Triploids and why they cannot pollinate

The cultivated apple, Malus domestica, has a base chromosome number of seventeen. A normal diploid cultivar carries thirty-four chromosomes, two of each. A triploid carries fifty-one, three of each, and arises when an unreduced gamete — a pollen grain or egg cell that failed to halve its chromosome number — takes part in a fertilisation.

Having three copies of everything is not a problem for the growth of the tree. It is a problem only at meiosis, the cell division that produces pollen and egg cells, because three sets cannot be partitioned into two equal halves. The chromosomes segregate unevenly, and most of the resulting pollen grains carry an unbalanced complement and fail. Enough viable pollen occasionally survives that a triploid is not absolutely sterile, but far too little to rely on.

Triploids are not rare accidents. A significant minority of long-established apple cultivars are triploid, and several of the most familiar are: Bramley’s Seedling among culinary apples, Blenheim Orange and Ribston Pippin among dessert apples, and Bulmer’s Norman among widely planted cider varieties. Triploids are often noticeably vigorous with large fruit, which is part of why they were selected and kept.

The rule follows directly from the biology, and it is the one piece of pollination arithmetic every orchard planner needs. A block containing a triploid needs two compatible diploid cultivars in flower at the same time: one to pollinate the triploid, and a second so that the first diploid is itself pollinated. Two varieties, one of them a triploid, will leave one of them setting nothing.

The two diploids also have to be compatible with each other, which is not automatic. Cultivars that share a parent, or that are sports of one another, may be cross-incompatible even though they are distinct varieties. Where doubt exists the pragmatic solution is a crab apple pollinator, which has no shared parentage with a cider cultivar and typically flowers over a long enough period to bridge several groups.

None of this shows up until the orchard has been in the ground for five years and the crop is disappointing, at which point correcting it means grafting a pollinator branch into existing trees or planting between rows. Since a triploid cultivar’s status is a documented fact about the variety, checking it before ordering trees is the cheapest step in the entire planting.

Given the inconvenience, it is fair to ask why triploid cultivars persisted in orchards at all. The answer is that the same extra chromosome set that ruins the pollen tends to produce a bigger, more vigorous tree with larger fruit and often better disease tolerance. For a grower selecting by eye over generations, those are exactly the traits that get a tree propagated, and the pollination problem is invisible as long as the orchard is a mixed one.

Traditional standard orchards were almost always mixed, containing a dozen or more cultivars planted over decades, so triploidy caused no difficulty. The problem is a modern one, created by the practice of planting large single-variety blocks for mechanical harvest. A twentieth-century planting model made a fifteenth-century genetic quirk into an operational constraint.