The main theme for the recent RFC South West Chapter Meeting in Busselton which I attended was apple grafting. I have an extensive collection of apple varieties (heirloom and others), and in the course of various discussions I briefly spoke about polyploids. As this is not a familiar topic to most, I was asked if I would follow up by writing something to describe what it’s all about.
Introduction to polyploidy
Most species whose cells have nuclei are diploid, meaning they have two sets of chromosomes - one set inherited from each parent. This is more common in animals than plants. Polyploidy is the state of a cell or organism having more than two paired sets of chromosomes; it refers to a numerical change in a whole set of chromosomes, whereas organisms in which a particular chromosome or segment is under or over represented are said to be aneuploids. Polyploidy may occur due to abnormal cell division, either during mitosis, or more commonly during metaphase in meiosis. In addition it can be induced in plants and cell cultures by some chemicals; colchicine is the best known one, and it typically causes chromosome doubling to produce tetraploids. Almost all flowering plants (angiosperms) exhibit evidence of polyploidy throughout their evolutionary history, but over time most of these duplications subsequently become diploid.
Many fruit trees of RFC interest areas are diploid eg red & white mulberry, most Actinidia, custard apples, coconuts, date palm, durian, papaya, most avocados, carambola, cherries, Japanese plums and most citrus. Examples of higher ploidy species include:
triploid (3chromosome sets) - some apples, seedless watermelon, most cultivated bananas;
tetraploid (4 sets) - southern highbush blueberry and jackfruit
hexaploid (6 sets) - kiwifruit and persimmon
octoploid (8 sets) - strawberries and sugar cane
decaploid (10 sets) - certain strawberries
dodecaploid (12 sets) - some hybrid sugar cane
and the record holder, black mulberry which has 44 sets.
Polyploidy in apples
There are over 7500 recorded varieties of apples (Malus domestica) across the world, most of them from countries with temperate climates. Australia has approximately 900 varieties with the biggest collection of around 700 at the old Grove research station in Tasmania (around 850 pome fruit all up). Modern molecular techniques have demonstrated several of these varieties have been given separate names but are not unique.
Apples are generally diploid, however certain varieties are polyploid. The most common of these have three or four copies of each chromosome. In 2007 to 2010 the National Fruit Collection in Brogdale Kent was DNA tested. A total of 2162 apple specimens in the collection were tested and of these 304 were found to be polyploid (a comparable result was found with pears - 559 sampled and 48 polyploids). The vast majority of these were triploid, with relatively few tetraploids. Tetraploids are found in a number of wild species of Malus eg M. coronaria, M. glaucescens, M. platy-carpa and M. glabratca.
Triploids in apples
So why are triploids important in apples? Primarily because some important commercial varieties are triploid (see below). In general, triploids have the following morphological characteristics: more vigorous growth, larger fruits, large blossom (often very attractive), and large leaves that can be thick, stiff and leathery. Fruit can also have different shapes such as being more rounded, and leaves can have off-centre leaf tips or be partially folded or crimped. Additional triploid features include:
- Better disease resistance, due to increased genetic material which may produce stronger defence responses or provide additional resistance genes.
- Better tolerance of difficult growing conditions such as increased salinity, due to extra vigour and increased expression of important genes.
- Cropping is erratic if not pollinated with diploid varieties as triploids produce very poor pollen. But if this condition can be satisfied then cropping is adequate.
- Increased tendencies for fruit disorders such as bitter pit, due to poor seed content of the fruit if not pollinated by diploid varieties.
The disease resistance in some triploid apples is slowly diminishing. Bramley is a variety that has been so widely grown for over two centuries in temperate climate countries that pathogens have had sufficient time to adapt and cause more serious damage. As a consequence, disease resistance nowadays is only average.
Common Triploid Varieties
Pollination is a crucial part in growing quality apples. The vast majority of apples require cross pollination that is normally achieved by bees moving pollen from a pollen source to flowers on a recipient tree.
Apples that do not receive adequate pollination and fertilisation can become malformed as they develop, or can result in early fruit drop. Apple ovaries are typically divided into five chambers, each containing two ovules available for pollination. A fully pollinated apple will contain 10 seeds, however a minimum of 6-7 seeds per apple will succeed in good fruit development.
The average blossoming period for apples when pollination can take place is around 9 days. This can be affected by the weather with cool weather extending the period, whereas warmer weather will shorten the period. Triploid apples have an additional reproductive barrier in that the three sets of chromosomes cannot be divided evenly during meiosis, yielding unequal segregation of the chromosomes (aneuploidy). Even in the very unusual case when a triploid plant produces viable seed (as apples can) it happens infrequently and seedlings rarely survive.