Genetic markers identify the sex of yams and accelerate breeding
Researchers have identified the genetic markers that distinguish the sex of yam plants, saving time and resources for future breeding efforts.
White Guinea yam (Dioscorea rotundata) is native to West Africa, where it has been a food security crop for centuries. As Africa is rapidly urbanizing, yams are now being grown commercially. This labor-intensive crop provides a source of rural employment. The large tubers are traded in open air, city markets and are increasingly sold as prepared food, such as ‘pounded yam’ in popular, moderately-priced restaurants.
However, productive yam cultivation is constrained by various pests and diseases and by the lack of improved varieties. Yam has been an orphan crop with progress in breeding limited by a lack of genetic and genomic tools. This may be about to change. For the first time, researchers have sequenced the genome of the white Guinea yam, the most important food yam in West Africa. Using the sequence information, and characterizing lines segregating for different traits, the team identified genetic markers, including those for the sex of the yam, which has male and female flowers on separate plants. The markers will allow plant breeders to separate male from female plants at the seedling stage, greatly improving the efficiency of planning and carrying out crosses in breeding programs.
Farmers usually grow yams from tubers or pieces of tuber. Although the crop is conventionally reproduced vegetatively, crossing two individual plants to obtain new genetic combinations is the basis for breeding programs. The plant is highly heterozygous, so when it is crossed, the resulting genetic combinations are quite diverse.
The team of researchers from the International Institute of Tropical Agriculture (IITA) Yam Improvement Program and partner institutions in Japan, Nigeria and the UK assembled a genome of 579 million base pairs and identified 26,198 putative genes and over 22,000 candidate genetic markers, including one for sex in yam. Sex in yams is determined not by X and Y chromosomes, as in humans, but by genes, ZZ for male and ZW for female. As an added complication, some ZW individuals are non-flowering, and some change their sex over generations.
“In yams it is important to establish male and female crossing blocks in separate and isolated plots to avoid pollen contamination during hand pollination. In addition to avoiding unwanted pollination by insects or any other means, it is important to know the flower sex of the parents to design a crossing plan as male and female flowers in most of our parental lines are in separate plants,” explained Dr. Robert Asiedu, Director, Research for Development, for IITA-West Africa, who conceived and supervised the study.
Other genetic markers identified by this research will help in the future to locate genes to breed for resistance to pests and diseases, and to improve other desired traits, such as yield. Much work remains to be done, but future plant breeding will be enhanced by the new genome map, genetic markers and the ability to separate male and female plants. This should make possible the release of improved varieties leading to greater yam harvests, with improved rural employment, greater food security and providing a nourishing, culturally appropriate food for the growing cities of Africa.
Photo: A yam farmer in Nigeria, where the crop is a source of rural employment. Credit: IITA
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