Hybrid vigor, or heterosis, describes the increased vigor of plants or other organisms when compared with parents that were unlike in one or more inherited characters. Although there is no single, fully acceptable genetic definition of hybrid vigor (Ashton 1949), it may be observed in the offspring in terms of increased size, uniformity, volume, quality in earliness, or resistance to unfavorable environmental factors.

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. This undefinable earliness factor and, likewise, other intangible factors not measurable by yield alone may be ascribed to heterosis or hybrid vigor.

Neither hybrid vigor nor its qualities can ever be predicted. They can only be established or proven through testing of the F₁ for each parental combination. Hybrid vigor cannot be maintained at its maximum because it starts reducing with the first generation in which self-pollination may occur. For maximum vigor, it must be created anew each season. The potential use of hybrid vigor in plants is always tantalizing to the breeder because it promises a new plateau of productivity. The problem is, first, the finding of this factor then, second, the development of a method of utilizing it economically under commercial conditions. In contrast to hybrid vigor, the inbreeding of a normally cross-pollinated plant not only results in an isolation of biotypes but also in a loss of vigor of the individual plant (Hawthorn and Pollard 1954*), which can make it more susceptible to unfavorable environmental factors. The inbreeding effects on a normally cross-pollinated plant are roughly the opposite of hybrid vigor.

The classic example of the use of hybrid vigor in plants is in hybrid corn production. The monoecious characteristic of corn makes it a simple plant for use in this manner because the male part, the tassel, and the female part, the ear, are widely separated on the plant, and, more importantly, the pollen is transported by wind. The only steps necessary after an appropriate cross is decided upon is to alternately plant rows of the two parental selections, then mechanically remove the tassels on one of the rows before flowering begins. Pollen may then be carried by the wind from the row with its tassels intact to the silks of the ears of the detasseled row. All of the grain produced on the detasseled row will be hybrid seed, and, likewise, the grain on the pollen-producing row will provide inbred seed for the next production season.

Unfortunately, in most other plants, the male and female parts are intimately associated within the same flower (complete flower) rather than being separated as in corn. When the male parts cannot be removed with dexterity, other means are explored for fertilizing the flowers of a plant with the desired pollen. One method is to use a self-incompatible parent with a suitable combiner. In incompatibility, which is widespread among plant families (Lewis 1949), the pollen and the ovules of both plants are independently functional, but because of some incompatibility between the maternal tissue and the pollen tube development, the pollen nuclei fail to unite with the egg nucleus and thus complete fertilization (Allard 1960). If plants possessing the genetic mechanism based on incompatibility are wind pollinated or anemophilous, the only action required to produce a hybrid is to interplant rows of the two cultivars and all the seed will be F₁ . If they are insect pollinated or entomophilous, arrangements must be made to have sufficient pollinating insects available to transfer the pollen. If pollen falls upon the stigmas of flowers of its own maternal origin, no fertilization occurs. If it falls upon compatible flowers, a hybrid results.

Within recent years, a simple method has been found for obtaining 100 percent cross-pollination on a large scale in plants that normally have both sexes within the same flower. The method utilizes biological emasculation of the plants, in which the pollen grain either fails to develop or is not viable. Such plants are referred to as being male-sterile. Male sterility of some form has been found in many crops, and breeders are always on the alert for such plants among their selections. Male-sterile plants appear unexpectedly even in long-established commercial cultivars.

Two types of male sterility have recently become economically significant, and are used by plant breeders: cytoplasmic male sterility and genetic male sterility (Duvick 1967). In the former, sterility is carried in or influenced by the cytoplasm. In the latter, it is carried in or influenced by the germ plasm of the nucleus, which contains the genes or hereditary characters. Because of their importance and relationship to insect pollination, they are discussed below in some detail.

Cytoplasm is the material of a cell that is transmitted from parent to offspring only through the egg, or the maternal side, independent of the cell nucleus. Characters influenced by the cytoplasm respond the same as in the female parent. Cytoplasmic male sterility is, therefore, carried through the maternal side of the line. The genes present in the nucleus are derived from both parents; therefore, genetic male sterility is influenced by both parents.

