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Showing posts with label strawberries. Show all posts
Showing posts with label strawberries. Show all posts

Tuesday, June 14, 2016

How to Make a Better Strawberry

Drawing of wild strawberry plant, with smaller drawings illustrating details of flower and other part structure.
Over the last few years I've collected numerous isolates of the Woodland Strawberry (F. vesca) from around Minnesota and Wisconsin. Most of the isolates went into a strawberry tower I had picked up at a garage sale, with each isolate being given a separate pocket of soil. An isolate I found in the far back of our property was planted in a pot all on its own away from the others. I put all the containers near my garden and then proceeded to mostly ignore them for the rest of the year. I wasn't going to baby them along, so only the real survivors would prosper.


It was soon obvious that one of the isolates in the strawberry tower was dramatically better at spreading. It took over the tower and then spread to every nearby bit of exposed soil. This plant originally came from a patch of sandy soil above a large boulder adjacent to a lake in northern Minnesota, fully exposed to the sun. This plant was obviously a determined survivor.

The isolate from our property grew well. It produced large leaves and occasional robust runners which it dropped over the edge of its pot. I didn't think much of it at the time.

At the onset of winter, I continued my strategy of not caring for the plants. The planters containing the strawberries were left out to experience the full brunt of (the admittedly mild) winter. I treated some nice Garden Strawberry (F. x ananassa) plants I had received midsummer much better. Their pots were covered in leaves and then placed up against the basement wall under a porch packed with other items being stored for the winter. I had previously had difficulty overwintering strawberries, so I took efforts to protect them.


Once the earliest hints of spring arrived, the Woodland Strawberry plants started to wake up. The aggressive spreading isolate was found to be just as aggressive about surviving. (The other F. vesca isolates in the tower were not so much.) Every pot that it had spread into last year was soon full of new growth. This plant's thin runners didn't seem to let it spread into the tall grass around the pots last year, however, as there are no lawn strawberries where those pots were kept. Right now the plants are putting out flowers in impressive numbers. I expect any berries will be much larger than the tiny examples the plant produced last year, as the whole of the plant is much larger and happier looking this year.

Out in the yard where I kept the isolate from the back of the property, I found some robust strawberry plants growing. Some of this plant's runners had settled into the tall grass around its pot and found their way to the soil where they were able to survive the winter. The parent plants in the pot itself, however, did not survive.

The Garden Strawberries that I took efforts to protect... completely died out. One bit of vital green did make it to spring, but it soon turned brown and joined its conspecifics in the afterlife. Fortunately, I can easily get more plants from garden centers and other sources.


It would be really nice to have a plant that combined the extreme hardiness of the Woodland Strawberry and the larger berries of the Garden Strawberry, so I started thinking about hybridizing them.

All known strawberries (genus Fragaria) have a basic chromosome number of seven. Wild species range from diploid (2n) to decaploid (10n) with every even number of chromosome sets represented. There are also the occasional naturally formed sterile hybrids with an odd number of chromosome sets.

The two species of strawberry in my collection have a different number of copies of each chromosome.
F. vesca (Woodland Strawberry; 2n)
F. x ananassa (Garden Strawberry; 8n)
If the two species are crossed, sterile pentaploid plants result. To get around this, researchers have used colchicine to double the chromosomes in F. vesca. Crossing a tetraploid F. vesca with an octaploid F. x ananassa should result in a fertile hexaploid plant. The resulting plants ended up being decaploid instead, perhaps by the involvement of unreduced gametes from the octaploid parent with normal diploid gametes from the tetraploid parent (8n + 2n -> 10n). With this model, the hybrid plants would be 20% F. vesca and 80% F. x ananassa. Back-crossing this hybrid to either parent would result in changes in chromosome number with sterility being the result for most cases (x 2n F. vesca -> 6n; x 4n F. vesca -> 7n; F. x ananassa -> 9n), so this plant is essentially a dead-end with respect to further breeding for improvements.


Chromosome doubling in F. vesca can theoretically be done twice to produce octaploid plants (2n -> 4n -> 8n) that could then cross with F. x ananassa to produce an octaploid hybrid.  The hybrid would be 50% F. vesca and 50% F. x ananassa. It would also be able to be back-crossed to either parent without producing progeny with sterility issues due to odd numbers of chromosomes. This would make the hybrid far more useful for further breeding projects, such as combining the extreme hardiness of F. vesca with the tasty and large berries of F. x ananassa.

