Molecular mapping for millet

Since the 1950s grain yields for pearl millet grown in India have more than doubled from 300 kg/ha to 800kg/ha in recent years. However, the reality behind this promising increase is that several years of prosperity have been succeeded by ruin as disease and/or drought has struck and yields have plummeted. Whether in Africa or in Asia, pearl millet varieties affected by Downy Mildew can be wiped out and farmers' losses can be devastating. Losses are particularly hard to bear when pearl millet is the staple food grain and it is the only cereal that can be grown in areas of low rainfall. Even with resistant varieties that are released, farmers can grow them for several years but the fungus begins to adapt to overcome the resistance until one year, a complete explosion of the disease occurs and much of the crop is lost. New hybrids with a different source of resistance then have to be found.

Credit:ICRISAT

However, molecular mapping has proved a vital and promising step in the on-going battle to improve pearl millet. In 1993 the first genetic map of the pearl millet genome was produced and within a year scientists were able to map genes that conferred resistance to downy mildew. Since this time, more than 600 molecular markers for a variety of traits have been developed and mapped for pearl millet and, using a process known as marker-assisted selection, new resistance genes have been used to strengthen the crop's natural resistance to disease. One such hybrid, HHB 67, has proved exceptionally successful in India and has yet to succumb to the onslaught of downy mildew.

The case for Africa is rather different as, in some areas, it is literally the only crop that can be cultivated. In the past, farmers have offset risk by growing a mixture of varieties, which has kept downy mildew at moderate levels although grain yields are generally poor. But, with increasing populations and decreasing land availability, farmers are desperately in need of better seed (see also 'More from millet') and traditional breeding techniques are unlikely to produce improved seeds at the pace and to the extent that they are needed, particularly as pearl millet is a highly variable outcrossing species. But, using genetic markers, landraces grown by Nigerian farmers, have been analysed for traits that may be useful in breeding programmes.

Disease resistance is not the only trait to be of importance in reducing pearl millet yields; drought and excessive heat also have a major impact. By using molecular makers, the locations of genes that control much of the variation for heat and drought tolerance have been identified and expensive and unreliable field screening trials have been replaced by marker-assisted selection. In the future, genetic variability for traits such as nutrient uptake may also be better understood. This is particularly important for pearl millet grown in Africa where nutrient-poor soils are a primary constraint. For instance, sandy soils are generally deficient in phosphate and, if genes responsible for phosphorus uptake were mapped, it should be possible to have marker-assisted selection for improved nutrient uptake from locally available rock phosphate.

Tools and knowledge already developed within other cereal crops can also greatly benefit pearl millet. Establishing the genetic similarities between grass species allows transfer of information from one crop to another. Genes found to control drought tolerance in rice, for example, may predict those genes that play an important role in pearl millet. Comparative mapping of rice, foxtail millet, sorghum and pearl millet has demonstrated the genomic relationship between the crops and, importantly, pearl millet is seen as a 'bridge' between various cereals.

The battle against downy mildew will never be won and there will always be room for improvement for pearl millet for a number of traits, besides disease. However, as more genetic knowledge is mapped and biotechnology tools are refined, it means that advances in plant breeding can be made not only at a lower cost but, most importantly, the results can be transferred to farmers fields' more quickly.

Back to Menu