One explanation of cytoplasmic male sterility (used as a teaching device by L. S. Stith, personal correspondence, 1972) is shown in fig. 3 and is similar to the explanation given by Briggs and Knowles (1967). Here the ovule of the milo group (female) of Sorghum vulgare L. [=S. bicolor (L.) Moench] is fertilized with pollen from the kafir group (male) of the same species. The cytoplasm and half of the genes in the nucleus are thus from the milo (female) and half of the genes are from kafir (male) in the F₁ . However, in the presence of the milo cytoplasm, the kafir genes produce sterility and approximately 50 percent of the F₁ are male-sterile. When these male-steriles are backcrossed to kafir, a higher ratio of sterile- fertile plants appear. Likewise, by the sixth backcross generation, near complete male sterility (99 percent) is established. Fertility can be restored at any time by reversing the mating and backcrossing the sterile plants to milo.

[gfx] FIGURE 3.--Probable inheritance of cytoplasmic male sterility in the Milo (M. male) group of Sorghum vulgare L. [=S. bicolor (L.) Moench.] when its ovule is fertilized by the sperm in pollen of the Kafir (K. female) group. Explanation: op= operon or operator gene--a genetic unit consisting of adjacent genes that function together under the joint control of an enhancer and/or a repressor factor: bc= backcross. Ratios indicate probable proportion of fertile to sterile genes. (After L.S. Stith, personal commun., 1972.)

The teaching device may leave something to be desired as an explanation for plant breeders or geneticists, but it does visually demonstrate incompatibility between nucleus genes (represented by a square) and plasma genes (represented by a circle). An explanation based on the DNA-RNA concept is simple and easily understood if one assumes that the Operon and structural genes controlling sterility are not identical in the milo and kafir group. By continual backcrossing to kafir, sterility is increased but fertility is restored when the plant is backcrossed to the milo group. The DNA-RNA molecular system simply explains partial sterility because DNA may be carried in organelles in the cytoplasm.

Cytoplasmic male sterility, therefore, is concerned with the incompatibility between factors in the cytoplasm of the cell and the genes of the nucleus.

Genetic sterility is that form involving only the genes in the nucleus of the cell, independent of the cytoplasm. The gene contribution is from both parents, with male sterility being the result of homozygous recessive genes or factors.

The cytoplasmic-genetic male sterility is the result of an interaction between the genetic and cytoplasmic systems. Under this system of male sterility, the double recessive genes (ms ms) in the nucleus produce fertile progeny (F) in normal cytoplasm but produce sterile progeny (S) when acting in a cytoplasm that has undergone change (Briggs and Knowles 1967).

The cytoplasmic-genetic male sterility system differs from cytoplasmic male sterility in that the offspring of the male-sterile plants may be male-fertile when crossed with certain selections that merely change the cytoplasm. Again, based on the molecular theory, the male sterility becomes a function of the DNA code in the nucleus of one parent being unable to activate the RNA system in the cytoplasm of the other parent.

Jones and Davis (1944) were the first ones to report the use of male sterility in the production of a commercial crop (onion seed), and they used the cytoplasmic-genetic system. After finding a male-sterile 'Italian Red' onion, which was propagated by its bulbils until the system could be understood, crosses and repeated backcrosses were made between the 'Italian Red' and a 'Crystal Wax' cultivar until the sterility was transferred to that commercially desirable cultivar.

The breeding research revealed two types of cytoplasm--fertile (F) and sterile (S). Those plants that had the (F) factor produced viable pollen, those with (S) cytoplasm did not. When a restorer gene (R ) was introduced from the male parent, the dominant gene (Ms or Rf) action produced fertile progeny, thus both genetic and cytoplasmic inheritance were involved. In commercial production of onions, 4 to 12 rows are planted with a male- sterile type for each one to two rows of male-fertiles (fig. 4), and they must both flower at the same time. Bees transfer the pollen to the male-sterile heads, and the hybrid seed is produced on these heads. The male-fertile flowers may be destroyed or harvested separately after pollination is completed. The seed that is harvested, being hybrid, produces an onion superior both in yield and flavor.

[gfx] PN-3741 FIGURE 4.--Hybrid onion seed production. Note the 2 pollinator rows (center, with larger flower heads), which supply pollen for 6 male- sterile rows ( 3 on each side) to produce the cross-pollinated onion seed.

The utilization of hybrid vigor is enticing. For example, its use was estimated to increase the yield per acre of corn by 35 percent (Jenkins 1936). In cotton, Stith (1970) estimated that production might be increased 20 to 25 percent by use of hybrid vigor, which he estimated would be worth $275 million per year to our growers, or the same annual production could be obtained from 20 percent less acreage. He believed this would result in no additional expense to the grower except for the increased harvest cost. Corn is wind pollinated but insects, primarily honey bees, would be required to cross- pollinate cotton.

