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Taking a peek into sorghum’s evolutionary history

Published online 8 April 2019

Sorghum did not go through the same bottleneck that drastically reduced the genetic diversity of other food crops when they were domesticated.

Rieko Kawabata

Sorghum is a staple food crop that supports the livelihood of millions of people in sub-Saharan Africa.
redbrickstock.com / Alamy Stock Photo

Today’s food crops are not as genetically diverse as their wild ancestors. This is often linked to a ‘domestication bottleneck’: a model that describes a drastic reduction in genetic diversity during the early stages of domestication, which began thousands of years ago.

New archaeological and genetic evidence now suggests that sorghum, one of the top five cereal crops in the world, does not fit into this conventional model. Instead, it underwent a steady decline in genetic diversity over time, according to findings by a team of scientists from the UK, Denmark, the US, Australia, and Egypt.

The team compared DNA from modern wild and domesticated sorghum with nine wild and domesticated samples recovered from Qasr Ibrim, a major archaeological site located in Lower Nubia, Egypt. The samples cover a timespan between 400 and 1,805 years ago.

They were surprised to find a linear downward trend in genetic diversity that “runs contrary to expectations for a domesticated crop”.

A technique called genome evolutionary rate profiling further revealed a build-up of deleterious mutations up to the present day. The researchers suggest that this mutation build-up over time was probably a consequence of dynamic selection pressures, rather than a collapse of diversity caused by domestication.

They also observed a striking build-up of mutations within the past century, which was likely due to intensive breeding, according to plant evolutionary geneticist Robin Allaby of the University of Warwick, UK.

The study also showed that different sorghum lineages have preferentially rescued each other in the past through the exchange of adaptive traits. It is important to keep wild ancestors of sorghum alive, as their ability to adapt to their surroundings through this rescue process could be crucial for finding ways to breed domesticated crops that can adapt to the threat of climate change.

“Increasingly, we are seeing that selection pressures have been going on long before the emergence of domesticates,” says Allaby, “incorporating more diversity and plasticity than we had previously understood to be the case.”

The team plans to study more crops to uncover trends related to the domestication process on a larger scale. “This is important not only to better understand our past, but also for our future food security,” Allaby says.

doi:10.1038/nmiddleeast.2019.52

Smith, O. et al. A domestication history of dynamic adaptation and genomic deterioration in Sorghum. Nat. Plants https://doi.org/10.1038/s41477-019-0397-9 (2019).