon their ability to ‘nick’, in addi- tion to the agronomic potential of the progeny. Parents of hybrids that ‘nick’
in one location do not necessarily flower at the same time or duration at another. Nicking may also vary from one season to another. Synchronization of flowering between parent lines of some vegetable species can sometimes be assured by sowing time of the pollen parent in relation to the female as well as successive sowings of the male parent in order to ensure a satisfactory overlap of anthesis. Information of these requirements should be provided by the plant breeder supplying the inbred parents, but it will also be learned from experi- ence in a specific location.
PHILOSOPHY AND POLICIES RELATING TO PRODUCTION AND USE OF F1HYBRID CULTIVARS Hybrid seed is several times more expensive than open-pollinated seed for a given crop. This is because the hybrid seed producer has to recover the investment incurred for developing it and pass on to the consumer the increased costs of hybrid seed production. In some crops the potential F1 seed yield is lower per unit area than for open-pollinated crops of the same species. One main advan- tage of an F1 hybrid to the organization developing and marketing it is the relative difficulty with which competitors can reproduce the cultivar; further protection is afforded with the introduction of plant breeders’ rights. Regard- less of legal constraints, a grower cannot successfully use seed saved from F1 hybrids due to segregation in the following generation.
It is sometimes suggested that F1 hybrids provide commercial seed com- panies with a hold on the market because once growers have found an F1 cultivar to be acceptable, they will be obliged to continue using it in subsequent years. However, it is the author’s observation that commercial vegetable producers worldwide are very keen to adopt F1s that have been demon- strated to be beneficial provided that they can afford the extra seed cost. This
assumes that there are no other constraints to maximize the cultivar’s potential. None the less, it must be emphasized to subsistence farmers that hybrid cultivars’ uniformity of maturity in crops from which there is only one harvest per plant (such as cabbage) may not provide the continuity of supply that they expect from some open-pollinated cultivars. There are some schools of thought among organic vegetable producers that resist the use of F1 hybrid cultivars.
It is stressed that individual hybrid cultivars should be judged on their agro- nomic and economic advantages to the grower.
Double-cross hybrids
The double-cross hybrids are produced by crossing two pairs of inbred lines.
The two resulting F1s are then crossed to produce a double hybrid. This tech- nique has been used in the production of some Brassica and sweetcorn culti- vars, and is considered better than single-cross hybrids because the seed yield is generally higher, although the maintenance of the appropriate lines and crosses increases the isolation requirements.
Triple-cross hybrids
This type of hybrid was described by Thompson (1964) as a method of over- coming the relatively high costs of maintaining inbred lines. Although not of major significance in vegetable crops, it has become important in the produc- tion of some fodder brassicas.
Synthetic hybrids
A synthetic hybrid is produced by the mass pollination of several inbred lines selected for their satisfactory combination ability. Random cross-pollination occurs between the different inbred lines, resulting in a mixture of hybrids.
The individual inbred lines to be used for the production of these synthetic hybrids are determined by the breeder. Because of the random crossing that takes place, there may be some variation from one season to another when the same parents are used. Cross-pollination is normally assured because of the individual inbred lines’ relatively high levels of self-incompatibility. This system has been used for some cultivars of Brassica species, including sum- mer cabbage.