If a rose is confirmed to be triploid, how many sets of chromosomes does its pollen contain?
ie. 1 set only, or some ratio of 1 set : 2 sets? …is there any predictable general ratio that can be assumed here, or is the ratio highly cultivar specific, or maybe even totally unperdictable and random within the same triploid clones??
Can external/environmental factors (ie. factors outside the plant itself) influence/shift pollen ploidy production to higher or lower numbers, in the same rose clone?
Yes, all of the above. Pollen can also be 3x. One challenge is that a lot of the grains are not viable and are aneuploid, incomplete sets. People just apply heavily. In potatoes and we can gleen in roses as well comparing some studies, the amount of pollen applied can influence ploidy of offspring too. The diploid pollen may have a competitive advantage (growing a bit more quickly) over the 1x pollen and if we apply pollen heavily the competition would lead to more of the eggs being fertilized by the 2x pollen. So, there is a lot of factors that interact. Also there is some preference it appears as we look at some segregation ratios in some studies for gametes of equal ploidy being more apt to produce viable offspring. Of course crosses with gametes of different ploidy do survive.
Is it safe to say that some triploids are completely sterile or does every triploid have a degree of fertility? I’m thinking of ‘Belinda’s Dream’ for example. I’ve read that she is completely barren as a seed parent but was wondering if he pollen might offer some possibilities as a pollen parent or is there complete sterility?
That’s a great question Rob. Some sure seem like they are completely barren. Hopefully we can prove that wrong for roses we particularly see value in using. Hopefully we can at least get a seedling from that that would be a bit more fertile to get their genes into more accessible breeding lines.
I’ve found that with the triploids I’ve used that from most I was able to get offspring from using them as a male if I tried hard enough. I haven’t tried Belinda’s Dream as a parent. Hopefully you can get some offspring from it.
My experience from working with triploids also is that some prefer one type of pollen over another. For example Carefree Delight prefers diploids. Knockout prefers tetraploid. Has anyone else noticed this?
For sure some roses are mechanically barren, like Cardinal de Richelieu.
“For sure some roses are mechanically barren”
I’ve always wondered if one could induce a single sport from those though (like what supposedly happened with the centifolias).
From my experience I would say that most triploids have some sort of fertility. I have 10 plants that I know are triploids and I
Freedom is mechanically sterile. The hips/peduncles are just …whacky inside.
Thanks David, Rob, Jon, Paul, and Michael for sharing all your collective knowledge and experience…it does help a lot to know about it all!!
Wow, it amazes me how roses behave genetically, (and not just the triploids!).
I have an old horticulture textbook from 1972 that I got from a book sale. In the genetics section it discusses chromosome assortment in a triploid spinach at anaphase 1 of meiosis, where n=6, 2n=12 and 3n=18. The possible chromosome combinations at anaphase 1 are 6-12, 7-11, 8-10 & 9-9. The seven types of gametes in terms of chromosome number would be (6,7,8,9,10,11,12) with a frequency of 1:6:15:20:15:6:1, so n=(6) and 2n=(12) occur in only 2/64 of the gametes.
Extrapolating this to n=7, 2n=14 and 3n=21 for roses I came up with chromosome combinations of 7-14, 8-13, 9-12 & 10-11. The eight types of gametes in terms of chromosome number would be (7,8,9,10,11,12,13,14 ) with a frequency of 1:7:21:35:35:21:7:1. So n=(7) & 2n=(14) would occur in only 2/128 (1.56%) of the gametes. This may explain why triploids can be so infertile and why it would be better to use them as pollen parents.
I wonder how stable the pollen of Rosa primula and Rosa roxbughii are? I know it sounds silly but I swear they act like a “genetic band aid phenomenon”.
Paul, thanks a lot for thie infobit.
Thanks all for the response to my question. Unfortunately due to space limitations I had to pull Belinda’s Dream but afterward I wondered if I had made a mistake and should have made more of an effort to use her pollen.
Now I’m thinking of a ‘Pink Meidiland’
Hi Paul G.
