Heterosis Breeding in Vegetable Crops

Heterosis breeding in vegetable crops Introduction: Most people in the world depend on rice or wheat as their staple food. However, rice and wheat alone cannot sustain health. There are more than one hundred different kinds of vegetables in the world, and they have become a necessary part of our daily diet.There are two main reasons for this. Firstly, they supply us with many kinds of nutrients needed for our health, e.g., vitamins, trace elements, amino acids, fiber and oil. Secondly, they appeal to the five senses. The taste of vegetables may be sharp, hot, pungent, bitter, sweet or sour. Pickled vegetables, processed with the assistance of microbes, have a different taste than the natural one. Moreover, the flavor, color and crispness of vegetables is enjoyed by our nose, eyes, ears, teeth and tongue. The numerous food cultures of the world use the various characteristics of vegetables in a wide variety of ways.In vegetable breeding, there are various breeding objectives for consumers, such as eating quality, color and nutrient content. For producers, there are breeding objectives such as high yields, early maturity and disease resistance. Even if a variety has a good flavor and high yields and is disease resistant, unless the seeds are stable and can be relied on to produce the same characteristics every year, it cannot become a good variety. Seed is also an agricultural product, and a good variety must have the characteristic of good seed production Hybrid: In general terms a hybrid is an off-spring from the mating of parents of different genetic makeup. The differences may be minor or they may be large. However, to the farmer or seedsman, a hybrid is the result of crossing inbred strains of a particular crop. The maximum degree of hybrid vigor is to be found in the first generation (Fx) following the cross, with reduced hybrid vigor in each subsequent generation. For this reason the term hybrid may be misleading unless it is qualified by the terms Fi or F2 generation. To the average person a magical quality is associated with hybrids, whether they be in flowers, vegetables, or animals. Apparently some breeders have been impressed by this magic, for they have at times introduced poorly adapted hybrids poorer in performance than good standard varieties. It should be emphasized that not all hybrid plants show hybrid vigor, even though the general public is often led to believe that all do. Hybrids which do well under some

conditions or in some localities may do poorly inothers. As in conventional variety breeding, the breeder who wishes to utilize hybrid vigor must have specific aims and purposes. The performance of a hybrid depends upon its genetic constitution, and therefore is a result of the hereditary makeup of its parental inbreds.The plant breeders' major task is the development or selection of suitable inbred lines to use as parental material. In the development of these lines, the effectiveness of selection for particular characteristics, the accurate measurement of these characters, and the extent of their heritability are highly important. There are no short cuts for this complex job. The secret of the success of good adapted Fi hybrids is that all the plants of a given hybrid are good producers. In open-pollinated varieties a few plants which may be superior to a good hybrid can be found, but many are inferior. Uniformity of production makes the big difference between performance of good hybrids and open-pollinated varieties.

Hybrid vigor: Hybrid vigor and what it is have puzzled man for centuries. He has used it even though he could not explain it. The hybrid vigor of the mule was known to ancient man. The mule has been called an animal without pride of ancestry or hope for posterity. Darwin (2) was an early observer of hybrid vigor in plants and noted that the progeny of two individuals were frequently more vigorous than either parent. Darwin stated that nature abhors con tinued self-fertilization since she has made many provisions to insure cross-fertilization. In 1909 Shull (15) stimulated great interest in the possible use of Fa hybrid seed when he advanced his hypothesis for the production and utilization of Fx single-cross hybrids in corn. Shull had noticed that when two in bred lines of corn are crossed, the vigor of their progeny increased greatly. He suggested that the term heterosis be given to the physiological stimulus of heterozygosis. Even today, 50 years after Shull's paper describing hybrid vigor was published, the causes of heterosis are not fully understood. An explanation of hybrid vigor resides in the fundamental chemistry of the plant, with the hybrid being more able to synthesize needed metabolites for growth and reproduction. As is often the case, there was a considerable time lag between the demonstration of hybrid vigor in corn and its practical application. Following Shull's fundamental research, it remained for Jones (3) in 1918, to place on a practical basis the utilization of inbreds in the production of hybrid corn. He suggested the double-cross method as

