Many of the world’s most significant crops have extraordinarily complex genomes created by repeated rounds of whole-genome duplication and hybridization. These so-called polyploid genomes consist of multiple sets of chromosomes inherited from different ancestral species. However, determining how these genomes were assembled could be extremely difficult, especially when the unique ancestral species are extinct or unknown.
A brand new study introduces a genome-wide approach to unraveling these complex genetic histories. The method takes advantage of the evolutionary signatures left behind by long-repeat retrotransposons, a variety of mobile DNA sequence. By comparing patterns of similarity between these elements across chromosomes, researchers can discover distinct subgenomes and estimate when major genome integration events occurred. When applied to cultivated octoploid strawberries, the technique revealed a step-by-step evolutionary history shaped by multiple rounds of allopolyploidization, providing recent insights into how plant genomes grow to be complex and diverge over tens of millions of years.
Why polyploid genomes are obscure.
Whole-genome replication has played a crucial role in plant evolution, facilitating innovation, adaptation, and the emergence of many crop species. In allopolyploid plants, chromosome sets originate from different ancestral genomes. These chromosome groups, generally known as subgenomes, proceed to evolve and interact long after the unique hybridization events.
Identifying these subgenomes is critical to understanding how a species evolved. Traditional approaches often rely on comparing polyploid genomes with known diploid ancestors. The problem is that many native species are either extinct or haven’t yet been identified.
Transposable elements offer one other source of data. Long-lived retrotransposons accumulate in characteristic patterns inside specific evolutionary lineages, preserving molecular evidence of past events. Although scientists have long recognized their potential value, reliable methods for converting these samples into accurate subgenome assignments have been limited. As a result, recent tools are needed to reconstruct polyploid genome evolution without counting on known progenitor species.
The recent method reconstructs the history of the genome.
Researchers from the U.S. Department of Agriculture and cooperating agencies describe such a tool within the journal Science. The team developed a bioinformatic framework able to reconstructing the evolutionary history of complex polyploid genomes.
To display the mechanism, they reexamined cultivated octoploid strawberries (Fragaria × ananassa). Using a sequence similarity matrix constructed from long terminal repeat retrotransposons, the researchers elucidated the subgenome structure of strawberry and uncovered multiple ancient genome integration events that contributed to modern species. The findings help resolve long-standing questions on the strawberry’s evolutionary origins.
This framework follows the evolution of genomes in three broad stages: before the ancestral species diverged, during their divergent evolutionary histories, and after their genomes merged. Retrotransposons that expand in periods of divergence retain signatures unique to specific subgenomes.
By calculating similarity matrices for these elements across chromosomes and examining how they cluster at different similarity thresholds, the researchers created what they call a “serial similarity matrix.” This approach captures evolutionary signals that accumulate over different periods of time.
Examining perspectives in crops
Before applying the technique to strawberries, the team tested it in well-studied allopolyploid crops, including teff and cotton. In each cases, the strategy successfully distinguished known subgenomes and segregation events that occurred before and after polyploidization.
The researchers also evaluated the approach using artificially constructed polyploid genomes. These tests confirmed that the strategy is sensitive to each divergence times and the abundance of transposable elements.
What the Strawberry Genome Revealed
When the strategy was applied to the octoploid strawberry, it identified 4 distinct subgenomes and discovered evidence for 3 sequential allopolyploidization events that occurred roughly 3.1–4.2 million years ago, 1.9–3.1 million years ago, and 0.8–1.9 million years ago.
The results support an in depth evolutionary relationship between the 2 strawberry subgenomes and species. At the identical time, the outcomes challenge previous models that proposed additional diploid progenitor species.
According to the evaluation, some contributors to the strawberry genome are extinct or remain unsampled, illustrating the complexity of polyploid genome evolution.
“This work shows how transposable elements can act as evolutionary timestamps embedded in plant genomes,” said a senior writer of the study. “By focusing on when and where these elements spread, we can reconstruct genome history even when direct ancestral references are missing. This method provides a powerful new lens for studying polyploid crops and moves beyond reliance on incomplete progenitor data, offering a more objective and reproducible framework for evolutionary evolution.”
Implications for crop research and breeding
Potential applications extend beyond strawberries. Many economically necessary crops, including wheat, cotton, and sugarcane, are polyploids with similarly complex evolutionary histories.
More accurate identification of subgenomes can improve gene annotation, trait mapping, and comparative genomic studies. Those advances, in turn, can support precision breeding efforts and help speed up crop improvement.
By making it possible to reconstruct genome evolution with out a known ancestor, the serial similarity matrix approach adds a precious recent tool to the study of biodiversity, speciation, and adaptation. This framework may additionally be useful for investigating other complex polyploid organisms, helping to integrate evolutionary biology with practical agricultural research.
This work was supported by National Institute of Food and Agriculture (NIFA)–Specialty Crop Research Initiative (SCRI) Grant 2022-51181-38241 to QY.