Scientists reveal the hybrid origin of the white Guinea yam

18 December 2020

An international team of scientists has revealed the origin of the most popular food staple in Africa, the white Guinea yam.

Yam is a major staple widely consumed in West Africa. This starchy tuber is an important food crop that contributes to the sustenance and sociocultural lives of over 300 million people in the region. Many species of yam are grown in Africa, of which white Guinea yam (Dioscorea rotundata Poir) is indigenous to West Africa. Despite its importance, relatively little is known about the white Guinea yam, especially at the genetic level, leading to its being branded as an “orphan crop.”

Yam cultivation is hampered by many challenges, including pests and diseases, postharvest losses and the need to develop more sustainable systems of farming for the crop. To efficiently improve Guinea yam, the genetic diversity of the species and its relationships with wild relatives need to be understood to allow breeders to identify genes controlling useful traits that could be rapidly transferred to elite cultivars by cross breeding.

In 2017, Iwate Biotechnology Research Centre (IBRC), Japan in collaboration with the International Institute of Tropical Agriculture (IITA) succeeded in decoding the whole genome sequence of a Guinea yam plant consisting of 600 million letters of genetic information, setting the stage for improving the crop using genomics information.

To further understand the genetic diversity of Guinea yam, the IITA-led AfricaYam project funded by the Bill & Melinda Gates Foundation, together with IBRC, Kyoto University, Japan International Research Centre for Agricultural Science (JIRCAS), Japan as well as other institutions joined efforts to expand the genome sequencing to 300 Guinea yam landraces and compared them with the genomes of putative or reputed wild ancestors of Guinea yam.

A paper on this breakthrough titled “Genome Analyses Reveal the Hybrid Origin of the Staple Crop White Guinea Yam (Dioscorea rotundata)” was recently published in an open access journal of the Proceedings of National Academy of Science, USA.

The study showed that Guinea yam likely originated as a hybrid between two wild species, Dioscorea abyssinica found in the savannah region and Dioscorea praehensilis common in the rainforest region.

“The finding also suggests the importance of wild yams that are found in savannahs and rainforests. They have much wider genetic diversity than the cultivars and may be used as parents for cross breeding of yams to overcome the many challenges facing yam farmers in Africa and other parts of the world,” said Professor Ryohei Terauchi of Kyoto University and IBRC, the lead scientist of the study. He said that systematic efforts are needed to introgress the beneficial alleles from wild species into cultivated species.

According to Dr Patrick Adebola, the IITA scientist leading the AfricaYam project, such alleles will increase disease resistance and abiotic stress tolerance to improve crop resilience and productivity, a useful resource for agriculture and farmers in Africa.

“Cracking the genetic origin of the crop and its relationship with wild relatives would facilitate the crop’s genetic improvement through breeding,” said Dr Asrat Asfaw, IITA yam breeder and program lead. Dr Robert Asiedu, IITA R4D Director for West Africa, said the information on genetic origin of yam will help increase the efficiency of the breeding program.

According to the study, the D. abyssinica and D. praehensilis accessions in the IITA GeneBank account for only 1.6% of the total Dioscorea accessions maintained as of 2018. Therefore, it will be important to collect and preserve wild Dioscorea species as genetic resources for improving Guinea yam, as findings from the study suggest that new alleles of loci, such as the SWEETIE gene, were introgressed from wild yam into cultivated Guinea yam multiple times, which likely conferred the plants with phenotypes preferred by humans.