That math is interesting.
Also, I must look up anaphase and study it a little more, so I can fully re-grasp phase 1…I have a general idea about meiosis, but need to refresh the memory on that one, to be real honest with you!!
Hi Paul G.
Thanks for your additional comments, yeah I totally understand and appreciate where you are coming from.
Also, David Z points out there are apparently some definite 21 chromosome pollens that some triploids can throw off as well…the frequency of those exta large pollens would be and ineresting thing to observe.
Could fertility occur with these large pollens from triploids??
ie. is it possible to then get a situation like:
21 chromosome pollen from triploid father rose + 7 chromosome egg from diploid mother rose —>tetraploid rose??
I think it would certainly be possible for that to happen, just like when a diploid or tetraploid produces un-reduced gametes. Robin Hood is noted for producing 2n eggs and pollen. David also found 4n pollen when he was researching for his paper on pollen diameter. So it is possible, you’d have to do some research or testing to find a triploid variety that was prone to doing that.
Great points Paul!! It is amazing the variability out there. It seems like ‘Tangerine Jewel’ has pollen the diameter expected for unreduced or 2n pollen. n refers to life stage and x to sets of chromosomes or ploidy. So n= the sex cell or gamete and 2n= the sporophyte or plant we see. So when we say 2n pollen we kind of mix the terms and say a gamete (it is pollen) and that is has the same chromosome count that the parent has.
When the original cells starts out (microspore mother cell for pollen or megaspore mother cell leading to the egg) it is a normal cell and has every chromosome duplicated. It is kind of like a cell with duplicated chromosomes ready to divide into two daughter cells for two identical copies (mitosis). However, for meiosis there are two rounds of division I and II as mentioned before. During I copies of like chromosomes (contains the same genes generally, although the alleles or forms of the gene can differ) find each other and sometimes exchange arms for crossing over and added variability. At the end of Meiosis I the matched chromosomes get divided up and pulled to opposite poles. With three of each chromsome (triploid), as mentioned it doesn’t happen evenly for complete sets of all the chromosomes moving together. One side may have one each of every chromosome plus an extra copy of just some.
In Meiosis II those chromosomes which are of course duplicated with the copies stuck together have the two copies split apart to the four final gamete cells. Typically then each gamete has half as many chromosomes as the parent plant. At the end of meiosis II the poles that all the chromosomes get pulled to and spindle fibers are typically at a 60 degree angle. Cytokinesis or cell division and membrane development typically happens at 90 degree angles to the spindle fibers and directional movement of the chromosomes. Each group of chromosomes are being pulled slightly different directions and we have four gametes that form (four pollen grains form typically and on the female side all but one survive to eventually make the egg and associated cells).
In meiosis II if the spindle fibers separating the two groups fo chromosomes after meiosis I ar parallel instead of at 60 degrees, then as cytokinesis occurs and we know it happens perpendicular to the direction of movement of the chromosomes, we can get just two gamete cells forming and no matter how the chromosomes divided in meiosis I we should have three complete sets in each of these two gametes which is nice. This is called 2n gamete formation due to parallel spindles. There are other mechanisms too. This helps triploids have fertile balanced gametes at times which have 3 sets of chromosomes.
4n gametes can arrise if the starting cell with its duplicated chromosomes just let them separate and everything stays in the one cell and it becomes a gamete. So the gamete then has twice as many chromosomes as the starting parent plant.
These are just ways for higher ploidy, balanced gametes that can help triploids (other ploidy levels too) to have balanced gametes. There are other ways too and of course we know triploids can have 1x or 2x pollen too.
It is really fun to think about and the genetic consequences of transferring much of the parent plants genetics directly into resulting offspring.
Hopefully I was complete enough in the description for people to follow where I was going.
Thanks Paul G again for your additional ideas, and thanks also David Z again, especially for taking the time to write all this refresher on meiosis stages/events. This represents high quality scientific knowledge, and it is very higly appreciated by me, and surely others here!
Awsome, elegant stuff!!!
Thanks David for the explanation and clarification.