POTENTIAL VALUE AND METHOD OF PRODUCING HAPLOIDS IN THE APPLE TREE, MALUS PUMILA (MILL.)
Mutagenesis is a very suitable method to improve a species such as apple, which is propagated vegetatively. Many mutants have been selected following irradiation with gamma rays. Obviously, irradiation of haploid plants would enlarge the range of mutants.
After doubling the number of chromosomes, the plants obtained will be the first completely homozygous apple clones. Producing totally homozygous clones is only feasible by using the haploid method. Homozygous apple clones should be valuable tools for the breeder, serving as parents which can transmit desirable genes to each gamete.
The method of producing apple haploids involve the use of a genetic marking system, an hybrid derived from Malus p. niedzwetzkyana. This hybrid is homozygous for a dominant marker gene, R, which determines the purple colour of leaves, stem and fruit. The green phenotype seedlings, presumably of a maternal origin, were selected and the chromosomes were counted. At present, 5 apple haploids have been selected from different varieties or hybrids : 1 from Topred Delicious, 1 from Erovan, 1 from Querina (a scab resistant variety) and 2 from a selection originating from a second generation of selfing Golden Delicious. The haploid raised from Topred Delicious was doubled by in vitro culture ; it is the first homozygous apple clone.
We try to get other haploids from different varieties and advanced scab resistant selections. We use the same technique combined with delayed pollination and pollen irradiation.