Genetic Improvement - CIF/FEM Collaboration
The first cultivar obtained by crossing was attributed to Thomas Andrew Knight (1759–1838). Another method to obtain new cultivars consisted in the selection of mutations and chimeras (Janick et al. 1996); these develop shoots with a stable variation when they are propagated vegetatively. The crossing of two parents is now, as it always has been, the main method in apple breeding (combination breeding).
Currently, the main characteristics of cultivated apples are (1) size over 100 g or 70 mm as a minimum for the market; (2) colors: yellow, green, red, bicolor, and brown in susceptible apples to russeting; (3) acidity: sweet apples when malic acid is lower than 4.5 g/L and bitter when it is over that limit; (4) tannins: sharp apples are those with more than 2 g/L of tannic acid; (5) sweetness: most of the cultivars contain between 12,8 and 18,8 °Brix; (6) harvesting period from August to November; and (7) resistance to diseases and abiotic stress.
Plant Breeding
The eating quality is difficult to measure objectively (Hampson et al. 2000). Contribution of crispness accounts for about 90% of the variation in texture liking. Juiciness, aroma, sweetness, and sourness change their relative importance from year to year. They account for about 60% of variation in flavor liking. Sweetness and sourness are better predictors of liking than analytical measurements of soluble solids and titratable acidity.
Some special characteristics can be important in the use of specific cultivars, as (1) sensibility to russeting, which produces a brown aspect that is specific in some cultivars such as ‘Reineta Gris de Canada’, ‘Boskoop’; (2) growing habit, spur types, and weeping; (3) late blooming; (4) high cold hardiness; (5) resistance.
A modern apple breeding program consists of 4 different steps:
Definition of a prototype (ideotype) of plant variety combining quality traits important for the consumer, with a long term storability of the fruit and the adaptation to a particular environment, resistance to the major diseases and insects included;
Production of hybrids crossing available varieties showing those traits identified during the definition of the ideotype;
Cultivation in greenhouses and fields of the hybrid progenies with several replications in different years and locations;
The complete process may last for 15-20 years and the selection step involve tens of thousand of plants. In a modern breeding program, like the one currently carried out by the E. Mach Foundation, the selective step is assisted by the use of molecular markers. To introduce in apple breeding new traits predicted to be necessary in the future, frequently the elite varieties are crossed to wild apple accessions showing positive attributes, like disease resistance and new skin colors, but with a small size of the fruit, frequently also associated to poor edibility. It is from similar crosses that, starting from about 30 years ago, several cycles of backcrossing to high quality varieties has made today possible to release varieties resistant to the scab disease elicited by Venturia.
A further innovative approach concerns clonal selection: the identification of clones derived via mutagenesis from superior varieties. These clones are genetically very similar to the original variety, but differ because of a better fruit (color, size) or plant taits (vigor). For instance, several derived clones are available for the varieties Red Delicious and Gala.
The role of molecular genetics in apple breeding
The approach to apple breeding based on molecular genetics has a major impact in reducing the time necessary to carry out those steps indicated as crossing and selection; the new proce- dure results in what it is known as “marker assisted selection, MAS”. Genomic data and the mapping position of apple genes in the 17 chromosomes of the species, allow to individuate in the chromosomes of specific varieties or wild accessions those genes which are responsible for superior traits.
The approach to apple breeding based on molecular genetics has a major impact in reducing the time necessary to carry out those steps indicated as crossing and selection; the new proce- dure results in what it is known as “marker assisted selection, MAS”. Genomic data and the mapping position of apple genes in the 17 chromosomes of the species, allow to individuate in the chromosomes of specific varieties or wild accessions those genes which are responsible for superior traits.
The priorities assigned to applied apple molecular genetics consider the identification of the genes responsible for i) resistance to biotic and abiotic stresses such as fungal diseases (scab and mildew), bacterial diseases (fire blight) ii) quality traits of the fruit (juiciness, crispness, firmness, sweetness, texture, acidity, skin and flesh color) and nutraceutical characteristics; iii) fruit storage; iv) plant architecture (plant vigor and shape).