a desirable means of overcoming the high seed cost of Shull's singlecross. Since the advent of the commercial use of field corn hybrids in 1933, interest in the use of Fi hybrids in other crops has increased tremendously. At this point it may be well to ask why have such great strides been made in the use of Fi hybrids in corn, whereas until recently comparatively little progress had been made in this respect in other crops. There are several reasons for this: namely, hybrid vigor is pronounced in corn, which is a monoecious plant, having the male and female flowers separated on the plant. Of course it is not to be inferred that these are the only reasons why such strides have been made with this crop, though they are very important ones. It is immediately apparent that the mechanical operations of emasculation, pollen collection, and cross- and selfpollination can be performed with relative ease. The combination of hand-detasselling and wind pollination makes the system commercially feasible. Male-sterility simplifies the process further. The primary limitation in the application of heterosis in the production of hybrid vegetables has been our inability in most cases to overcome the high cost of hand emasculation of plants which have perfect flowers. Most of our vegetable crops normally have perfect flowers, i.e., bear both male and female organs in the same flower. Production of hybrids: The principles for producing hybrids of all plants are much the same. Inbreeding is first practiced until the material is relatively pure or homozygous. It is often accompanied by a reduction in vigor, which is associated with the number of recessive deleterious genes car ried by the plant. Inbred lines must be evaluated by the breeder and undesirable ones dis carded. Experimental hybrids are produced by combining inbred lines. After a hybrid has been produced, it must be compared and tested against the best available standard varieties. These tests involve yield, uniformity, disease resistance, quality, and shipping ability, as well as other characteristics. Such tests should be conducted for several seasons and in as many locations as feasible. In other words, a hybrid should be given as strict an evaluation as would be given a conventional variety.

Expression of Hybrid Vigor The amount of hybrid vigor expressed varies with the crop under consideration. In some crops hybrids are much more vigorous than conventional varieties, while in others hybrids show little or no increase in vigor over open pollinated varieties. For example, watermelons

exhibit less hybrid vigor than do hybrid onions. It is difficult to generalize on the subject, but usually the greatest hybrid vigor is found in crops which are the most highly cross-pollinated and the least in those which have the least cross-pollination. Fx hybrids of tomatoes are not as dramatic in their yield increases as are hybrids of sweet corn or spinach. A notable exception to this general rule is eggplant. Even though this is self-pollinated, Odland and Noll (10) reported a yield increase of 68 per cent for six F± hybrids over the average yield of their parents. An understanding of these differences is complex, but it has to do with the amount of genetic variability initially present in the species and in turn the degree of genetic differences between the developed inbred lines which are crossed.

Methods of Effecting Hybridization The primary limitation in the application of heterosis in producing hybrid vegetables has been the inability to overcome the high cost of hand-emasculation of perfect-flowered plants. However, there are tools or aids which the plant breeder can use in overcoming this difficulty, at least in part. These aids are often based on the use of male-sterility in any one of its several forms or on self-incompatibility. Methods of hybridization: Hand-emasculation and pollination: In a very few cases where there is a very high seed set in each fruit, this form of hybridization may be feasible. The eggplant is an example of a crop in which seed yields per fruit are very high. Kakizaki (7) reports that the seed count may be as high as 2,500 per fruit. For this reason hybrid seed of this crop could be priced at tractively even if it is produced by hand-pollination.

Hand-emasculation followed by natural crosspollination: This method of producing hybrids has found its greatest application in monoe cious plants. In this case the male and female lines are planted in alternate rows, or blocks of rows, in the field. Sweet com is by far the best example in this category. The tassels are removed by hand from the female line and crossing with the male line is effected by wind pollination. Summer squash hybrids are produced by this method also. The male buds are picked from the female lines every few days and crossing with the male parent is done by insects.

Functional male-sterility: This form of malesterility is not used commercially though it could be used in the omato. Larson and Paur (8) have described a functionally sterile flower mutant in tomatoes and have tentatively advocated its use for producing hybrid seed. Even though this mutant is fully self-fertile, its an thers fail to dehise; for this reason crosses may be effected without resorting to expensive hand-emasculation. It has been estimated that the time required to produce an ounce of Fx hybrid seed by using this mutant character is 1.5 hours as compared with 13.1 hours for the normal non-mutant female. Nuclear controlled gametic male-sterility. Malesterility involving failure to form gametes, and expressed through the action of a single recessive gene, is widely found in plants. Rick stated that genes for such sterility have been reported in 19 different species. Among the several vegetable crops in which nuclear malesterility has been reported are muskmelons, tomatoes, summer squash and winter squash. The application of nuclear male-sterility in the production of hybrid seed is essentially the same in all crops. The primary limitation to the commercial application of this form of male-sterility is the difficulty encountered in maintaining malesterile lines. The lines are maintained by the backcross method in which the male-sterile, homozygous recessive is crossed with a hetero zygous male. The expected ratio following this backcross is one male-fertile to one malesterile. A further handicap in the use of this form of sterility is that the plants must be grown until they flower so that the male-fertile and male-sterile plants can be identified. This handicap can be overcome by the use of a marker gene, which will enable the breeder to identify and eliminate the male-fertile plants.in the seedling stage. However, it is difficult to obtain such a favorable combination in most species. Cytoplasm-nucleus interaction: Hybrid onions are today being extensively grown in this country. These hybrids have been made possible by the discovery of a male-sterile plant in the variety Italian Red (6) in 1925. Subsequent work with this male-sterile type revealed that sterility results from an interaction be tween a cytoplasmic factor (S) and a recessive nuclear gene (ms).

