Hi Mark,

You are correct that the F1 hybrids would be seed-sterile, due to the odd number of chromosomes. However, the usual thing to do in that case is to double the number of chromosomes (using colchicine or an alternative) to create a 62-chromosome plant, called an allotetraploid, that can set viable seeds.

The Profusion, Pinwheel, and Zahara zinnias are commercial examples of that being done. Zinnia violacea (2n = 24) was crossed with Zinnia angustifolia (2n = 22) to obtain sterile F1 hybrids (2n = 23). Colchicine was use to double the chromosome count, resulting in some fertile, true-breeding allotetraploids (2n = 46). These allotetraploids were designated as a new species named Z. marylandica in honor of the University of Maryland, where much of the original research was done.

"...meaning any variability I would see could only come from this first generation, as plants from the F1 generation could not be further crossed to select for certain traits..."

Unfortunately or fortunately, depending upon your point of view, that tends to be the case. Zinnia violacea (frequently still referred to as Zinnia elegans) are naturally heterozygous and highly variable, except in the highly inbred commercial F1 cultivars. Many gardeners appreciate that the Profusions and Zaharas do come true from open-pollinated seeds. I think that you can make crosses between allotetraploids to produce F1 allotetraploids, but the recombinations that "shake out" in the F2 generations seem to be more restricted than you might want. In other words, genes from one original parent don't seem to recombine with genes from the other original parent. However, with persistence you might be able to get some recombination. Also, in the case of zinnias, there have been some interesting results from back-crossing the allotetraploids to both Z. violacea and Z. angustifolia.

So, I say, "damn the torpedoes" and make the cross and then double the chromosomes and then continue experimenting with seeds grown from the allotetraploids. For one thing, you can try crossing different allotetraploids, as well as back-crossing.

Another approach you could try would be to create tetraploid versions of both original species, which would have chromosome counts of (2n = 64) and (2n = 60) and make F1 crosses between them, which would have a chromosome count of (2n = 62) and could potentially be fertile. Also, you could try crossing some of these 62-chromosome F1 tetraploids with the 62-chromosome allotetraploids. New territory there. In fact, this whole line of experimentation is pretty much new territory. Good luck.

ZM

Mark,

Make the cross, if you can, and see what happens. A difference in chromosome number does not necessarily mean that the hybrid will be sterile. Much depends on why there is a difference.

In Lycoris aurea, for example, there are different chromosome numbers. In some specimens, two chromosomes have become linked end-to-end. So, where one parent may contribute two chromosomes, A and B, the other passes along a single chromosome, AB. "Pairing" is a bit odd, since both A and B hook up with AB, but it all works out.

Furthermore, a hybrid of L. aurea (2n = 14) and L. radiata (2n = 22) yielded a fertile hybrid that was then backcrossed to both parents.
http://www.actahort.org/books/430/430_83.htm

Viola tricolor has 13 chromosomes, V. arvensis has 17. Clausen studied the hybrids (some fertile, some sterile) of these species, and found an assortment of chromosome numbers in progeny.

Make the cross. Worry about consequences after you succeed. There was a lot of successful plant breeding going on before anyone knew about chromosomes.

Karl

Here is a link that might be useful: Clausen: GENETICAL AND CYTOLOGICAL INVESTIGATIONS ON VIOLA TRICOLOR L. AND V. ARVENSIS MURR

Thanks for the reply. This makes me wish I was still in university and could pop on over to the Life Sciences Dept. and try and convince someone to treat the F1 generation with colchicine. Alas, that is no longer the case, but maybe something like nicotine sulfate would work. Ideally, something really nice will come out of the F1 generation and further breeding will be more of a luxury, rather than a necessity.

Mark

Hi Mark,

"Ideally, something really nice will come out of the F1 generation and further breeding will be more of a luxury, rather than a necessity."

That would be nice. However, in my experience breeding zinnias, the F1 crosses are usually somewhat disappointing, and the really interesting things have come from recombinations in successive generations. And making hybrids between hybrids has yielded a few good results for me. This is an example of a zinnia that appeared last year in my garden in the recombinants from hybrids of hybrids of hybrids. You won't find anything like this in a zinnia seed packet. And I doubt seriously that it could be obtained by making an F1 hybrid. There just isn't any available starting material that would produce this as an F1 hybrid.

{{gwi:17244}}
"Alas, that is no longer the case, but maybe something like nicotine sulfate would work."

Do an Internet search on Oryzalin (Surflan) and trifluralin. There are several alternatives to colchicine for chromosome doubling and they can be more effective, less hazardous, and more available.

ZM

Colchicum autumnale - colchicine

All parts of the plant contain toxins. The greatest concentration of toxins is found in the seeds and the bulb (corm) (Cooper & Johnson, 1984; Frohne & Pfander, 1983).

Colchicine is present in the flowers (0.1 to 0.8% in fresh flowers; up to 1.8% in dried flowers), in the seeds (0.2 to 0.8%) in the bulb (corm) (0.4 to 0.6%). The leaves contain very low amounts of colchicine (Gessner & Orzechowski, 1972).

Here is a link that might be useful: IPCS Inchem

Karl,

Thanks for that very informative link about colchicine. I think it is interesting that it said, "Intoxication is always due to oral absorption of parts of the plant or extracts."

So, if you can just resist the temptation to eat the stuff, you should be safe. They didn't say anything about absorption through the skin, which I think is significant. Of course, you will take colchicine orally if it is prescribed to you by a physician for use as a drug. I think that, historically, castor beans have been more dangerous.

ZM