Triploid Varieties

A number of ‘iconic’ varieties, such as Cox’s Orange Pippin, Oxbo, Ashmead’s Kernel, were reportedly bred from parents, one or both of which were triploids.  Yet it has long been known that pollen and seed of triploids have low viability. It added a mystique, perhaps because it was also inconsistent.

DNA has revealed that there are few if any examples of triploids as parents.  Cox’s Orange Pippin, like other progeny of a purported triploid is actually from a pair of diploids, in this case Margil x Rosemary Russet.

While triploid varieties may have desirable properties, such as a greater disease/pest resistance, crop yield, and size, they have little or no use for further breeding.  Howard et al. (2022) have further argued that triploids are essentially an evolutionary dead-end.

Triploid varieties have three, rather than the usual two of diploids, copies of all 17 chromosomes. Considine et al. (2012) and others have shown this arises from one parent (often the mother) donating all its DNA and the usual half (haploid) set from the other parent.  However as triploids have 51 chromosomes, the normal process of cell division into two by meiosis is messed up and results in both pollen and ovules with gametes likely having more than a haploid set .  Hence the low viability.

One parent can be identified with considerable confidence when a triploid contains a complete copy of the entire fingerprint of a diploid variety; Ashmead’s Kernel does contain the entire fingerprint of Nolan Pippin.  However the second parent is often more difficult than normal to identify.  Even if one or both parents are lost or haven’t yet been fingerprinted, if two triploids share at least two alleles for each of the marker-pairs, they are likely full-siblings with the same diploid gamete donating parent.  This aids identifying relationships; results of several such studies are reported here.