Resultados de entrevistas a dependencias gubernamentales

onions, sugar beet, Brussels sprouts, kale, castor, tomatoes and cotton, although dra- matic increases in yields are not attribut- able to these hybrids. One of the driving forces behind the search for and develop- ment of hybrid crops lies in the economics of seed production and sales. Although hybrids may be expensive to develop they have the economic advantage that the grower cannot use harvested seed for the next season’s crop, but must return to the

wholesaler to purchase the F₁hybrid seed.

This commercial scheme seems to have worked well in developed countries where

the necessary infrastructure exists to take advantage of it. Such an approach in devel- oping countries may be less appropriate, especially since population improvement techniques have shown equal potential and are often less expensive.

5.4.5 Clones

Clones are groups of individuals that have descended from a common parent by mitotic division alone, meiosis has not been involved. Hence, clones are geneti- cally identical plants that have been vege- tatively propagated from a single parent. Vegetative propagation occurs naturally in the form of asexual reproductive structures such as stolons, rhizomes, tubers, bulbs and corms or by well established horticul- tural techniques such as budding, grafting, leaf cuttings and leafless stem cuttings. A wide variety of crops are produced as clones including perennial vegetables, potato and sweet potatoes, tree crops, apples, peaches and rubber trees, shrubby crops, bananas, sugar cane and pineapple, soft fruits, strawberry, blackberry and rasp- berry and certain species of grasses.

Breeding clones involves obtaining crosses between the flowering heterozy- gous clonal parents and then selecting

among these F₁ individuals, and in subse-

quent vegetatively propagated clonal gener- ations for individuals having favourable characteristics. The most valuable feature of clonal breeding is that favourable char-

acteristics produced in the F₁ progeny of

the sexual cross, are immediately fixed because subsequent multiplication is vege- tative, producing only genetically identical individuals. However, one of the major problems with clonal breeding is that the plants used usually have reduced fertility thereby restricting the ease of the sexual cross.

There are basically two types of clonal crop: those that produce a vegetative prod- uct and those producing a fruit. Crops grown for their vegetative products can suffer from various degrees of reduced flowering and fertility making sexual repro- duction and selected crosses difficult. Wild

potato species flower readily while many potato cultivars either do not flower at all in the field, e.g. King Edwards, or their flower buds fall off prematurely. Pollen sterility is also a major drawback in hybridization preventing many desirable crosses (North, 1979). Although flowering is obviously not a problem among fruit pro- ducing clones, pollen sterility does provide a major drawback. Also a number of spe- cial reproductive problems exist, e.g. parthenocarpy in bananas, which have to be overcome before cross breeding is possi- ble. Further problems for the breeder are encountered due to carry-over of virus dis- eases in clones, lengthy generation times of some perennial plants and a slow rate of multiplication of some cultivars (North, 1979). These problems are, however, being solved is some cases by the use of in vitro techniques.

In vitro methods involve the cultivation

of plant tissues under sterile conditions in the laboratory using various nutrient media to promote growth and development. Meristem cultures utilize meristem tips of developing plants to rear virus free clonal stocks and for the rapid multiplication of some species having a slow rate of vegeta- tive propagation. Embryo cultures, where developing embryos are removed from the young fruit and grown in artificial media, are used in situations where the embryo normally fails to develop, such as in the hybridization of related species. Cell and tissue culture, the most recent advances in

in vitro methods, have provided some spe-

cific cases of potential uses in clonal multi- plication, e.g. freesia, production of haploids from pollen cultures and hence homozygous diploids as an aid to inbreed- ing lines e.g. tobacco, and in vitro hybridization, e.g. soybean. The full poten- tial of cell and tissue cultures is yet to be fully realized, but they will definitely play an important role in future developments in genetic manipulation.

5.4.6 Backcross breeding

Backcross breeding is used when breeding with inbred lines, open-pollinated popula-

tions and clones. It provides a method to incorporate a desirable trait into an other- wise acceptable variety of a crop plant (Mayo, 1987). Backcross breeding transfers a single or a few genes that have readily identifiable and desirable characters from a donor plant (often poor in general agro- nomic ability but having a useful trait such as pest resistance) to a generally superior variety lacking only the desired trait. The superior variety is referred to as the recur- rent parent and the ultimate value of the new improved variety will be largely dependent on the quality of this parent. Hence, recurrent parents are usually estab- lished varieties that have a proven ability but lack a character that could potentially make them of more value. The agricultural value of the donor parent, for characters other than the one under selection, is unimportant but the trait to be transferred must be highly heritable and the intensity of its expression maintained throughout a series of backcrosses. Hence, traits that are used in backcrosses are usually controlled by major genes. Disease resistance, plant height and earliness are traits normally considered suitable for incorporation by backcross breeding (Mayo, 1987).

