By reducing fertility, by the way, I really meant self-fertility, as in field-performance of the plant. Fertility for a particular cross, particularly an inter-specific cross, can certainly be improved by changing ploidy. Wasn't sure in hindsight which you were refering to.

Well, I guess an inter-specific cross where the offspring is say triploid and not very (self)fertile. How would one go about doubling the ploidy in an effort to restore fertility?

Treatment with colchicine ought to work, theoretically anway. It's been a long while since I've seen this done, so rather than tell you how, I'd just recommend Googling around a bit...there should be a fair amount out there on it.

Just remember the colchicine is seriously nasty stuff.

What Eladazal means by seriously nasty stuff is that what colchicine does to plant cells it also does to lung cells if you inhale the powder while mixing it. It can be used safely, but you must realize that inhaling the dust can kill you. And I don't mean a lot of dust. Get help from a chemist who has worked with poisons and is stll healthy if you are going to do this.
Getting it on your hands is not so dangerous, if washed promptly, and the stuff on your hands doesn't dry and become dusty. but still, it is better not to touch it. Get help with it if you want to do it.
Which ploidy to use as female depends on the plant family. In many, maybe most plant families, the loewr ploidy is most successful as feale. but in wheat, where I first did genetic research, the higher ploidy parent is best used as female.
In Sorghum, where I did later work, a ploidy cross of diploid x tetraploid can give mostly fertile tetraploids if the diploid female has genes for male sterility.
The reverse cross, or the diploid x tetraploid without using a male sterile female, gives almost all sterile triploids.
So try both ways until you find out how your plants do.
Walter

I've done some chromosome doubling over the years with roses and some herbaceous perennials. I recommend using trifluralin instead of colchicine. It is the active ingredient in some preemergent herbacides and is more active on plant microtubules than colchicine which in general is more active on animal microtubules. Trifluralin slows and stunts cell division of the radicle of germinating seedlings and is why it is used as a preemergent herbacide. If used carefully on shoot tips, one can recover polyploid shoots. Preen is a common product which is just Treflan (~43% trifluralin) soaked pieces of ground corn cobs. I use treflan itself since I can mix known concentrations instead of trying to leach it out of corn cobs. I found ~0.08% trifluralin in the end to work well. I like to use a drop of soap in the solution as a surfactant and about 1% dimethyl sulfoxide to help make cell walls more permeable. I prefer putting a drop of this mixture on germinating seedlings right between their cotyledons before true leaves are present. Using seedlings seems to allow for a greater conversion rate than axillary buds or shoot tips later in development since there are not many leaf primordia over the growing point yet and the chemical penetrates the growing point more readily.

Good Luck,
David

How do you find out what ploidy a plant has?

Well, technically to determine the ploidy of any particular plant you'd have to do a chromosome squash and count chromosomes, I'd guess. The easiest way to come up with the ploidy of species in general would be to look it up, however there are frequently different ploidy levels within a species. A little googling will turn up standard ploidy levels for most things...ask around here and people will be able to help out, too.

In many (I'd say most, but I don't really know enough species to say so with confidence) species, ploidy increases of more than doubling are pretty rare. Polyploidy messes up lots of stuff. Some species are more tolerant than others...Darrow's book has a picture of a relatively healthy looking 32X strawberry, something that would be pretty much impossible in many species.

Thick, oversized stems and leaves and poor health are possible signs of ploidy increases. Cross incompatibility with standard types would be another hint.

Ethane, it is my impression that looking at the pollen under a microscope may be useful as a "first guess". Apparently there is enough overlap between say diploid and the next higher up (in this example triploid) so that it is not useful for definite determinations.

Since triploids seldom have viable pollen, the next practical step up is tetraploid. Since a tetraploid will have about twice the cell volume as a diploid, the linear length of a pollen grain from a tetraplooid will be about 1.3 times the length of a pollen grain from a diploid.
That is about the cube root of 2. And it was done in my head so it is not exact. But close.
The pores on the bottoms of leaves give good correlation with ploidy in the grasses. I have no experience with looking at pores of other plants.
Walter

This is all so fascinating. I am a seasoned gardener but a novice to this sort of culture. Can someone suggest very basic simple language and diagramed reading material on the subject?

Thanks,
Toddy

Walter Pickett wrote, "Since triploids seldom have viable pollen".

This rule is not so broadly applicable as you suggest. Most triploid roses are at least partially fertile and can be used as pollen parents if you use plenty of pollen. And in the Hyacinths, triploids are just as fertile as the diploids -- and are even self-compatible, which the diploids usually are not.

It is also useful to keep in mind that three "types" of triploid are possible. Autotriploids have all triplicate chromosomes, presumably from the same species. Instead of pairing, many of the chromosomes may attempt trio-ing, which is messy and can lead to much inviable pollen.

Auto-allotriploids have two sets of matching chromosomes along with one set of unmatched chromosomes that don't pair as well with the others. These triploids can behave almost like diploids, though with reduced percentage of viable gametes and occasional exchanges with the dissimilar chromosomes.

Finally the fully allotetraploids have three unmatched sets of chromosomes. Call them A, B and C. Sometimes some of the chromosomes of C (for example) will find partners among the B set while the others will take their mates from the A set.

Karl

It also can make a great difference whether the triploid is derived from a diploid species, a cross between a diploid and a tetraploid species, a cross between a diploid and a newly formed tetraploid, or as a haploid of a hexaploid species.

Higher polyploids may carry "genes" that inhibit chromosome pairing. Otherwise they'd end up with trivalents, quadrivalents and other tangles that would reduce viability of the gametes.

There are also "genes" that increase pairing frequency. The balance of enhancing and inhibiting factors keeps the chromosomes of a species pairing properly. When we cross species with different chromosome numbers, the degree of fertility of the hybrids may be influenced by the new balance of factors affecting chromosome pairing frequency.

For example, a cross between a diploid cherry-plum and a hexaploid domestic plum gave a tetraploid with nearly normal chromosome pairing. On the other hand, a triploid derived from a hexaploid wheat behaved like a haploid with no chromosome pairing.

Karl

Here is a link that might be useful: Genetic control of chromosome pairing

Does anyone know why when clicking on links posted by members,a commercial site shows up,almost every time?

You can change the chromazone count by using colchasine which is dangerous , or growing the plants you want to change amoungst CROCUSES they have colchasine in them and have mutagenic proprerties . good luck