Like most higher organisms, apples generally have two sets of chromosomes. This is a useful arrangement, providing each gene with a backup, and allowing such helpful features as sexual reproduction. (This ought to be a review of high school biology for most of us, but I assume that for people who don’t deal with this daily, as I do, it may have snuck away to wherever it is my knowledge of trigonometry has gone). The large majority of plants conform to this arrangement (diploid), but the occasional error in cellular mechanics has resulted in an array of species with more than the standard two sets. This phenomenon, called polyploidy, is an important mechanism of evolution, and it exists in many fruit crops.
As long as the number of sets of chromosomes remains even, things can continue to function pretty well (though it’s by no means guaranteed…plants are complex things). Reproduction can occur normally, with each parent contributing half of its complement of chromosomes. Uneven ploidy numbers pose a problem for reproduction, though (three sets of chromosomes doesn’t divide neatly), and as a result such plants are often at least partially sterile.
Since sterility equals no fruit, one would think that apples with three sets of chromosomes (triploids) would be of no value to anyone. While not fully sterile, these trees produce very little viable pollen, and set a fruit on a fairly small proportion of flowers. Yet there are an amzing number of triploid apples. A trip to the grocery store up the road from me would turn up at least two (depending on the time of year), ‘Jonagold’ and ‘Crispin’ (aka ‘Mutsu’). The early American cultivars ‘Tompkins County King’ and ‘Rhode Island Greening’ are both triploid, as are the British apples ‘Bramley’ and ‘Ribston Pippin’ (mentioned in an earlier post). The earliest known triploid apple is ‘Gravenstein’, known as early as the seventeenth century in Denmark, and a popular apple of the colonial era, still grown occasionally today.
The reason why such trees have been popular, despite their obvious problems, is simple: size. Polyploids are generally characterized by larger anatomies, and this includes the fruits. ‘Jonagold’ and ‘Mutsu’ are among the largest apples in commerce today (averaging over 150 grams, with the occasional 300 gram fruit). I, personally, feel these particular apples are often too large, pushing the lower limits of meal-sized, rather than snack-sized, but there are plenty of firm believers in the “bigger is better” school of thought out there.
The fertility problems do complicate things a bit, though. The lower number of viable flowers is rarely an issue (apples produce far more flowers than they could ever actually bear fruit, and normally require thinning anyway), but the lack of viable pollen is an issue. Most apple cultivars are self-sterile, a problem overcome by planting two genotypes, allowing each to pollinate the other. Because triploid apples produce to no viable pollen (or at least very, very little), two other diploid apple cultivars are required (the two diploids pollinate each other, as well as the triploid…if only one diploid is used, it will pollinate the triploid, while remaining unpollinated itself.) And of course, one must also make sure that the three cultivars are all flowering simultaneously. Luckily, years of research has provided growers with a wealth of information on the ploidy of various cultivars as well as their flowering habits, making monster apples a reality with just a little extra orchard planning.
Polyploidy has been a major force in both evolution and breeding, providing both opportunities and difficulties. As some one who works with strawberries (octoploids), ploidy issues are something I spend a lot of time with. Future articles will likely deal with ploidy issues not only of apples and strawberries, but also brambles, blueberries, and stone fruits.