Reducing Genetic Drift in Gene Bank
The nightmare of a gene bank curator: a collection of 25,000
precious, unique samples of maize seed - one of the world’s most extensive. The seed is stored carefully, keeping it cold and dry, but eventually it becomes non-viable. The genetic diversity it once embodied is lost to humanity forever. To keep this bad dream from becoming a reality, Suketoshi Taba, head of maize genetic resources at CIMMYT, and his team constantly monitor the germination of collections. When it drops below 80-85 percent, they take viable seed from the endangered accession (the term for individual, registered samples in the bank), sow it under controlled conditions, and harvest enough from progeny to replenish the accession. Known as “regeneration,” the process sounds simple, but in fact must be done painstakingly to ensure the integrity of the genetic diversity from the original accession.
The Crop Science Society of America recently bestowed the honor of “2004 Outstanding Paper on Plant Genetic Resources” on an article by CIMMYT biometricians that provides models for proper handling of repeated cycles of regeneration. Their work is particularly relevant for outcrossing, genetically diverse crops like maize, legumes, or sorghum, to name just a few. “For maize regeneration, we use artificial pollination, to avoid out-crossing with pollen from other maize fields,” says Taba. “But even the individuals in a maize population or accession are genetically diverse. How can we decide on the best way to pollinate the plants, or how many ears we need to harvest, or how many and which seeds to choose from each ear?” According to Taba, the danger is ending up with a sample that differs from the genetic make-up of the original. And with each successive cycle of regeneration, it can drift further.
Building on a strong body of work in this area by CIMMYT biometricians since the 1980s, the award-winning paper refines and expands the statistical model and provides reliable computer simulations. “Among other things, the simulation model shows exactly how many alleles are likely to be lost through various sampling and regeneration strategies,” says Jiankang Wang, CIMMYT biometrician who is first author of the study. “It describes how different strategies can affect the conservation of alleles and gives gene bank curators options that can be tailored for specific types of accessions.” (CIMMYT, 2/2/06).
