Revolutionizing sweetpotato breeding by exploiting hybrid vigor

Genetic studies allow breeders to identify sweetpotatoes that are more genetically distant, in order to separate them into different groups. Crossing genetically distant parents from the separate groups leads to offspring that have much higher “hybrid vigor” or heterosis. In other words, the offspring are dramatically superior to the parents in key traits such as yield, yield stability, stress resistance and earliness.

In sub-Saharan Africa, plant breeders are developing more nutritious sweetpotato varieties, especially those that are rich in vitamin A and ones that have more iron but also give higher yields. The large and heterogeneous genome of the sweetpotato makes this a challenge. Traditional sweetpotato breeding has involved hand crossing two lines to obtain true seed, which is then planted. Each individual plant that is grown from the true seed is potentially a new sweetpotato variety. Many parents are crossed at once and their offspring are grown and evaluated over generations, to painstakingly select the best lines. These lines which may then be crossed with each other and their progeny for more generations, until a new, improved variety is finally chosen for release to farmers.

In spite of all this work, the results can be disappointing if the parental plants are too similar genetically to begin with. Ideally, breeders want to start by crossing sweetpotato varieties which are genetically different from each other so that their offspring will have more hybrid vigor (heterosis) and a lot more yield. Finding genetically distant parental groups can be difficult with traditional search methods. Even sweetpotato varieties from distant geographic areas can be genetically similar. Outward appearance is also a poor indicator of genetic differences. Researchers needed a better way to evaluate the genetic differences in sweetpotatoes.

In Uganda, researchers from CIP recently used 31 genetic markers to study the relatedness of 141 sweetpotato parents, mostly from East Africa. Together with colleagues from IITA and the Universidad Nacional Agraria La Molina
(in Peru), they characterized the varieties in a dendrogram, with genetic relatedness displayed like the branches of a tree. Most of the East African sweetpotatoes fell into two, genetically distinct groups (Clusters A and B). Consequently, in planning crosses for new hybrids, the parents from Cluster A will be crossed with parents from Cluster B. This new method to cross parent lines in sweetpotato as well as other RTB crops is called the heterosis exploiting breeding scheme (HEBS).

Research teams led by Wolfgang Grüneberg at CIP have demonstrated that HEBS works well and, within one breeding cycle of five years, leads to nutritious, improved breeding lines with great increases in yield, disease resistance and early maturity. “The resulting new hybrid populations were higher yielding, had more roots and more iron, thus exhibiting very high rates of genetic gains,” Grüneberg explains. Indeed, the hybrid vigor obtained by crossing different parent groups is so high, that to reach such increases it would probably take 36 years of conventional crossing and selections, instead of just five years needed if HEBS is applied.