|1. Burbank's Fragaria x Rubus.|
There is a lot that Luther Burbank didn't know about plants. His exuberance for performing crosses and doing selections let him produce some wondrous results, but his lack of knowledge was a limitation.
Chromosomes were discovered in the 1880s, but the process of meiosis wasn't made clear until 1905-1911. The dates suggest it is possible that Burbank was aware of meiosis, even if he wasn't aware of the consequences for his work. Fortunately, such knowledge is now widespread and biologists are well aware of the consequences.
|2. Meiosis and failures of meiosis.|
If two species with different chromosome counts are crossed, the resulting hybrid can have an uneven number of chromosomes and will be generally unable to generate gametes (fig. 2B). (Example: 2n x 4n => 3n; this is how seedless watermelons are made.)
If two species with the same number of chromosomes are crossed, but the chromosomes are too unrelated, the resulting hybrid will also fail to generate gametes (fig. 2C). In this case the hybrid will have an even number of chromosomes, but they won't line up during meiosis and the result will be a haploid with an increased basic chromosome count. This can be caused by a high level of structural rearrangements in the chromosomes of strawberries vs. raspberries, even if the genes are otherwise compatible.
Because Burbank performed the cross with whatever strawberry and raspberry plants were convenient and the cytogenetics of the parent plants wasn't examined, either of the above scenarios could be responsible for the hybrid infertility that he saw.
I have one raspberry (Rubus occidentalis, isolated in my yard in Minnesota) and one strawberry (Fragaria vesca, isolated in central Wisconsin) in my collection and I think I will set about crossing them during this year. Both species have been examined in detail and happen to be diploid with 14 chromosomes, so the first incompatibility mechanism isn't a concern.
|3. Meiosis after allotetraploidy.|
|4. Tragopogon spp. hybrids.|
If everything works out, it will be a few years before I have a fertile strawberry/raspberry cross. I wonder what the fruit would taste like? I'll keep you informed as it goes.
- Burbank plants: mentalfloss.com/article/57818/10-crazy-creations-plant-wizard-luther-burbank
- Luther Burbank cross: bulbnrose.x10.mx/Heredity/Burbank/Burbank_raspXstraw.html
- Strawberry chromosome counts : strawberryplants.org/2011/02/genetics-of-strawberry-plants/
- Raspberry chromosome counts : hortsci.ashspublications.org/content/30/7/1447.full.pdf
- Rubus occidentalis: en.wikipedia.org/wiki/Rubus_occidentalis
- Fragaria vesca: en.wikipedia.org/wiki/Fragaria_vesca
- Colchicine: archive.org/stream/useofcolchicinei3424derm/useofcolchicinei3424derm_djvu.txt
- Colchicine and Oryzalin: hortsci.ashspublications.org/content/43/7/2248.full
- Tragopogon 1: www.massey.ac.nz/~jtate/index_files/Page573.htm
- Tragopogon 2: nzprn.otago.ac.nz/wiki/NZPRN/PeopleSymonds
- Tragopogon 3: scienceblogs.com/observations/2010/10/20/evolution-watching-speciation-1/