Ploidy or roses by DNA counts (not chromosome counts)

Found this interesting paper on ploidy levels in roses as determined by DNA counts (rather than actual chromosome counts):

I hope that link works.

I found it especially interesting that both davidii and fedtschenkoana are assigned to being hexaploid by their DNA amounts. If chromosome counts confirm this, it would certainly change my breeding strategies with a few of the species cross seedlings that I already have…

rugosa X davidii and
palustris X fedtschenkoana
would probably both be tetraploids rather than triploids.

davidii X virginiana would be a pentaploid rather than tetraploid

and davidii X fedtschenkoana would be hexaploid rather than tetraploid.

Of these hybrids, only rugosa X davidii and davidii X virginiana have bloomed. Rugosa X davidii was surprisingly fertile - set lots of OP hips and seeds germinated pretty well - for what I’d been assuming was a triploid. So maybe it really is a tetraploid hybrid?

Davidii X virginiana has been incredibly infertile. I’ve only ever found 2 seeds from hundreds of OP flowers. So, pentaploidy (rather than tetraploidy) might not be so far fetched.

Excuse my novice question, will this change the thinking of “ploidy” ? and place different classes of roses into other ploidy classes or the chromosome counts still stand ?

Hi David,
I’m no expert either, but I think I’d trust actual counts of chromosomes the most. To me, this sounded like a quicker/easier(???) and maybe more cost effective way to assign ploidy to a relatively large number of samples. In cases where the ploidy indication by DNA amount conflicted with actual chromosome counts, I’d probably side with the chromosome count.

One interesting aspect though, I think they found roses with mosaic/chimeric states where some parts had one ploidy and other parts another. Just doing chromosome counts from “root tip squashes” might miss oddities like this.

And in the absence of an actual chromosome count, it sounds like it would give a pretty good indication of the likely ploidy.

But as I’d mentioned… my biggest question after skimming through, was about the actual ploidy of davidii and fedtschenkoana. I’ve been operating under the idea that both of these are tetraploid. Does anyone know any details about actual chromosome counts done on these species?

Hi Tom,

I haven’t counted those two. I agree that direct chromosome counts are valuable and take precedence. Staining DNA and running nuclei over a laser to look at florescence of the dye to estimate amount DNA per nucleus is a good indirect way to estimate ploidy and usually provides a pretty trusted estimate. There are discrepancies and it would be valuable to count davidii and fedtschenkoana again to confirm. I counted ‘Bonica’ multiple times as well as its sport ‘Royal Bonica’ and there are clearly 21 chromosomes in the root tips- triploid. Flow cytometry friends have run led for them to estimate it as tetraploid and they did not get mixoploid results suggesting multiple ploidy levels in their cell samples. I came across a book years ago highlighting that sometimes interspecific crosses can lead to hybrids that have more DNA than what would be expected from the contributions of either parent. They didn’t go into the reasons why that may be from my memory. Perhaps to stabilize the combined genomes some sections of DNA become duplicated along chromosomes???

Flow cytometry is a nice method to quickly screen through plants for ploidy and then it doesn’t hurt to use direct chromosome counts to help confirm ploidy for those plants one needs a stronger confirmation. The benefit of flow cytometry is that you just need plant tissue such as leaves to eventually purify nuclei from to work with. For chromosome counting we need cells in cell division/mitosis and at metaphase when the chromosomes are condensed and separated and easier to individually spread and count. This brings in more art and skill to get tissue at the right stage and typically takes much more time.

Tissue cultured cells often give rise to aberrant ploidy. Wide crosses may do the same. In some systems there seem to be controllers of chromosome number. Depending on the dye used, the effectiveness in labeling chromosomes may not be uniform with tissue from different parts of a plant. More or less of the DNA is open and accessible at different stages in the life cycle. So it would seem reasonable that reliability of staining nuclear DNA might vary between species, or crosses. In the caninae, not all of the chromosome sets are fully available all the time, for usual cellular functions. Whether they take up stain in the same way, I don’t know. So I’d be cautious in making interpretations based on flow cytometry.

There are so many other influences to consider and contend with as a breeder that it seems to me to be of little relative value to know the ploidy of a rose. It works for you or it doesn’t, and the only reliable way to find out is to try it.

Perhaps someone (or a group of someones) could write an article for the newsletter on ploidy with an emphasis on the practical benefits of knowing the ploidy of their breeders and hybrids?

Here’s another article that discusses the differences among the sections.

Annals of Botany 85: 557-561, 2000
Nuclear DNA Amounts in Roses
Yokoya, et al.

“In the Pimpinellifoliae, DNA amounts of tetraploids were disproportionately larger than those of diploids which suggests that they originated as hybrids with species of sections with larger DNA amounts.”

Hurst regarded R. gymnocarpa and R. willmottiae as close relatives of the diploid Pimpinellifoliae. So, I copied the data from the above article, added Hurst’s septet formulae for the species, then sorted on DNA quantity. It is interesting to see how well the data agree with Hurst’s classification.

There are a couple of oddities in the chart. For one thing, ‘Félicité et Perpétue’ has a higher DNA content than might be expected from its supposed ancestry. Also, the pentaploid R. canina has more DNA than the hexaploid R. moyesii, which happens to carry two B septets.

And one more, though not rose-related.

Heredity (1993) 71, 300–304
Heterosis and nuclear DNA content in maize
D P Biradar and A Lane Rayburn
Twenty-five F1 maize hybrids were analysed with respect to their nuclear DNA content. Twelve of the hybrids have been reported previously to have a low heterotic response [e.g. hybrid vigor] while 13 have a high heterotic response. The nuclear DNA content of each F1 hybrid was compared with the midpoint DNA amount of its respective parents. In nine of the hybrids with the low heterotic response, the observed nuclear DNA amount exceeded the expected DNA amount by approximately 5 per cent. In 12 of the 13 hybrids that had a high heterotic response, the observed nuclear DNA amount was not significantly different from the expected DNA amount. These results demonstrate an association between heterotic response and nuclear DNA content inheritance in F1 hybrids of maize.