I have a new branch on another sterile rugosa hybrid that has all the hallmarks of polyploidy: increased thickness of leaflets, petals and leafstalks; increased width/length ratio of leaflets; darker green; and puckering of the leaflets. All of these traits are present throughout the 5-6 inch branch, which is a continuation of last year’s suspicious lateral growth from a trifluralin treated shoot tip. Yesterday, the first bloom (of 4) opened, with petals thickened, rounded and overlapping. But, there still doesn’t seem to be much pollen. I’m thinking it’s going to follow the pattern of my previous possible conversion (without the dieback hopefully). The suspicious looking Rugosa x xanthina branch, in spite of having more substance, and larger parts, had only a little more pollen than flowers from the other branches. And no seeds formed from selfing. I’m thinking that maybe the conversion was chimeric, affecting the outer layer, but not the LII layer (source of gametes). Or the plant was simply not self-compatible. Or conversion quadrupled instead of doubled the chromosome number (octoploid would not be very fertile). Now, with the newly formed branch in question having produced so little pollen, I’m wondering what’s up. Does anyone know if converted polyploids need time to “settle down”, before becoming fertile? Or does anyone have any other possible explanations for this behaviour. I know that the flower may be forming seeds as I type this. But, I’ve got a sinking feeling that it’s gonna abort. Time will tell. I’ll have to get some pictures posted so noone thinks I’m fooling myself about the branch. It really does appear to be polyploid. Until then, I still have pictures of the deceased rugosa x xanthina branch at my website. See links page of RHA.
Hi! What do you mean by sterile rugosa hybrid? Is it straight rugosa? In general rugosas are self-incompatible and will not set hips without pollen from another rose (rugosa or otherwise). Does it just not make hips? The percent of stainable pollen (looking at pollen under a microscope with a stain that is absorbed by generally viable plump pollen grains making such grains very visible)went way down in my doubled polyanthas verses their diploid counterparts. Hopefully you will have more pollen produced in further bloom cycles. In general the pollen you have with an autotetraploid can have reduced viability with a greater proportion being empty aborted pollen grains. This is generally attributed to pairing difficulties in meiosis with autotetraploids and the greater likelihood of incomplete sets of chromosomes going to the gametes. In general breeders try to double several genotypes and then concentrate on using those with better fertility. Out of my 15 or so doubled polyanthas I have a few that have much better fertility and vigor than others and have and will continue to concentrate on using them as parents. I’m excited to try to start using some of these as female parents this year. So far I just used them as males.
Please let us know how your breeding work continues and if this rose is fertile as a female.
The hybrid is rugosa x arvensis (from what I’m told); it’s not one of mine. It has never set a single open-pollinated hip for me, and I’ve never heard of any offspring at all from it. With such an “unfertile” (“sterile” does seem so definitive) hybrid, I’m expecting that the doubled version should be relatively fertile. Difficulties in pairing between the genomes of the two species, should help prevent quadrivalent formation in the tetraploid version, allowing for more normal meiosis (allopolyploid/amphidiploid type plant). I guess, time will tell. I’m hoping that, even if the conversion is chimeric, that eventually some tetraploid cells will get displaced into the LII, and result in gamete formation.
Inspired by this branch, I’ve started treating a hybrid of my own (rugosa x multiflora foundling). My rugosa hybrid has never produced hips either. It has pink, extremely double flowers (old rose appearance) inherited from the multiflora pollen parent (a 20-30 petal found wild multiflora variant). It also has a strong, old rose scent. I would love to have a converted version of this rose, to use like R. kordesii has been used. Along with the double flowers, and strong scent, this rose also has the added advantage of repeat bloom in the fall (reliably for 3 years now). The poor substance of the flower petals, which often results in “balling”, would hopefully be corrected by polyploidy. I’ll keep you informed of my progress. Thanks for the responses. Tom