Monday, February 29, 2016

Tetraploid Arils, Anyone?

by Tom Waters

'Tadzhiki Eclipse'
If you're a gardening enthusiast with a keen interest in irises or other perennials, you've probably come across the term tetraploid. There are tetraploid daylilies, for example, and in the iris world you'll see the term used to describe some miniature tall bearded (MTB), Siberian, and other types of iris. But what is a tetraploid and why does it matter if a variety is tetraploid or not?

Plants and animals are made up of cells, and in the nucleus of each cell are structures called chromosomes that carry the organism's genes. These chromosomes come in sets. For almost all animals and many plants, there are two sets of chromosomes in each cell. Humans, for example, have two sets of chromosomes, 23 in each set. At conception, each person receives one set of chromosomes from their mother and one from their father. One can't really think of any simpler way for an organism to inherit genes from both parents.

There are more complicated ways, however, especially in the plant kingdom. Some plants have four sets of chromosomes, two sets coming from each parent instead of one. These are tetraploids. (Plants with just two sets are called diploids.)

What are the consequences of having four sets of chromosomes instead of two? One consequence is that tetraploids are frequently (not always) larger than their diploid counterparts. In the 19th century, garden tall bearded irises were all diploids. Around 1900, plant collectors found tetraploid tall bearded species in the Middle East, and brought them into European and American gardens. The new plants attracted attention because of their taller stems and larger flower size. (There was no knowledge of chromosome numbers at the time, so iris growers and breeders were aware only of the difference in appearance of the new plants.)

The tetraploids were so popular with iris breeders that by the middle of the twentieth century, virtually all new TB varieties were tetraploid. This is still the case today.

One must be cautious, however, in associating tetraploids with large size. Size is determined by many factors. One of the very smallest bearded irises, the dwarf species Iris pumila, is a tetraploid although the blooms are only a few inches above the ground!

The great importance of tetraploidy in plant breeding is not that tetraploids are sometimes larger, but rather that more genetic combinations are possible with tetraploids than with diploids. With four copies of every gene instead of two, there are just many more possibilities!

One extraordinary aspect of tetraploid breeding is that hybrids between different species or types may be fertile if the parents are tetraploid, but sterile if the parents are diploids. The standard dwarf bearded irises (SDBs), for example, are fertile hybrids between tetraploid TBs and the tetraploid dwarf Iris pumila. So tetraploidy makes possible whole new types of fertile hybrids that simply could not exist otherwise. This is the main reason some hybridizers have taken such an interest in tetraploids.

'Bronze Beauty Van Tubrgen'
So now we come to the topic of this post: tetraploid arils. I love aril irises, with their exotic color patterns and dramatically shaped blooms. Most aril species are diploids. (I'm referring to pure arils here, not the arilbreds, which are a mix of aril and bearded ancestry.) In fact, the oncocyclus arils (the type that usually comes to mind when one thinks of arils) are all diploid. Crossing them with bearded irises, when it can be done at all, produces sterile hybrids.

Think of the possibilities that could open up if there were a family of tetraploid arils, like the family of tetraploid TBs!

Aril hybridizers have been thinking of that possibility for decades, and there are in fact some tetraploid arils, just not very many yet.

First, although all the oncocyclus species are diploids, there are two Regelia species that are natural tetraploids: Iris hoogiana and Iris stolonifera. A classic hybrid from these two species, 'Bronze Beauty Van Tubergen' (Van Tubergen, R. 2001) is also tetraploid. These are all lovely irises, but being Regelias, they do not have the oncocyclus characteristics, like huge globular blooms and big velvety black signals, that are the allure of aril irises for most people.

Second, a few arils with some oncocyclus ancestry are "accidental" tetraploids. Occasionally, a diploid parent produces a tetraploid seedling by chance. 'Persian Pansy' (Austin, 1960) and 'Tel Hashi' (Kellie, 1968) came about this way, but both seem to be lost now. 'Dunshanbe' (Wilkes, 1977), a seedling of 'Persian Pansy', is still being grown however, and is quite lovely with a dark sheen and nice veining. Its makeup is still more Regelia than oncocyclus, however.

Third, some breeders have created tetraploids deliberately using the chemical colchicine, that interferes with cell division, sometimes converting a diploid plant to a tetraploid. Usually, the plants created this way are unstable and sometimes unhealthy, but they can be crossed with other tetraploids to produce stable, reliable plants. 'Werckmeister's Beauty' (Werckmeister, 1992) came from this approach. In the 1980s, Sam Norris and John Holden created many tetraploid arils of predominantly oncocyclus makeup using this method. Alas, few have survived to the present day, and none are available commercially. Lawrence Ransom used one of these to produce 'Tadzhiki Eclipse' (Ransom, 1997) and 'Tadzhiki Bandit' (Ransom, 1997), both of which are still being grown in Europe and the US. 'Tadzhiki Eclipse' is, I think, the most oncocyclus-like tetraploid aril one is likely to be able to acquire today.
'Werckmeister's Beauty'

Through a little patience and diligence, I have acquired a small collection of tetraploid arils from the plant sale of the Aril Society International, occasional offerings of commercial growers, and the generosity of fellow aril hobbyists. I encourage others to do the same, especially those who live in the western US or other regions where pure arils can be grown without heroic measures being taken on their behalf. These are arils, not arilbreds, so they may need a little special attention regarding soil and siting. I have found them easier than most other pure arils, however.

If you are a hybridizer, you can cross them with each other to expand the family, or cross them with tetraploid bearded irises to create brand new types of fertile arilbreds.

Even if you are not a hybridizer, you can enjoy their bloom and know that you are helping preserve a very important group of plants, which may someday become the progenitors of new breeds of irises we can only imagine today.

If you can grow aril irises, have you tried any of the tetraploids? I think they are worth seeking out.

'Tadzhiki Bandit'

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