Mexico
July, 2007
 Source:
CIMMYT E-News, vol 4 no.
7 - July 2007
Two decades ago,
CIMMYT scientist
Jose Crossa began to
apply sophisticated statistics and population genetics to the
management of seed collections in gene banks. Now, the
methodologies developed at CIMMYT for the maintenance and
classification of genetic resources are used throughout the
world.
Germplasm banks (also called gene banks) often seem like museums
or bank vaults, keeping precious treasures locked away for the
next generation. Banks like the Wellhausen-Anderson Plant
Genetic Resources Center at CIMMYT are certainly built to
withstand disasters such as earthquakes, hurricanes, and power
failures. But there’s much more to it than security. Germplasm
banks are really more like a zoo, and a batch of stored seeds is
more like a cage full of monkeys than the Mona Lisa. Zoo animals
require constant maintenance. Old animals die and have to be
replaced, often by careful breeding to ensure that the genetic
diversity of a captive population is maintained.
The scientists who manage collections of genetic resources face
similar challenges. Even at the very low temperatures and
humidity levels used in germplasm banks, seeds can’t be stored
indefinitely. Over time, they lose their ability to germinate:
how fast this happens depends on species and storage conditions,
but all the seeds in a collection will eventually become useless
for breeders or farmers. Before this happens they have to be
planted, grown to maturity, and a new generation of seeds
harvested for return to the bank. CIMMYT’s seeds are monitored
every 5-10 years, and scheduled for regeneration when their
viability drops below 80%.
This process is relatively straightforward in self-pollinating
crops like wheat, where the offspring are genetic copies of the
parent. But it is much trickier in cross-pollinated crops like
maize, where the offspring have a jumbled-up mixture of the
parents’ genes. A single maize ‘accession’—a batch of seeds from
a single variety or race of maize—contains seeds with quite
different combinations of genes. When these genes are recombined
in the next generation, the risk is that some, rarer, genes will
be lost.
Twenty years ago, Crossa had recently arrived at CIMMYT as a
biostatistician, when colleague Suketoshi Taba, now Head of the
Maize Germplasm Collection, approached him with a problem. He
wanted to know the best way to regenerate accessions to retain a
high level of genetic diversity, including how to work out how
many seeds to plant, and how to manage the pollination process.
“I had no idea,” says Crossa, “but I started looking into it. I
really wanted to work on genetic resources because I knew how
valuable they were. I’d been working in the US, and everyone
spoke about how unique and important CIMMYT’s collection was.”
And so began a long and fruitful collaboration.
Crossa realized that ideas from population genetics held the
key, but these were not being applied to genetic resources. The
crucial concept was effective population size (EPS), a measure
of the number of parents that contribute to the next generation.
A larger sample is likely to contain more of the population’s
genetic diversity, so the progeny are likely to represent the
original population better. Therefore the number of seeds
planted for regeneration should be as large as possible—but in
reality this is limited by the capacity and funding available.
However, the effective population size of the parent sample can
also be maximized by carefully controlling the regeneration
process so that each parent contributes equally to the progeny.
The plants are not allowed to cross-pollinate freely. Instead,
crossing is done by hand, with each plant being used to
pollinate one other, ensuring that the male reproductive cells
or “gametes” (i.e. pollen) from each plant are represented
equally. So that the contributions of female gametes are also
equal, a fixed number of seeds is taken from each plant, rather
than simply harvesting all the seeds.
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Some of more than 20,000 maize
samples currently held in CIMMYT's Wellhausen-Anderson
Plant Genetic Resources Center, which holds seeds of
wheat, maize and their relatives in trust for the world. |
The models developed by Crossa and
his colleagues allow scientists to make informed trade-offs
between genetic diversity and regeneration costs. For example,
to have a high probability of retaining a gene variation that
occurs in 3% of the population, making it fairly rare, an EPS of
200 is needed. Using systematic regeneration to maximize EPS,
this means planting around 250 seeds, to allow for some
regeneration failure.
“I think the work has made a difference,” says Crossa. “People
used to use much smaller samples, but now they are more aware of
the genetic erosion caused by not using appropriate sample
sizes, and the need to control male and female gametes.” The
methodologies developed at CIMMYT have shown scientists around
the world how genetic diversity can be managed successfully, and
are used to ensure the preservation of many national and
international collections. The team’s models have been extended
to species with any degree of self- and cross-fertilization—even
wheat, since in reality accessions are never completely
homogeneous.
Crossa continues to apply the tools of statistics and population
genetics to the field of genetic resources. His team has done a
great deal of work on core collections, small subsets of
accessions that represent as much as possible of collections’
overall genetic diversity. In the case of maize, they have
grouped farmer landraces into racial groups, and generated core
collections for each one. These allow researchers to study a few
tens of accessions rather than trying to select from the
thousands available. The team has developed ways to combine a
large amount of data in order to select the most varied subsets,
including data from molecular markers and data on physical
traits, both quantitative and qualitative. This way of
organizing and combining many types of data is now being applied
as a valuable tool for selection for plant breeding. Crossa has
also worked on the challenges faced by researchers out in the
field collecting samples for germplasm banks, developing
methodologies to efficiently capture the genetic diversity of
farmers’ crops.
“When Taba first asked me about seed regeneration I knew nothing
about it, and there wasn’t much work in the area, so the
challenge really appealed to me,” says Crossa. “Twenty years
later I’m still happy I can make my contribution to preserving
genetic diversity. There aren’t many people working in this
field—because, although each gene bank is extremely important,
numerically there really aren’t very many—so every advance we
make has a big impact.”
For more information: Jose Crossa, Head, Biometrics and
Statistics () |
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