Yearly genetic gains in cassava have more than trebled in Nigeria, Africa’s main cassava producer, with superior varieties now available to smallholder farmers.
Although RTB crops are the linchpin of food security in sub- Saharan Africa, the genetic gains made in these crops have been less than that in others. With the embrace of genomic selection, this has recently changed, with a tripling of genetic gains being reported in late-stage field trials of cassava and with superior varieties of this crop already being released in Nigeria. Demand creation trials are helping to speed adoption among Africa’s smallholder farmers. RTB Breeding is further accelerating and mainstreaming genomic selection in banana, sweetpotato, and yam.
Cassava is the quintessential African crop and plays a crucial role in food security and resilience for myriad smallholder farmers. Its resilience to heat and drought and tolerance of marginal soils makes it a staple food for over half a billion people worldwide. However, the crop’s long crop cycle of 12–36 months, depending on farmers’ needs and desires, its low multiplication rate and low seed set per cross, as well as a phenotype-based breeding cycle of 4–6 years, used to represent major bottlenecks to cassava’s genetic improvement. Until recently, these challenges hindered the ability of breeders to regularly deliver superior genetic gains, to rapidly respond to emerging biotic and abiotic constraints, and to sustain the sustainable expansion of this remarkable crop into harsher marginal environments.
One CGIAR Centers and national breeding programs have embraced advances in genomics that have already revolutionized plant breeding in other crops, making tens of thousands of SNP markers available and affordable. These advancements underpin genomic selection, namely the ability to predict the performance of untested genotypes, thus facilitating the selection of parents with superior genetic merit and the identification of clones with superior performance. This step change in cassava breeding enables transitioning to predictive breeding that delivers enhanced efficiency and genetic gain.
Continuous deployment of genomic selection and refinement of prediction models has not only produced unprecedented annual genetic gains of up to 2–3 percent for dry yield but also cut cassava’s breeding cycle in half. Above all, the first release of new varieties of the crop by market segment has occurred in Nigeria. Building on this success, the first phase of RTB Breeding is further mainstreaming genomic selection through a close-knit partnership of scientific talents from IITA, the Alliance of Bioversity and CIAT, and several national programs such as NaCRRI (Uganda), NRCRI (Nigeria), TARI (Tanzania) in Africa and the Brazilian Agricultural Research Corporation (Embrapa) in Brazil. The power of genomic selection is being further enhanced by the steady expansion of on-farm testing through tricot as well as updates of CassavaBase, an open-access, multifunctional breeding database for efficient management of massive amounts of phenotype and genotype data points (https:// cassavabase.org), and use of plant growth regulators, pruning of young branches, and LED lighting to induce flowering and enhance seed set.
Although cassava was the RTB crop first enhanced by genomic selection, other RTB crops are quickly catching up. For instance, yam breeders at IITA and NRCRI have combined genomic selection with novel propagation techniques to hasten parental recycling and reduce breeding cycles. All of these represent significant changes in several components of the breeder’s equation.
It is perhaps in polyploid RTB crops where genomic selection is most impactful. Banana breeders are exploiting it for faster parental recycling, hence decreasing breeding cycles in diploid, tetraploid, and tetraploid by diploid parental combinations. First results of collaborations between IITA and NARO breeders in Uganda are encouraging, with prediction accuracies in the 0.67–0.71 range observed for yield components in matooke. In sweetpotato, CIP scientists working with colleagues from NaCRRI and IIAM have progressed to the second cycle of population training.
The small RTB breeding community was an early adopter of “breeding modernization” tools. Their market research defined product profiles that meet consumer needs. The combined impact has been dramatically accelerated genetic gain providing higher yielding varieties with high consumer ratings. RTB crops are entering their ‘Green Revolution’ moment.
Jim Lorenzen, Senior Program Officer, Bill & Melinda Gates Foundation
Header photo: Umudike seeds fields at NRCRI, HQ Umudike, Nigeria. Chiedozie Egesi/IITA.