Boosting sweetpotato’s productive potential with hybrid breeding

Orange-fleshed sweetpotatoes bred in Uganda are improving nutrition across East Africa (CIP/H.Rutherford).

Sweetpotato has great potential for improving food security, nutrition, and income, which is why scientists at the International Potato Center (CIP) and an array of partners have helped more than six million African households adopt improved varieties over the past decade.

CIP has worked closely with agricultural research institutions across Africa and Asia on breeding and selection to ensure that more farmers get access to sweetpotato varieties with essential characteristics such as high yield, nutritional value, and climate resilience. This collaboration has included innovations in genomics, statistics, and field-trial protocols that have accelerated the development and dissemination of new varieties. To enhance this process and respond to the global challenge of feeding a population of more than 8 billion in a changing climate, CIP scientists have been evaluating the potential of population hybrid breeding.

Hybrid breeding focuses on crossing genetically different parents to take advantage of hybrid vigor, or heterosis, which refers to the fact that the progeny of genetically different parents can be superior to either parent. Hybrid maize is one of the great agricultural success stories of the 20th century, making a major contribution to food availability, and crop improvement programs and enterprises across the globe are working to repeat it with other crops.

Sweetpotato is more challenging, partly because it is autopolyploid and highly heterozygous – meaning that instead of having paired genes like maize (diploid), its genes are arranged in groups of six (hexaploid) – making it more difficult to predict the results of a breeding cross. However, an article by several CIP scientists published in Frontiers in Plant Science details an approach to population hybrid breeding of sweetpotato that has had impressive results – yield improvements ranging from 97% to 132% relative to the foundation (grandparents) – indicating that population hybrid breeding could become a game changer for sweetpotato in coming years. Based on trials in Peru, the article provides the latest evidence that years of efforts to implement hybrid breeding in that root crop have been a worthy investment.

“When I first considered testing hybrid breeding two decades ago, it was a pipe dream, but it has since evolved into a highly successful breeding scheme,” said CIP senior scientist and main author Wolfgang Grüneberg.

He explained that his endeavor to use hybrid breeding on sweetpotato was inspired by French plant breeder André Gallais, who wrote about the method’s potential in a textbook on quantitative genetics and breeding in autopolyploids. With Grüneberg’s guidance, breeders in Peru, Uganda and Mozambique undertook experiments in hybrid breeding in several sweetpotato breeding populations. They began by dividing each population into two genetically different groups based on sequencing data, then crossed individuals among groups to identify parental combinations that produce the advantage of hybrid vigor. An important component of the approach is “reciprocal recurrent selection,” through which breeders evaluate the progeny of crosses to identify the best parents – those most likely to produce offspring with hybrid vigor – for use in subsequent breeding.

Farmer in field holding one sweet potato in each hand.
Average yield of future variety D20(L) compared to Uganda’s most popular orange-fleshed variety NASPOT 8 (R) from field trials in Twebitaba, Uganda. (plot size: 3md, 10 plants per plot) (Credit: Girisom Bwire)

Though still relatively new, population hybrid breeding has produced eight candidates for release as new sweetpotato varieties in Uganda, 30 in Peru and potential varieties for Mozambique. CIP has also sent the seeds from hybrid sweetpotato crosses in Peru to Asia for planting and evaluation.

Average yield of future variety D20(L) compared to Uganda’s most popular orange-fleshed variety NASPOT 8 (R) from field trials in Twebitaba, Uganda. (plot size: 3md, 10 plants per plot) (Credit: Girisom Bwire)

“The yield increases gained through six years of hybrid breeding would have taken 30—50 years to achieve using the traditional breeding approach,” says Grüneberg.

According to CIP breeder Reuben Ssali, one promising product of hybrid crosses in Uganda that breeders call D20 is undergoing on-farm evaluations and should be released as a new sweetpotato variety soon. That future variety produced an average of 18 tons of sweetpotatoes per hectare in trials in five of Uganda’s agroecological zones – compared to the African average sweetpotato yield of 7.0 tons per ha reported by FAO reported for 2021 – and its attributes include a taste and consistency local men and women want, and high resistance to sweetpotato viruses.

“Given the results of population hybrid breeding so far, breeders could easily develop varieties with almost double the yield of the sweetpotatoes currently being grown in Africa,” Ssali said.

Global benefit

Grüneberg explained that another advantage of population hybrid breeding is that it can be used for large-scale crossing of parents from groups that produce high-performing offspring so that significant amounts of seed are produced that could be sent to agricultural institutes in other countries for evaluation and selection of varieties.

Sweetpotato is a vital crop for food security and nutrition in East Africa.

The common approach to disseminating the products of regional sweetpotato improvement programs has been a laborious process that limited a program’s ability to share potential varieties with national agricultural programs. It consists of selecting variety candidates with optimal combinations of traits, growing them as tissue culture plants, checking them for virus infection, cleaning them of any viruses discovered and, finally, shipping tiny plants in test tubes to national partners for multiplication and evaluation. This process is slower and more expensive than shipping seeds and allows partners to evaluate only a small sample of potential varieties for local needs and conditions.

By sending botanical seed from elite crossing rather than in-vitro plantlets, breeders can forgo the time-consuming process of virus elimination, because viruses don’t infect botanical seed the way they do tissue culture plants. It also allows breeders to share larger selections of potential varieties with partners for evaluation on the ground – in varied environments and changing climates – than they could with in-vitro plantlets.

Grüneberg noted that yet another benefit of the progress made on population hybrid breeding of sweetpotato is that the lessons learned could be applied to other root or tuber crops.

“We hope that this population hybrid breeding approach will serve as a model for similar studies in other root and tuber crops, that it attracts more breeders, and contributes to affordable food security in this decade,” said Grüneberg.

He noted that applying the approach to a new crop needs to be done carefully, within a pilot phase or program first, but that progress with sweetpotato has been positive enough that he encourages wider use of hybrid breeding in that crop.

“Our population hybrid breeding approach guarantees greater breeding progress in sweetpotato and we hope to extend it to other countries,” Grüneberg said.

This research was supported by the Bill & Melinda Gates Foundation and USAID.

Blog by David Dudenhoefer, Joel Ranck (contributor).