Origin and evolution of strawberry (instead of one diploid genome, 7 chromosome-pairs, …. it has four subgenomes, 28 chromosome-pairs)

This GEITP topic has to do with how domestication and selective breeding of a wild fruit (foraged by our hunter-gatherer ancestors) has resulted in such a succulent, fragrant, and tasty fruit available in today’s grocery stores and supermarkets (i.e. the environment over tens of thousands of years has changed the genome). This process of domestication — has actually transformed wild plants into new species. Authors [see attached article] report on the origin and evolution of the strawberry. Their study reveals a complex history (geographic and genomics details) that exists worldwide, involving both natural processes (weather, climate) and human intervention (agricultural breeding programs).

The genus of strawberry, Fragaria, has many species. They bear small seeds that are easily dispersed; consequently, the genus is native to both Old and New Worlds (even the isolated islands of Hawaii have a species). Moreover, the species themselves are wide-ranging, and they hybridize easily. These hybridization events have formed polyploid species (‘diploid’ (2X), i.e. chromosome pairs, is normal for eukaryotes; quadruploidy (4X) is not normal, but it can be viable in e.g. plants and fish) — whose nuclei contain essentially complete chromosome sets (each chromosome set theoretically forms a ‘subgenome’).

The nuclei of strawberry diploid species (2X), with the “usual” number of chromosomes, contain seven chromosome-pairs — tetraploid species (4X) have 14 pairs, hexaploid species (6X) have 21 pairs, and octoploid species (8X) have 28 chromosome-pairs. Fitting with the general trend (among plants) that “polyploids are favored during domestication”, the modern cultivated strawberry, Fragaria X ananassa, has the highest of these ploidies (shouldn’t the plural be ‘ploidys’?). To clarify species relationships, authors carried out a comprehensive phylogenetic analysis — comparing the sequences of expressed genes in multiple lineages of all diploid species to the genome of Fragaria X ananassa; their analyses identified all four diploid species ancestral to Fragaria X ananassa, thus providing strong support for the two previously identified ancestors: a species endemic to Japan, Fragaria iinumae, and a species broadly distributed across the Northern hemisphere, Fragaria vesca.

Furthermore, they were able to prove that a particular subspecies, Fragaria vesca ssp. bracheata, endemic to the western part of North America, was also involved in the hybridizations. Authors provided evidence of other chromosome sets — another species from Japan, Fragaria nipponica, and the Eurasian species Fragaria viridis [see the diagram map in the editorial]. The octoploid stage has been estimated to have occurred more than 1 million years ago. The final hybridization occurred only ~300 years ago, courtesy of market gardeners in Europe. Pathway analysis showed that certain metabolomic and disease-resistance traits are largely controlled by the dominant subgenome. These data, and the reference genome, should serve as a powerful platform for future evolutionary studies and will enable further molecular breeding in the strawberry plant [what more could be needed?]. 🙂


Nat Genet Mar 2o19; 51: 541-547 [article] & pp 372-373 [editorial]

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