April 2024 - 9

The path toward homozygosity is
called fixation. Fixation is enabled
by several rounds of self-fertilization
during which the two copies of each
chromosome gradually become
identical to one another, Clot said.
" After fixation, you are sure of what
will be transmitted during crossing,
but it comes with a cost: inbreeding
depression, " he said. " Inbreeding
depression is the loss of vigor and
fertility associated with the fixation
of defective genes. One way to stay
away from inbreeding depression is to
limit the fixation to a set of important
genes while keeping the rest of the
genome 'unfixed.' "
This approach could produce fertile,
reliable diploid progenitors which
would ideally be crossed with tetraploid
varieties that have been improved for
quality traits and high marketable yield.
A PAIR OF MAJOR
DISCOVERIES
In Clot's quest to find a way to cross
tetraploid varieties with diploid parents
in a way that transmitted their fixed set
of traits, he made two major discoveries.
The first is that the gene allowing
self-fertilization, the Sli gene, is actually
present in many tetraploid varieties.
Until recently, researchers believed it
only existed in the diploid wild species,
Solanum chacoense. Breeders can now
use the Sli gene for fixation without
Achieving homozygosity is typically easier with diploid potatoes, like these used in breeding
research in Orono, Maine, because they only have two copies of each chromosome. Photo
courtesy of USDA ARS.
risking bringing wild traits into their
potato clones.
The second discovery facilitates
crosses between diploids and
tetraploids. Normally, crosses between
parents with unequal ploidy levels
are not possible, Clot said. When
reproductive cells are formed, their
genetic material is halved. The pollen of
the diploid parent will contain one copy
of each chromosome, and the eggs of a
tetraploid variety will carry two copies
of each chromosome. The resulting
triploid embryos usually abort before
becoming a viable seed.
However, if during pollen production,
the halving of chromosome number
fails, a diploid parent will produce
unreduced pollen - larger-than-usual
pollen grains that have the capacity to
fertilize the eggs of a tetraploid.
Although breeders exploited this
in the past, it was unclear how the
restitution of chromosomes during the
formation of the reproductive cells was
genetically regulated. That is, until Clot
localized the main hereditary factors
associated with this trait.
A NEW BREEDING SCHEME
Clot's discovery could be a game
changer for potato breeders. Using
the two-step strategy of fixation and
restitution, the goal is to exploit the
Spudman.com 9

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April 2024

Table of Contents for the Digital Edition of April 2024

Editor's Letter
Breeding breakthroughs
Psyllid watch
Trail by smoke
Digging up answers
Immense impact
Potato Association of America
National Potato Council
Potatoes USA
Calendar/Ad Index
April 2024 - 1
April 2024 - 2
April 2024 - 3
April 2024 - Editor's Letter
April 2024 - 5
April 2024 - 6
April 2024 - 7
April 2024 - Breeding breakthroughs
April 2024 - 9
April 2024 - 10
April 2024 - 11
April 2024 - Psyllid watch
April 2024 - 13
April 2024 - Trail by smoke
April 2024 - 15
April 2024 - 16
April 2024 - Digging up answers
April 2024 - 18
April 2024 - 19
April 2024 - Immense impact
April 2024 - 21
April 2024 - Potato Association of America
April 2024 - 23
April 2024 - National Potato Council
April 2024 - Potatoes USA
April 2024 - Calendar/Ad Index
April 2024 - 27
April 2024 - 28
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