Clonal selection and roguing

Many commercial pineapple cultivars are improved through selecting genetic variants that occur occasionally in experimental plots or production fields. Such variants are perpetuated as clones whose genetic make-up is identical and 'fixed' by vegetative propagation until new variation arises. Clonal plantings form the basis of modern pineapple culture, and many different clones are used throughout the world. All 'Smooth Cayenne' clones in Hawaii, including 'F200' and the 'Champaka' clones were developed through clonal selection (Py et al., 1987). In Australia, clonal selection began in 1950 with the selection of 100 plants from commercial fields. Four clones - 'C8', 'C10', 'C13' and 'C30' -were eventually released to the industry in 1975 (Groszmann, 1982) and, in conjunction with the Hawaiian clone 'Champaka F180', form the basis of the present-day industry. Several clones of the cultivar 'Queen' have also been established in Australia, including 'Ripley Queen', 'Alexander' and 'McGregor', through a process of clonal selection by priva te growers (Lewcock, 1940). In Malaysia, an improved cultivar, 'Masmerah', with higher vigour and yield, was obtained by clonal selection from a commercial field of 'Singapore Spanish' (Wee, 1974). Clonal selection has also been used in the Philippines to develop a new clone of 'Smooth Cayenne' less prone to the green-ripe disorder (Obrero, 1995).

Spontaneous mutations

At least 30 major mutations have been recorded in 'Smooth Cayenne' pineapple since the early 1920s. These include white flowers (as opposed to purple), foliar floret proliferation, multiple sepals and bracts, enlarged fruit segments, protruding fruit segments, increased or decreased trichome density, seediness, multiple crowns, a lower harvest index, misshapen fruits, smaller-diameter fruits, leaf spininess, reduced chlorophyll or anthocyanin levels, dwarfed habit, translucent fruits, collar of slips, increased or decreased slip number and fruits with an increased incidence of basal knobs (Anon., 1925, 1940; Kerns, 1928; Collins and Kerns, 1938; Groszmann, 1939; Dalldorf, 1975; Sanewski et al., 1992). Other measurable differences between clones likely to have been caused by mutations are sucker number, length of the fruit development period and peduncle length and width (Sanewski, 1997). Only two mutations - non-translucent fruit and resistance to mealybug wilt - could be considered substantially progressive (Collins and Carter, 1954; Obrero, 1995). Mutations can also be induced with a range of techniques, including tissue culture (Williams and Fleisch, 1993), exposure of in vitro leaf cultures to cobalt gamma rays (Cisneros et al., 1998) and the treatment of seed and plants with chemical mutagens (Singh and Iyer, 1997).

Despite the large number of mutations identified in pineapple cultivars, they are not generally considered to be unstable genotypes. The incidence of mutations is probably a function of the large number of plants propagated and observed. The type of planting material used may also determine the frequency of some off-types. As an example, if slips are used as planting material, then plants that produce the mutation near-collar

(slips arise from the peduncle very close to the base of the fruit) will be multiplied faster than normal types and hence the frequency will increase. Therefore, while the genotype is considered stable, some specific characters, such as leaf spininess, are considered unstable and the frequency of mutation for these characters is high (Collins and Kerns, 1938).

Roguing

As most mutations are regressive and considered to be off-types, the population performance will gradually decline if no selection is done to maintain its standards. Inferior types may be difficult to detect visually, especially those that have slightly reduced plant vigour or fruit size. With the large number of plants grown and the efficient propagation methods used, inferior types that escape detection can be quickly multiplied. Roguing can rapidly reduce the percentage of most off-types, where they are initially numerous, to low levels. Therefore, major pineapple producers routinely rogue inferior spontaneous mutants to maintain productivity and efficiency.

Roguing is sometimes called mass selection, although, strictly speaking, it is a form of culling rather than selection. Roguing aims to minimize the number of undesirable variants in a field by not replanting material from those plants. Roguing is, by necessity, an ongoing process and is best performed on well-grown plant crops. To identify the inferior plants the grower walks through the field before harvest and marks the crown or plant itself with paint. The exact process will depend on what type of planting material is used. In Australia, it is performed on summer plant crops, where the expression of defects such as collar of slips is greatest (Groszmann, 1945). The main characteristics targeted in roguing are described below.

fruit weight. Fruit weight is strongly affected by environment and cultural operations. Also, subjective assessment of fruit weight in the field is difficult even where the differences are as much as 10%. For these reasons, roguing against decreased fruit weight is considered not to be very effective

(Kerns, 1928). There is, however, a tendency for the very heavy and very light types within a field to be genetically different. Roguing can therefore be used to discriminate against very small-fruited types (Anon., 1925). Where more objective methods of assessment of fruit weight are used, such as in a clonal selection programme, gains in fruit weight are possible, provided measures are taken to avoid inadvertent selection for more translucent types. As an example, a 10-15% increase in fruit weight was achieved in the Australian clonal selection programme (Wassman, 1982).