The general principle of backcrossing can be illustrated by considering two geno- types: aa the donor and AA the recurrent parent. If these are crossed, the resulting

genotype is !fAA, !sAa, !faa. If this F₁genera-

tion is then backcrossed with the recurrent

parent, the resulting genotype will be #fAA,

!fAa, i.e. the donor parents’ contribution to the progeny will be halved in each succes- sive backcross. Eventually, the genetic con- tribution of the donor parents becomes insignificant except for the desired charac- ter which is maintained during this reduc- tion process by positive selection. The number of backcrosses that is required is dependent on the recovery of the essential characters of the recurrent parent, or alter- natively the redirection of the characters, other than the selected character, associ- ated with the donor parent. The recovery of the recurrent parent will be enhanced if in the early generations there is some

selection for parental type. The number of generations required to achieve a suitable level of recovery can be determined in a similar way to the number of generations required to achieve homozygosity through selfing. Figure 5.9 can be used to find the percentage of plants homozygous for a given number of alleles entering the cross from the recurrent parent. If no selection is practised, then for parents differing in, for example, five gene pairs five backcrosses will produce a population in which approximately 85% of the individuals will be homozygous and identical with the recurrent parent at all loci (Allard, 1960).

Backcross breeding in open-pollinated crops differs only in the number of plants that must be used as recurrent parents. This number must be sufficient to ensure the recurrent parents represent the gene frequency characteristic of that particular variety (Lawrence, 1968). The situation is similar in clonal crops when dominant genes are being transferred; more than one recurrent parent is required to avoid inbreeding, and enable the progeny to be left heterozygous for the transferred gene.

5.4.7 Breeding for horizontal resistance There are three possibilities to contend with in breeding for resistance to insects. The first is that the insect may exhibit gene-for-gene vertical resistance (although probably rare among insects) and the requirement for an appropriate breeding scheme to incorporate the genes into high yielding cultivars; backcrossing tech- niques should be appropriate. The second possibility concerns the identification of resistance characters controlled by major genes (preferably expressing characters that are beyond the insect’s capacity for change) and the introduction of these genes into appropriate cultivars. The third possibility involves the selection and breeding for polygenically inherited resis- tance characters.

Utilization of characters that are beyond the insect’s capacity for change will pro- vide durable resistance in crops plants and although such characters are often con-

trolled by major genes, the resistance is horizontal. Such major gene characters are amenable to traditional methods of breed- ing and can be readily incorporated into high yielding cultivars, but unlike vertical resistant characters, the resistance will be durable. In the short term the continued research effort for this durable major gene resistance is inevitable because at present it fits in with traditional methods of breeding. The basic difference between breeding for horizontal resistance and the more tra- ditional breeding techniques is that with the former outcrossing among individuals is actively encouraged while, in the more conventional approaches, inbreeding is considered the most useful form of fertil- ization. For horizontal resistance breeding, crops that are normally self-pollinating will have to be prevented from self-fertil- ization. The use of male gametecide sprays

is one possibility, e.g. MS₃ for sorghum

(Johnson and Teetes, 1980), although the frequent use of these will encourage resis- tant plant strains that do not respond to the spray treatment. The more conventional alternatives for preventing cross-pollina- tion are plant nuclear and cytoplasmic sterility. Nuclear male sterility is con- trolled by male chromosomal genes that are usually recessive, e.g. MsMS and Msms are male fertile and msms are male sterile. Cytoplasmic sterility is maternally inher- ited, i.e. a sterile female crossed with a fer- tile male produces sterile progeny (Mayo, 1987). Male sterility facilitated recurrent selection has been successfully used to develop resistance to stem borers in rice (Chaudhary and Khush, 1990).

The general method for breeding for horizontal resistance is one of mass selec- tion from a large random polycross. Plant populations must consist of thousands of individuals but need not cover an area greater than a few hectares if, for instance, cereals such as rice or wheat are being grown. Agronomically unsuitable and sus- ceptible plants must be removed before flowering and selections should be based on assessments or measures of the relative amount of resistance or conversely the

degree of susceptibility encountered. Horizontal resistance will have accumu- lated to useful levels after 10–15 genera- tions which could mean only 5–7 year cycles with two cropping seasons per year.

