First of all I would like to thank our contributing writers on the recent excellent article in our newsletter regarding some of the unusual chromosome counts that are generated
David Zlesak co-authored that article with me, so he is to be thanked as well. David is probably able to answer your questions better than I, so I hope he will chime in, Baxter.
Thanks,
Paul
Hi Baxter!
Great questions indeed. I wish there were more people interested in this type of research. In general what you mentioned in terms of pairing makes sense. However, there can be a great deal more complexity. The homology among the seven different chromosomes of roses can differ between genomes of the parents crossed to generate the triploid. Perhaps for chromosome one there is a lot of homology between the chromosome from the diploid parent and two from the tetraploid parent that generated that triploid, but not the case for chromosome 2. In general univalents (lone, nonpaired chromosomes) lag behind and can become lost. Ma et al. from the Texas research group under Dr. David Bryne looked at meiosis in ‘Basye’s Blueberry’ and found many configurations and verying homology.
I have triploids that generally produce 2x pollen from looking at pollen diameter of well stained grains from that parent and progeny chromosome number. For instance one is a cross of a small leaved polyantha x Champlain. All of its progeny that I’ve examined in crosses with tetraploids have been tetraploid and have at least a little influence of the poly parent which supports your theory of the diploid parent’s chromosomes coming through to some degree. On the other hand, a triploid of ‘Orange Honey’ x a poly produced a nice diploid seedling from open pollination and it’s pollen diameters are much more variable. Such a triploid may be producing 2n pollen (3x) as well- like I suspect from ‘Iceberg’ and Paul and I learned may be possible in ‘Out of Yesteryear’ from tetraploid progeny in crosses of a diploid x ‘OoY’. By saying 2n and having 3x pollen one can inadvertently say the pollen is unreduced, at least in chromosome number. 2n refers to the chromosome number of the sporophyte and n the gametophyte. x just means the number of chromosomes. When one says the pollen is 2n that means that the chromosome number of the pollen happens to be that of the parent which produced it.
Anyway, what you mentioned are all great questions and in general I agree and think that makes a lot of sense, but in practicality there is likely a greater degree of complexity depending on homology of specific chromosomes across species and other factors as well. Sometimes unbalanced gametes (aneuploid) may be able to eliminate chormosomes as well, or even add duplicate copies to make up for missing ones. I had a triploid potato hybrid from a wide cross and over time it increased in chromosome number until clonal descendants of it were a stabilized tetraploid. Another student analyzing the genome of it eventually could not prove it had cultivated potato chromosomes in it. That one set of chromosomes that were part cultivated potato (cultivated potatoes are tetraploid, but I used a diploid haploid with a tetraploid species) were probably differentially lost and replaced with copies of the species chromosomes. There are a lot of unanswered questions regarding that clone. Perhaps more work would reveal some cultivated chromosomes or sections are still in it. Somehow the fertile gametes from triploids tend to stabilize themselves as euploid. Perhaps there are means for chromosome elimination or duplication or perhaps mechanisms which promote it through the pairing you described Baxter, or perhaps just the lucky ones are euploid and that’s why there is generally less fertility among triploids- or perhaps a combination of all these with one or another being more influencial depending on the clone and environmental conditions. Aneuploidy is possible, but generally conferrs fitness problems. The one aneuploid I confirmed (20 chromosomes) in a rose I applied trifuralin to (it was diploid and had incomplete polyploidization or something odd happened), grew very slowly and had twisted petioles.
I think triploids are a wonderful way to go for many rose breeding objectives- less seed set and more repeat bloom, generally more vigor compared to tetraploids, good compromize in general for flower and plant size between diploids and tetraploids. Of course phenotypic ranges for traits can overlap between diploids, triploids, and tetraploids, but there are some general advantages which are possible with lower chromosome numbers. Many great landscape roses are triploid and support this- ‘Nearly Wild’, ‘Sunrise Sunset’… also ‘Iceberg’.
Sincerely,
David