Just like humans, most organisms that reproduce via sex are diploid, meaning they have two copies of chromosomes (one from mom and one from dad). With plants, however, things can get a lot more complicated, and several species—including many agricultural crops—have multiple copies of chromosomes. For example, the cultivated strawberries you buy at the grocery store (Fragaria × ananassa) are octoploid, meaning they have eight copies of chromosomes.
But how did they get such a complicated genome?
Polyploid plant species get their extra chromosomes through two different processes: autopolyploidy and allopolyploidy. Autopolyploid species just have multiple copies of their own chromosomes, whereas allopolyploid species originate from hybridization events in which two different sets of chromosomes are combined in one organism, which often becomes a new species. Though their genomes live on in modern allopolyploid plants, species that contributed their genetic material to these organisms may have since gone extinct. That makes unraveling allopolyploid genomes like the strawberry’s particularly tricky.
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To get a better handle on how the strawberry came by its eight copies of chromosomes, geneticists from Portland State University looked to the genome’s long terminal repeat retrotransposons. These little genetic elements are pieces of repeating DNA that can duplicate and insert themselves into other parts of the genome. By analyzing the patterns of their distribution, researchers can get a better picture of the plant’s evolutionary history. They published their findings in Horticulture Research.
By applying this novel method to strawberries, researchers were able to determine a couple of things. First, the strawberry’s genome comprises four distinct subgenomes (which makes sense given its octoploidy). Second, the modern cultivated strawberry experienced three distinct hybridization events that occurred around 3 to 4, 2 to 3, and 1 to 2 million years ago.
“This work demonstrates how transposable elements can function as evolutionary time stamps embedded in plant genomes,” explained one of the study’s senior authors in a statement. “By focusing on when and where these elements expanded, we can reconstruct genome history even when direct ancestral references are missing.”
These new findings have important implications for agriculture and plant breeding and researchers hope to examine the long terminal repeat transposons of other polyploid crops—including wheat, sugarcane, and cotton.
And if you’re wondering how octoploid strawberries rank among polyploid plants, they’re far behind the current known record holder: The black mulberry has 44 copies of its seven chromosomes, making it a tetratetracontaploid plant. 
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