Screening methods can then be devised to select plants according to their relative value measured in terms of numbers of insects present, their rate of development or growth, their fecundity and survival or the amount and type of damage they cause. Rarely are absolute measures of the above variables made; usually a scale or index scoring system is devised and a visual assessment of the variables translated into a relative estimate. Such visual assess- ments and scoring systems do not provide a very accurate evaluation of resistance but in most breeding situations large numbers of plants have to be screened quite rapidly, so other techniques are often too laborious and time consuming. Screening under con- trolled conditions in the laboratory may identify polygenically inherited resistance more readily than in the field. Among labo- ratory populations of organisms, selection occurs from among a relatively low number of individuals and because the distribution of phenotypes will be limited, the selection will tend to make use of existing common variation rather than drawing on novel variation from rare phenotypes (Roush and McKenzie, 1987). The application of insec- ticides to insects in the laboratory, where dosage and mortality can be precisely con- trolled, has resulted in the accumulation of polygenically inherited resistance to the insecticide by the small proportion of insects that survived each generation (Roush and McKenzie, 1987). Similar selection pressure to that exerted by the insecticide in the laboratory can be pro- duced in host plant resistance studies with closely controlled infestations of insects for a given number of plants. The infestation has to be just great enough to permit suffi- cient plant survivors for the next genera- tion. The work of Barnes et al. (1969) on laboratory screening of alfalfa for resistance to the alfalfa weevil (Hypera postica) repre- sents the type of methodology that would

select for polygenically inherited resis- tance; the weevils were allowed to feed until 95% of the plants were destroyed and the 5% surviving plants were grown on for further resistance evaluation. The surviv- ing plants in the above experiment were subjected to further tests and were shown to support significantly less adult feeding and produce smaller larvae than unse- lected plants of the same variety. The dif- ferences between the selected and unselected plants were small but this would be consistent with selection of poly- genes which would produce a small effect each generation and would need to be accumulated over several generations to provide useful levels of resistance.

The majority of screening that is carried out, however, is not in the laboratory but in the field, where conditions are more vari- able and where screening and selection techniques need to be simple and economi- cal of time and effort.

The selections must only take place dur- ing the esodemic if vertical resistance is suspected. Where vertical resistance is not present, the selection pressure (provided by the level of pest infestation) should, ide- ally, be both intense and uniform. Methods for increasing the level of infestation include planting susceptible varieties around and within the breeding plot (but not allowing them to flower) and the use of artificial infestations (inoculation from lab- oratory mass reared insects). Ensuring that regular severe infestations occur is impor- tant for two reasons: a patchy distribution of infestation will increase the likelihood of escapes, and horizontal resistance will be eroded in the absence of selection pres- sure. Plants that escape infestation are technically included as horizontally resis- tant but the greater the number of escapes, the slower the rate of accumulation of hori- zontal resistance since escapes that are selected may not contribute resistance genes to the gene pool. To prevent this, infestations need to be as uniform as possi- ble over the breeding population.

In the same way that genes for resis- tance can be gradually accumulated in the

presence of a positive selection pressure, resistance can also be lost or eroded in the absence of that pressure. The implication of this is that in situations where pest infestations are sporadic and highly vari- able between seasons, horizontal resistance will only accumulate at a slow or negligi- ble rate unless the level of infestation is artificially maintained. The methods men- tioned above are applicable. Susceptible varieties can be sown early around the block to encourage a quick establishment and build-up of the insect populations (Jackai, 1982). Infested plant material could then be distributed within the breeding block. As the infested plant sections wilt and die the insects will move on to adja- cent test plants (Lowe, 1973).

Breeding for horizontal resistance will be easier in some crops than others. The difficulties with self-pollinating crops have been mentioned above but these can be overcome. The greatest problem with such crops comes from the reticence of the breeders to consider alternative breeding techniques rather than with technical diffi- culties related to the methodology. Long term perennial outbreeders are another matter. Many of these crops are difficult to breed because they occupy such large areas, some have a very low level of seed production, e.g. coconut, and long genera- tion times, e.g. dates (7–10 years). Since there is little likelihood of perennial plants exhibiting vertical resistance to insects,

selection for horizontal resistance will be possible within existing crops. In forest trees, the selection for resistance may be hampered by the problems of screening such large plants. Where pests may be high in the crown developing appropriate sam- pling methodology will be difficult, although most trees can be vegetatively propagated by stem or root cuttings once resistant individuals have been identified. Recent advances in tissue culture methods with conifers and deciduous trees may stimulate vegetative production of insect resistant trees (Hanover, 1980).

Also little use has been made of mass selection for resistant phenotypes despite its successful use for other quantitatively inherited traits, including rust resistance. Mass selection has been used to improve crops of lobolly pine, Scots pine, black wattle and eastern cottonwood (Wright, 1976). The characteristics used were mostly related to growth and production such as growth rate, tree height, number of branches and diameter but work on slash pine (Goddard et al., 1973) and eastern cot- tonwood (Jokela, 1966) has shown that mass selection techniques are also highly effective at selecting for horizontal resis- tance to rusts. Since the principles of breeding for horizontal resistance to pathogens apply equally well to insects, mass selection for resistance to insects in forest stands has a high potential for success.

Case Study: Directed mass selection for leafhopper, alfalfa aphid and