Earlier I mentioned using colchicine to double the chromosomes of F. vesca. Colchicine is a toxic alkaloid originally processed from Autumn Crocus (Colchicum spp.). The compound interferes with the polymerization of tubulin into microtubules, thus preventing the migration of chromosomes to spindle-pole bodies during cell replication. The effect of this is that newly doubled chromosomes during cell replication aren't split into two new cells, thus a single cell remains with twice as many copies of each chromosome as it started with.

Plants with increased ploidy often have phenotypic changes, such as thicker stems/leaves and larger fruit. Even when the overall plant remains much the same, an increase in ploidy can be observed by noting an increase in the size of pollen grains. If there is no apparent change, even in the pollen, then one would have to count the actual chromosomes to be sure the chromosome doubling protocol has worked.


So, when will I set about doing this? I've got so many projects lined up, that I don't know when or if I will actually do this. I really like the idea, and the protocols aren't all that difficult (or dangerous), but my resources are limited, so we I'll have to stick with, "We shall see." for now.

Anyone else interested in taking a go at it in the meantime? If you do, please let me know of your progress.


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Wednesday, June 3, 2015

Strawberries Galore

I've been thinking a lot about what I wanted to do with my recently acquired property. Aside from the usual house and yard parties, and just relaxing under the shade of our lovely cottonwood tree, I've been thinking about installing some berry gardens.

Strawberries in particular are often in my thoughts. The plants produce numerous attractive flowers in early spring and the berries themselves are a wonderful treat. When I was a young child one of the neighbors had an amazing raised-garden full of strawberry plants, so there is also some nostalgia for me in the idea of picking ripe strawberries out in the yard.

Biologically, strawberries are in the same family as roses, raspberries, and blackberries. Remarkably, they can even hybridize with raspberries, though the reported hybrid have been completely sterile. The sterile hybrids produce wonderful flower displays, however, so they're not without value. As well, there remains the potential to correct the sterility and produce some [presumably] novel-tasting fruit. (http://the-biologist-is-in.blogspot.com/2015/01/hybrid-sterility-and-speciation.html)

With all that in mind, I've been collecting wild strawberry plants during my travels around the mid-west. I like the thought of discovering something 'new' and distinct from what I would get at the local garden center. I have also acquired a batch of modern domesticated plants from a fellow gardener in Minneapolis, because no matter how much I like the wild plants, I really do want to get some nice large berries out of this project.

  1. Wild strawberry from swamp in central Wisconsin.
    • Found growing on a raised trail in the center of a swamp in central Wisconsin. The plants filled in large areas of the trail and produced scattered small fruit held above the leaves. The flowers have petals facing more forward than the typical domesticated strawberry. The three leaflets of each leaf are narrower and more forward pointing than the typical domesticated strawberry.
  2. Wild strawberry from lake-shore in Brainerd, MN.
    • Found growing above the waterline in very sandy soil where they would experience periodic dryness. The plants formed wide colonies, but were small enough they could possibly be maintained in a mowed lawn. Ripe berries weren't seen, but several small ones are now developing.
  3. Wild strawberry from woodland in Brainerd, MN.
    • Found on the other side of the house from the other Brainerd sample, these were growing in deep shade underneath pines. The plants are large, similar to domesticated types.
  4. Wild strawberry from woodland in Duluth, MN.
    • Found growing in open woods. Large, flat-faced flowers are reminiscent of domesticated types. The plants are large, similar to domesticated types.
  5. Wild strawberry from open woods in Saint Paul, MN.
    • Found growing under tree cover adjacent to open area where it had spread across a low slope. The plants run wildly, but otherwise appear as a domesticated strawberry. Ripe berries have not been observed.
  6. Wild strawberry from Fairbanks, AK.
    • I don't actually have this plant yet, but I will be visiting central Alaska in the coming months and totally expect to do some botanizing while I'm there. The locals tell me that there are wonderful wild strawberries and I plan to pick up a few plants. They should be very cold-hardy and produce berries very early in the season. The big question is if they will be able to survive the heat of a mid-west summer.
  7. Domesticated strawberries (mixed varieties) from Minneapolis, MN.
    • These appear to be producing large berries, like you might find in stores. Some of the plants were described as being "alpine" strawberries, but their lack of distinction from the others makes me question this claim. For now the "alpine" variety is planted separately.
I've got some photos of the various plants, but a comprehensive photo-shoot will have to wait until I get them all planted in a soon-to-be-constructed garden bed (with some way to keep the deer at bay) for them.

The other day I made the pleasant discovery that one of my raspberry plants is in flower at the same time as the various strawberries. I've got too much going on (in and out of the garden) to attempt crossing my strawberries to it this spring, but it is definitely in the plans for next year.
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Thursday, January 22, 2015

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.


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.


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.

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