WHY IS TRIPLOID APPLE POLLEN SO POOR?
Triploid pollen is effectively sterile and cannot be relied upon to pollinate diploid apples because of chromosomal abnormalities. During pollen formation, meiosis is disrupted, leading to non-viable pollen grains.
The Genetic Explanation
Diploid vs. Triploid Chromosomes: Most apple varieties are diploid, meaning they have two sets of chromosomes (2n = 2x = 34). Triploid varieties, in contrast, have three sets (2n = 3x = 51).
Meiosis and Gamete Formation:
For successful pollination, an apple tree must produce viable gametes (pollen and ovules) through a process called meiosis. Meiosis requires chromosomes to pair up precisely.
The Problem with an Odd Set:
In triploid plants, the three sets of chromosomes make this pairing process highly irregular. During meiosis, the chromosomes cannot be evenly divided into functional haploid (one set) gametes. The odd set leads to the formation of gametes (pollen grains) that have an incomplete or unbalanced number of chromosomes (aneuploidy).
Pollen Inviability:
The vast majority of these aneuploid pollen grains are not viable and either fail to germinate on the diploid stigma or, if they do germinate, the pollen tubes grow very slowly and often stop before reaching the ovule.
Low Success Rate:
While it's over-simplifying to call triploid pollen "sterile" (a small number of viable grains are sometimes produced), the viability rate is very low, often less than 10%, compared to 70%+ in diploid pollen.
This makes triploid varieties useless as effective pollinators for other trees in an orchard setting.
Note for interested students...
Crane and Lawrence (ref. 1), in the years up to 1938, did extensive research on crossing apples and recording the number of fruit produced for both self-pollination and cross-pollination. Their book gives data for about 100 separate named crosses.
Cross-pollination gave ratios for (fruit produced)/ flowers pollinated ranging from 0% to 33%, depending on the degree of compatibility.
Self-pollination gave values between 0% and 5% for most varieties, with just five named apples reaching 7 - 10%.
It's interesting that triploids in the list included Blenheim at 1.27%, Bramley at 4% and Ribston at 4.5%. So if you're relying on self-pollination, triploids are no worse than the others.
REFERENCES
1. Crane, M.B. and Lawrence, W.J.C., The Genetics of Garden Plants, pp 200-203, MacMillan & Co, 1938.
Nigel Deacon / Diversity website