An explanation of this type of sterility can be made by assuming that there are two types of cytoplasm. All plants with normal cytoplasm (N) produce viable pollen and may be of the genotypes N MsMs, N Msms, or N msms. Plants which have S cytoplasm and the genes MsMs or Msms will also be fertile inspite of the S factor, for they carry the dom inant Ms gene for pollen fertility. Only plants which are S msms are male-sterile (5). Even though this system of producing hybrids appears complex, it is in practice very effective and considerably easier to use than is a single recessive gene for male-sterility. One form of male-sterility in maize has been found to be determined by the interaction of cytoplasmic particles and nuclear gene. This form of male-sterility can be expected to rapidly gain in use for producing hybrid corn, especially in conjunction with pollen restoring genes. Marker genes to identify hybrids: Genetic markers which can be used to identify hybrids are available in some crops. Generally these are seedling markers and enable the grower to rogue non-hybrid plants in the seedling stage. This system could find application in the production of triploid, or "seedless", water melons. Marker genes are also helpful in producing hybrid onion seed. Dioecious plants: Hybrid spinach is now a commercial reality, as is shown by the rising popularity of Early Hybrid No. 7. Spinach is a dioecious plant, i.e., the plants are most often either male or female. In actual practice the sex ratios are approximately 50% female :40% male : 10% bisexual plants. This unusual form of sexual expression has made it possible for plant breeders to exploit hybrid vigor in this crop. Self-incompatibility alleles: Under most environmental conditions natural selection favors cross fertilization over self fertilization. In recent years, most of the vegetables cultivated over a large part of the world have been F1 hybrid varieties, that realize high yields and early maturity through heterosis. Heterosis breeding is only possible if there is an efficient method of producing F1 seed on a large scale. For this, some control of reproduction or pollination is needed.In controlling pollination, self-incom-patibility (SI) has been used for the Cruciferae. These include many important kinds of vegetables, such as cabbage, radish, Chinese cabbage, turnip and broccoli. The study of SI in crucifer crops began in Japan, where it still continues. In 1949, a Chinese cabbage F1 hybrid variety, "Nagaoka Kohai I Go", was produced by Shojiro Ito, and in 1961 a radish F1 hybrid

variety, "Harumaki Minowase", was produced by a commercial seed company. Most cruciferous vegetables grown in Japan and in other countries are now F1 hybrid varieties, whose seeds are produced using SI.However, SI is not always stable. It can be easily overcome under various external and physiological conditions. Therefore, stable F1 seed production has been the subject of study for many years.SI is governed by a series of multiple alleles (hereafter referred to as the S-gene) (Bateman 1955). However, there are genetic variations in the level of SI (hereafter referred to as LSI). Therefore, it can be assumed that SI is also regulated by genes other than the S-gene.There are two major seed production methods using the SI system for crucifer crops. One method is a single cross, in which SI is overcome in parental seeds by treating them with CO2 gas. The other method is a double cross, in which parental seeds are produced using a pair of isogenic lines for the S-gene.There are genetic variations in the reaction level of selfincompatibility (RLSI) to a 4% CO2 gas treatment. Thus, the parental lines in the parental seed production in a single cross have to show a marked reaction to CO2. In F1 seed production on both a single and a double cross, the parental lines have to show a high LSI. It is therefore important to know the genetic relationship between these characteristics. Heterosis in carrot: It is a well-proven fact that F1 hybrids show a great promise in getting uniform and high yields in crops. The first report of heterosis in carrot upon crossing of two inbred lines was made by Poole (1937). Inbreds producing roots averaging 12.8 and 24.9, respectively, produced hybrids with roots weighing 80.5 g upon hybridization. The degree of sterility in carrot depends more on cultivar than meteorological conditions and male sterility was mostly confined to primary umbels. The hybrids obtained from male sterile forms yielded 34.4-44.9 percent more and contained higher amount of dry matter and sugars compared to intervarietal pollination. Katsumata and Yasui (1965) reported that F1 hybrids among the cultivars Yokono, male sterile Sansun and Kuroda possessed good root form, high carotene content and were high in quality. Most of the F1 hybrids exhibited heterosis for carotene content. The male sterile forms of the cultivars Nantes 14 and Chantenay 2461 were successful in the production of heterotic hybrids (Litvinova, 1979) and heterosis was 20-22 percent higher than in hybrids produced with out using CMS (cytoplasmic male sterility). These F1 hybrids showed better uniformity of morphological characters.

