Hybrid Sterility and Speciation

1. Burbank's Fragaria x Rubus.

Luther Burbank (1849-1926) was a widely renown botanist and scientist. He bred numerous interesting plants. He liked to attempt wide crosses; crosses between distantly related species. One of the more unexpected crosses he attempted was the cross between a strawberry (Fragaria spp.) and a raspberry (Rubus spp.) (fig. 1). To the then (and now) commonly held Victorian ideal of plant species, this cross shouldn't have had a chance at all of working. However, the cross appeared to succeeded and fruit developed. The plants that developed from the seeds grew with a combination of characters from the parents, thus showing their hybrid nature. The hybrids flowered abundantly in the second year, but no fruit was ever produced. Burbank found that at most a few seedless drupelets (fruit segments like in a raspberry) would form and so he abandoned the project.

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.

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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.

Strawberries and raspberries show a diversity of genome sizes, but they all have a basic chromosome count of 7. They species range from diploid with 14 chromosomes to decaploid with 70 chromosomes. An even number of chromosomes is found in all cases, as this is required for the formation of gametes (fig. 2A).

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.

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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.

The second incompatibility mechanism can be overcome by inducing tetraploidy in the hybrid. This would be done using Colchicine or Oryzalin, herbicideal compounds that interfere with cell division and result in a doubling of the number of chromosomes in treated tissue. Induction of tetraploidy generally produces one branch that has larger fruit, thicker stems, and other visible features to distinguish it from the original diploid parts of the plant. Because this tetraploid would contain two full copies of genomes from different species, it would be referred to as an allotetraploid.

4. Tragopogon spp. hybrids.

This process has been observed to happen naturally. Three species of Tragopogon (T. dubius, T. porrifolius, and T. pratensis) were introduced into the Pacific northwest region of the USA in the early 1900s from Europe. By the 1950s, scientists realized there were two new species of Tragopogon to be found in the region (T. mirus and T. miscellus). The new species were fertile allotetraploid hybrids between pairs of the introduced species. The hybrid species have even been recreated in the lab. In this case, it appears the allotetraploids came about because the parent species occasionally produce aberrantly diploid gametes which merged to form the fertile allotetraploid. The precise pathway is different than what I expect would be going on with the strawberry/raspberry cross, but it is a wonderful case-study for hybrid speciation.

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.

References:

• 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/