The Biologist Is In: How to Make a Better Strawberry

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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