2.1 Steps in selection

The first step is to select dura female parents in dura x dura crosses, which segregate into 100% dura; and to select pisifera male parents in tenera x tenera (25% pisifera) or in tenera x pisifera crosses (50% pisifera).

Breure et al. (1987) describe the history of phenotypic parent selection from bunch yield and physical bunch analysis in early commercial plantings to the present method whereby all components of oil extraction rate, growth and leaf-Mg status are also taken into account. The method of growth recording was developed in Malaysia (Hardon et al., 1969; Corley et al., 1971), and an early account on how these measurements were applied in breeding work elsewhere was given by Breure et al. (1982).

The standard procedure is to select first dura x dura families. Individual dura female parents are then selected within these selected families based on more detailed recording, in terms of bunch analysis, growth and sometimes leaf-Mg level.

Family selection in the source of pisifera male parents is handicapped by the occurrence of sterile pisifera. Yield and bunch analysis records are therefore restricted to tenera palms only. Once elite families are identified, pisifera selection can only be based on growth and magnesium level.

The genotypic performance of palms selected for phenotype (step 1 in the selection procedure) is still masked by all sorts of errors. Indeed, the correlation between phenotypic characters, in particular yield, of the dura parents and their tenera offspring is usually poor, while as mentioned before, pisifera cannot be selected for yield per se.

Fortunately, as in other crops, in oil palm the performance of the offspring can be quite accurately estimated by adding the genotypic effects of the female and male parents (Breure & Bos, 1992). Hence the expected yield of the tenera offspring, E(y), can then be described as a constant + the genotypic effect of the dura female parent + the genotypic effect of the pisifera male parent. These additive effects of the parents are in quantitative genetics termed General Combining Ability (GCA) values. Reliability of selection can therefore be greatly improved by selecting parents according to GCA values estimated from results of dura x pisifera crosses (step 2 in the selection procedure).

The additive model does, however, still not fully predict the performance of the tenera offspring; crosses may perform better or worse than estimated by adding GCA values of the parents. This deviation is due to the effect of Specific Combining Ability (SCA); but remember this SCA effect is usually much smaller than the effect of GCA.

To exploit both GCA and SCA effects the pa- rents must be crossed in all combinations. Elite families selected among these crosses can be reproduced by crossing specific parents (step 3).

Le Guen et al. (1991) reported a considerable increase in yield and extraction rate of tenera clones above the mean of the palms of the dura x pisifera family from which the tenera was selected. This is not surprising as tenera can be considered as relatively heterozygous while the dura and pisifera parents are relatively homozygous genotypes. Recombination of favourable alleles in response to the cross can therefore generate an excess of heterozygous and thus superior genotypes. Cloning these elite tenera palms yield thus palm populations consising of homogeneous and heterozygous genotypes.

The final phase in improving planting material therefore appears to clone outstanding tenera selected within elite families of step 3.

The female parents tested in step 2 and step 3 are reproduced as dura selfings. Selfings of all parents tested are planted at the same time as the step 2 parent testing program is established. Female parents for seed production are then selected in selfings of dura which are selected on the basis of the outcome of the test crosses.

It has been shown in maize (Hallauer & Miranda, 1981; p. 281-283) that there is a sufficiently large correlation between performance per se of the inbred lines and their GCA value calculated from all crosses obtained from these lines.

For this reason the dura selfings are also recorded in detail to support results obtained from the dura x pisifera test crosses.

The aim is to clone all pisifera tested in step 2 and step 3 ; as a safeguard, in case cloning fails, pisifera are crossed with an elite tenera in the same family. Either clones or pisifera selected in elite tenera x pisifera crosses are selected for seed production.