The heterosis breeding in carrot has been facilitated by the cytoplasmic male sterility (CMS) which is of two types, viz., (i) brown anther type, in which the anthers degenerate and shrivels before anthesis, based on S-cytoplasm and at least two recessive genes with complementary action and (ii) petaloid, in which five additional petals replace anthers, based on S-cytoplasm and at least two dominant genes with complementary action. In hybrid development petaloid steriles are employed more widely and then the brown anther type. If genetic and environmentally stable brown anther steriles were available,then they would be preferred over petaloids because of their higher seed yielding potential. Basically, there are three lines in heterosis breeding, namely the male sterile, male fertile sister line and the pollinator line which is male fertile and has a good combining ability with the male sterile line. The male sterile and the pollen parent lines are inbred for several generations for attaining uniformity. The loss in vigor in these can be restored by hybridization. The hybrids in carrot are normally three way crosses, (A_B) _C, because the hybrid vigor in a single cross F1 female seed parent normally results in much greater seed production than that of inbred male sterile parent. Single cross hybrids, A_B, are on an average more uniform than three way crosses. Moreover, they do not require an extra year to produce F1 seed parent stock. So the single crosses can be used, if their productivity is adequate. Reduced uniformity of three way crosses can be overcome if backcrosses are utilized as seed parents in hybrids as a result the final product attains the form [(A_B)_B] _C. Although compared to three way crosses, it consumes an additional year of seed parent production, but it permits utilization of less similar seed parent inbreds, A_B, than what is required.

Heterosis in radish: Pal and Sikka (1956) obtained high yielding hybrids, which gave 30-60 percent higher yield than the better parent. Significantly higher yield than better parent, earliness for 50 percent root formation and long sized roots were observed in radish hybrids (Singh et al., 1970; Singh 2003). Self-incompatibility and male sterility phenomenon in radish easen exploitation of hybrid vigor. Self-incompatibility system is of sporophytic type in which the papillae on the stigma surface disrupt the pollen tube penetration. Male sterility has been reported by Ogura (1968) in Japanese radish, which is governed by the interaction of a recessive gene ‘ms’ and S-

cytoplasm (Bonnet,1970). The hybrid breeding in radish, thus, became feasible using either incompatibility or male sterility. The first hybrid radish was developed by Frost(1923) who observed that the crosses between selfed lines were very vigorous and usually exceeded the better parent in root size and all plant characters. Radish superiority of F1 hybrids were superior over parental lines for earliness and productivity. Heterosis for root diameter and root length has also been observed. Heterosis in beet root: Heterosis breeding is at infancy in beetroot. Although with the introduction of CMS, the production of hybrids has been facilitated. Male sterility was governed by the segregation of a gene X in S-cytoplasm, with fertility dominant to sterility (Bliss, 1965). Another gene Z with partial male fertility and complete dominance over male sterility was also reported but it was independent of and hypostatic to X. Gabelman (1974) and Gabelman (1974) reported that monogerm character and cytoplasmic male sterility (CMS) were transferred from sugar beet. For hybridization, if dominant marker is available it can be used to avoid emasculation of female parent and the selfed plants can be rogued either at seedling stage or at root stage. If the pollen parent does not possess any marker genes it would be necessary to emasculate the flowers before crossing since these are hermaphrodite. During pollination care should be taken to avoid contamination from foreign pollen by wind. The bags covering the plants of the male and female parents should not be opened on a windy day and pollination must be done when the air is still, preferably in a glasshouse or plastic cage(Swarup, 1991).

Conclusion: Plant breeders express the degree of hybrid vigor of an agronomic character in different ways; the percentage increase over the best parent, over the midparent or average of the two parents, or over the best commercial cultivar in the area. The way the breeder chooses to express the hybrid vigor determines the percentage. For example, a cotton selection or line 'A' may produce 800 pounds of lint per acre, and line 'B' may produce 1,000 lb/acre. When crossed, the offspring or F1 (first filial generation) produces 1,200 lb/acre. The best commercial cultivar in the area also produces 1,200 lb/acre. Depending upon which way the breeder chooses to express the hybrid vigor, it may be 33 percent (over the midparent), 20 percent (over the best parent), or 0 percent (over the best commercial cultivar based on yield, but because the F1 or hybrid between

'A' and 'B' sets its crop of cotton on the stalk 3 weeks earlier than the commercial cultivar, thereby reducing irrigation and harvesting costs and insect pest problems, the hybrid is preferred

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Agric. Sci., 21 (4) (578-579) : 2008 Ahmed, N., Hakim M.A. and Gandroo, M.Y., 1999,

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