Developing genomic resources for banana
Jim Lorenzen, email@example.com
Banana and plantain (Musa sp.) are a very important staple food and cash crop in Africa. Although the principles of banana breeding and genetics were established decades ago, it is still a time-, land-, and resource-intensive process. A crew of several persons collects male flowers and pollinates female flowers while perched on ladders. When successful, seeds must be surface-sterilized and embryos removed for germination in test tubes (or else most won’t germinate), multiplied, and carefully “weaned” for field planting.
Large-sized plants require much field space, and new hybrids must be evaluated through two or three production cycles (about 3 years) before being selected for further testing, such as for disease resistance. Some essential attributes, such as resistance to disease or nematodes may also take several years to assess properly. It would be a huge advantage if early selection could be done, based on some associated marker or rapid test, to eliminate susceptible individuals without wasting resources on them. For other complex traits, it would be useful to have markers based on component genes to be able to select ideal “genotypes”.
One way to do early selection is to use molecular markers that are linked to the target traits (molecular-assisted breeding). This method is becoming common in cereal breeding, yet should be even more cost-effective for a large perennial crop such as banana that requires so much time and space to evaluate. The tools of DNA fingerprinting are applied, and by knowing which DNA markers lie near genes of interest, selecting for the markers will be equivalent to selecting for the trait a year or more later.
The problem is that we lack enough information on the banana genome to have molecular tools to map traits. One of our activities has been to map and characterize new molecular markers for use in banana breeding and genetics. PhD student Gaby Mbanjo from the University of Yaoundé, Cameroon, has been working in Uganda and Kenya to characterize and map a large new set of simple sequence repeat (SSR) markers, often called microsatellite markers. She is a scholar of the Biosciences for Eastern and Central Africa (BecA) program, with funding provided by the Canadian International Development Agency (CIDA).
Gaby is also working to develop other types of molecular markers based on small genetic differences (single nucleotide polymorphisms = SNPs) between alleles of genes involved in controlling plant defensive reactions. These will be used to try to map the genetic loci responsible for resistance to the burrowing nematode (Radopholus similis) and banana weevil (Cosmopolites sordidus) in the population she is studying. Markers of both types can be converted to semi-automated assays for hundreds or thousands of assays. This effort is expected to result in a DNA fingerprinting assay in which we can select the associated DNA markers and thus also select the target resistance without spending as many resources on susceptible plants.
The molecular markers will have other practical uses. Unfortunately, sometimes varieties get distributed with wrong names, or a batch of plants supposedly of a single variety may actually contain a mixture of varieties. Molecular markers can be used to “fingerprint” mother plants used to produce new planting material to ensure that they are of the proper variety. They can also be used to select diverse parents for maximizing the heterozygosity of offspring. Some of the markers are being transferred to a national research program for assessing varietal purity in their advanced selections.
Molecular markers are a way in which biotechnology and the rapidly expanding knowledge of DNA sequences in plant genomes can be used to make classical breeding more efficient. This should be especially helpful for large perennial crops such as banana and plantain.