Kinman (1970) reported the discovery of a fertility restoration gene for cytoplasmic sterility in sunflowers. This, he believed, was the final step required in the development of hybrid sunflowers. In personal correspondence, Kinman indicated that this male sterility and its restorer in sunflowers could result in doubled production of current cultivars. The effect of such an increase in production and potential profits on the future of this crop in the United States is unpredictable but will doubtless be great. Bees would be required to transfer this pollen from the fertile to the male-sterile plants.

Hybrid onions now command the bulk of the onion market. Growers use honey bees almost exclusively in transferring the pollen of the fertile plants to the male-sterile ones. Because there is no pollen for the bee to collect on the male-sterile plants, it visits the blossoms only to collect nectar. Onion growers frequently complain that honey bees are reluctant to visit the male-sterile flowers solely for the nectar. To produce hybrid seed, the flowers on the male-sterile onion row must be visited by nectar-seeking, pollen-coated bees that have previously visited the fertile rows.

The above discussion illustrates the need to consider the attractiveness of the plant to nectar- and pollen-collecting insects during the process of developing a male-sterile plant. It must be recognized that bees may visit a flower for its pollen, its nectar, or both, and in male-sterile plants only nectar is available. Bee breeders have made selections of bees that show preference for alfalfa pollen (see "Alfalfa"), but no selections have appeared that show preference for nectar. The plant breeder might approach the problem from another angle--by selecting plants that produce more nectar or, at least, more attractive nectar for the bees. Cooperative work between bee and plant specialists in this area may prove valuable.

Caviness (1970) stated that hybrid soybeans as a commercial crop was intriguing, but he doubted that it would ever materialize because the flowers were small and unattractive to bees, and had other discouraging characteristics, including the sparsity of nectar and pollen and the relative concealment of the flowers by the foilage. Male sterility has, however, been found (see "Soybeans") in soybeans. Also, other breeders are looking for ways to utilize hybrid vigor in this $2 billion crop because the potential profits are great with only a minor increase in production. The primary problem seems to be the relative unattractiveness to bees. Already there are leads in that area. Some plants show greater attractiveness than others.

The discovery of a strain of beans highly attractive to bees or the development of a way to attract bees to the flowers could almost assure utilization of hybrid vigor in this crop. This is an example of a crop on which cooperative research between bee specialists and plant specialists can no doubt make advances of benefit to both.

Rubis (1970) indicated that hybrid safflower was feasible based on differential separation of male and female parts, which he called functional male sterility. In this crop, the male-sterile plant produces pollen on the anthers inside the anther tube. The anthers release the pollen only after the style has elongated and pushed the stigma beyond reach of the anthers. Bees visit these flowers freely for nectar, bringing pollen from stigmas that have pushed pollen before them and out of the anther tube. In their collection of the nectar, they may also transfer pollen from the anther tube to the stigma of the same flower.

Davis and Greenblatt (1967) have reported the discovery of cytoplasmic male sterility in alfalfa with a restorer gene. Hybrid alfalfa is produced on a limited scale now, and the discovery of cytoplasmic male sterility may greatly enhance the use of hybrid vigor in this important crop. Because alfalfa is a perennial crop, the male-sterile plants could be used for several seasons.

Foster (1967) reported that hybrid muskmelons produced twice as much fruit as the commercial lines. Foster (1968) reported the discovery of male sterility in muskmelons. The plants are entomophilous and are freely visited by bees for nectar, so the future commercial use of male sterility and hybrid vigor in melons is bright.

Nieuwhof (1969, p. 231 ) stated that genetic male sterility had been found in Brussels sprouts, cauliflower, and sprouting broccoli, but a laborious task of thinning would be required to remove the (roughly 50 percent) male-fertile plants. He doubted that commercial utilization of hybrid vigor in this group was likely. Other breeders are searching for cytoplasmic male sterility in these crops through which complete sterility might be obtained. The cole crops and numerous other vegetable crops are insect pollinated.

An economical way of producing hybrid tomato seed is highly desirable. The few bees that visit current cultivars of tomatoes do so only to collect pollen. A male-sterile strain would therefore be of no interest to such bees. Possibly some of the primitive species of this family group produce nectar. If such a species could be found and this characteristic transferred to a commercial male-sterile cultivar, it would then attract the insect pollinators, and insect cross-pollination could be achieved. Here again, cooperative research between exploratory botanists, plant breeders, and entomologists might be productive to the public.

Regardless of the type of male sterility--incompatibility, or cytoplasmic, genetic, cytoplasmic-genetic, or functional sterility--if insect activity is involved, specialists should cooperate to utilize all factors in the development of more productive crops.

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