multiple crowns. Multiple or fasciated crowns can be heritable but the trait is also strongly influenced by environment and cultural operations (Dalldorf, 1975). Excessive nitrogen fertilization and high temperature during flower-cone development are known to increase the incidence of multiple crowns (Anon., 1925; Linford and Spiegelberg, 1933). Linford and Spiegelberg (1933) observed that the condition can start at the top of the fruit or, as evidenced by the change in eye pattern, at a position in the fruit itself. Replanting only single crowns from vigorously growing plants that flowered during the hotter periods of the year should reduce its incidence (Dalldorf, 1975). Roguing cannot completely eliminate this disorder.

collar of slips and knobbiness. Collar of slips is a condition where several slips emerge from the base of the fruit. This severe defect is caused by a dominant mutation (Collins and Kerns, 1938). The incidence of knobs on the base of the fruit is closely associated with collar of slips and is sometimes considered a milder expression. Collar of slips is highly heritable but influenced by environment. In Australia, collar of slips is usually expressed in summer fruit but not winter fruit (Groszmann, 1940). Also, plants propagated from the collar-of-slips type will occasionally produce about 10% normal types (Collins, 1933a; Groszmann, 1945). Fruit from collar-of-slips types is usually smaller and slip number is more than double that of normal plants (Collins, 1933a; Py et al, 1987). It is best to avoid using slips as planting material unless the parent plants have been checked before harvest for the incidence of collar of slips. Eliminating this off-type should minimize its incidence. Collar of slips is one of the main characteristics targeted in a roguing programme (Py et al., 1987).

The number of slips produced lower down on the peduncle is not considered the same as collar of slips, as it is a quantitative trait whose expression is more strongly influenced by plant vigour and environment (Collins and Kerns, 1950). Vigorous plants on the outer edges of fields commonly produce more slips. The clone 'F200', which is grown widely in the Philippines, reputedly does not produce slips. Roguing against increased or decreased slip number is not likely to make much difference, because of the large environmental and cultural influences. However, a well-conceived clonal selection programme should make progress in this regard.

fruit conformity. There are several distinct mutations that affect fruit shape or appearance. Long Tom or long, slender fruits is a heritable condition where the fruits are long with a small diameter. Other common mutations in fruit conformity include dry fruit and bottleneck. With these off-types, the flowers are absent from either the upper portion of the fruit or the entire fruit. The effect is a small malformed fruit with obvious bracts but poorly developed eyes. These genetic disorders should not be confused with the physiological disorder associated with sun damage (Broadley et al., 1993). Roguing against these off-types should eventually eliminate them.

spiny leaves. Genetic studies have shown that spiny leaf is recessive to smooth leaf and 'Smooth Cayenne' carries the gene for spines in a recessive condition (Collins and Kerns, 1946). The mutation of the dominant gene for smooth leaves to the recessive gene for spines can occur at any time in the growth of the plant. Depending on the developmental stage of the plant when the mutation occurs and the position in the plant of the mutating cell, it can result in a chimera with partially spiny leaves or a completely spiny plant (Collins and Kerns, 1933).

The mutation to spiny leaves should not be confused with the tendency of 'Smooth Cayenne' to develop spines as a result of stressful conditions and the cessation of leaf growth. As an example, a high night temperature increases the development of spines along the leaf (Friend, 1981). A genetic change is not thought to have occurred in these instances and new leaves develop normally once the growing conditions are favourable. Roguing is very effective in removing spiny-leaf off-types.

Clonal selection

While roguing is a form of clonal maintenance, clonal selection offers the possibility of clonal improvement. Clonal selection aims to identify superior individual plants that are eventually multiplied as a new clone. Clonal selection, however, is a slow process and is usually beyond the resources of the commercial grower.

It has been demonstrated that apparently normal plants can carry some undesirable traits, such as multiple crown. If enough plants of that genotype are observed over several environments, these traits will eventually be expressed and that line or clone can be eliminated. Clonal selection is therefore more successful than roguing against defects such as multiple crowns that are strongly influenced by environment.

The initial population in a clonal-selection programme is established usually by walking through fields of maturing pineapples and selecting the crown from superior types or marking the plant with paint and a stake to collect the entire plant later. These selections are then assessed and multiplied over several crops until only the most consistent clones remain. When sufficient plants are available, replicated trials can be established in different environments. The method of obtaining the initial population will, of course, vary depending on the specific characters in which the breeder is most interested. Novel methods of selection can be used for some traits. As an example, a different approach to selecting for larger-fruited types has recently been used in the Australian clonal-selection programme. Selection of larger-fruited types by sorting several thousand fruit using a weight grader appears more reliable than walking through fields and selecting plants by subjective assessment of fruit weight (G.M. Sanewski, unpublished results).

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