WHY ARE SEEDS FROM TRIPLOID APPLES SO POOR?
Whilst triploid trees can be pollinated well enough by diploids to produce good fruit, the seeds inside, if there are any, are often not viable.
The seeds are poor primarily due to aneuploidy, a condition where the resulting embryo and the surrounding nutritive tissue (endosperm) have an incorrect or unbalanced number of chromosomes:
The Mechanism of Aneuploidy
Normal Meiosis in Diploids:
Diploid (2x) apple trees have two sets of chromosomes (34 total, n=17). During meiosis, their cells divide evenly to produce haploid gametes (1x, 17 chromosomes in pollen and ovules).
Problematic Meiosis in Triploids:
Triploid (3x) apple trees have three sets of chromosomes (51 total). During meiosis, it is nearly impossible for the three sets to pair up and segregate evenly into functional gametes. This results in most ovules containing an unpredictable, unbalanced number of chromosomes (aneuploid), rather than a single (1x) or double (2x) set.
Fertilization and Unviable Seeds:
When normal, balanced haploid (1x) pollen from a diploid tree fertilizes an aneuploid ovule from a triploid tree, the resulting embryo and endosperm have severely mismatched chromosome numbers. But the developing seed requires balanced chromosome numbers in the embryo and the endosperm to develop properly. The vast majority of these chromosomally abnormal seeds either do not form, are aborted early in development, have limited ability to germinate, or the seedlings rarely survive after sprouting.
Note from ND:
For several years I tried doing crosses with Bramley as the seed parent. I tried a wide variety of pollens. Bramley was a good pollen acceptor; there was no problem getting hand-crossed fruit. Unfirtunately the seed count was generally very low (about 2 per apple) and those which germinated produced weak, sickly seedlings.
Nigel Deacon / Diversity website
