Strawberries Have 8 Sets of Chromosomes to Thank for Their Survival

Strawberries Have 8 Sets of Chromosomes to Thank for Their Survival

The strawberry is a treasured treat whose large red fruit and sweet flavor make mouthwatering jams, stand-alone afternoon snacks, or toppings to nearly any dessert. However, strawberries are more than just a delicious snack. Hidden beneath the surface of that bright red fruit lies a unique branch of the evolutionary tree. Strawberry’s genetic quirks are ripe for scientists to study and gain fundamental insights into how organisms can evolve new, complex and versatile features.

The first quirk in the strawberry genome is something scientists call polyploidy, meaning multiple sets of chromosomes in its cells. Humans are diploid, meaning we have two sets of chromosomes; every individual gets one set from a paternal sperm donor and one set from a maternal egg donor. Strawberry, meanwhile, is an octoploid, meaning it has eight sets of chromosomes. Seriously.

The second quirk is hybridization, where distinct species mate with each other and produce offspring that contains genomes from both species. (In most living things, the mating or breeding leads to a mash-up of genomes in which not everything gets ported over.)

In 2019, my colleagues and I published the first high-quality genome of the strawberry plant, which revealed the octoploid genome arose by a stepwise process. At some point over a million years ago, two ancient diploid species hybridized and produced a now-extinct plant species with four sets of chromosomes; that species hybridized with a third diploid species, resulting in six sets of chromosomes, and then with a fourth diploid species, resulting in eight sets of chromosomes. This ancient wild octoploid then spread throughout the Western Hemisphere, splitting into two species that European colonists collected in the 18th century; those plants underwent a final hybridization event in continental Europe around 300 years ago to create the strawberry you know and love in your grocery store or garden.

What this all means is that strawberry has, on average, eight copies of every gene and the genetic diversity equivalent of four different species in every cell. Genetic diversity is the engine of evolution, and with multiple copies of a gene one copy can perform essential functions while additional copies are free to engage with new activities and functions. Two recent studies speak to this advantage. First, a study led by researchers at the University of Pittsburgh found that polyploidy in strawberry species leads to changes that not only allow them to better survive and reproduce in favorable environments but also to better resist stresses in unfavorable environments. As the researchers note, their findings fit a hypothesis that polyploidy allows plants to be both a “jack of all trades” and a “master of some.”

Second, the process of domesticating wild plants inevitably leads to a substantial decrease in genetic diversity in general. Humans select only a small subset of the total genetic diversity of a wild species and then continually select small slivers of successive generations. This model matches well with the transition from the small, bushy and hard-cased wild teosinte to the single-stalked large-eared corn that carpets the landscape of the U.S. Midwest. However, because of the many hybridization events, this is not the case for strawberry. My colleagues and I, led by the strawberry breeding lab at the University of California, Davis, looked at the genomes of wild and domesticated octoploid strawberry and were surprised to observe that there is nearly as much genetic diversity in domesticated strawberry as there is in the wild relatives. This genetic variation is put to good use. The same study showed that different copies of genes inherited from the four different diploid parent species were all influenced by natural selection throughout strawberry’s history of early and modern domestication; each of these parent species provided a different stockpile of genetic fuel to help species adapt to diverse locations or meet the needs of plant breeders.

The final hybridization occurred between two wild octoploid species: one native to the temperate environment of North America and the other acclimated to the western coast of North and South America. The resulting hybrid was able to easily adapt to distinct environments. A different study from the U.C. Davis group showed that genes under selection in domesticated strawberry grown for coastal environments were more likely to come from the parent species native to coastal environments, whereas genes under selection for domesticated strawberry grown in temperate environments were more often from the parent species adapted to temperate ecosystems. By having two genomes from hybridization, strawberry is equipped with extra genetic diversity to survive in either environment to which the parental species adapted.

The evolutionary significance of polyploidy extends far beyond strawberry. The ability to create massive amounts of additional genetic material sets the stage for future adaptations to novel environments or the ability to persist in unusually harsh conditions. The sequencing and analysis of dozens of genomes across the tree of life revealed that, although many eukaryotic species currently have diploid genome structures, nearly every species possesses signals of ancient polyploidy events, where organisms experienced a whole-genome duplication and gained new sets of chromosomes. These events conspicuously occur before the evolution of major novelties like the spinal column in vertebrates, flowers in plants, and fermentation in yeast. Genes maintained in duplicate despite hundreds of millions of years of evolution are critical in the development of these traits, providing strong evidence that polyploidy led to the evolution of these novel traits. Additionally, polyploidy events seem to occur during mass extinction events, like the one at the boundary of the Cretaceous and Paleogene periods approximately 66 million years ago; polyploidy may have been critical to the survival of species during this time of massive climatic upheaval.

The next time you sink your teeth into a strawberry, remember it isn’t just a delicious snack. It’s a window into a unique genetic and evolution process that explains how species can evolve never-before-seen functions or survive unprecedented environmental changes.

This is an opinion and analysis article, and the views expressed by the author or authors are not necessarily those of Scientific American. 

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