Plant Genome I Conference
Town & Country Conference Center, San Diego, CA, November, 1992.
PG-I: 30pg1
ASSOCIATING MOLECULAR MARKERS WITH DESIRABLE TRAITS IN
CULTIVATED, CROSS-POLLINATED, TETRAPLOID POPULATIONS.
Daniel Z. Skinner, U.S. Department of Agriculture and Agronomy
Department, Throckmorton Hall, Kansas State University,
Manhattan,
KS 66506.
The methodology of establishing association of molecular
markers with a desirable trait primarily is based on
cosegregation of particular morphotypes of molecular markers and
expression of the trait. Requisite to the success of these
methods are the assumptions that the mapping population shows
measurable segregation of the trait; the segregation of
polymorphic markers within the population is within random error
of the expected; the sample I examined is large enough to
represent the genotypes apropos to the trait; and that gene
action in the mapping population is the same as in cultivated
populations. With cross-pollinated tetraploid plant species, one
of these assumptions necessarily are true. With alfalfa
(Medicago sativa L.) 'normally cultivated as a tetraploid crop,
development of molecular maps is being pursued in diploid
populations to simplify analysis and reduce the number of plants
needed. Segregation has been reported to differ significantly
from expected ratios with about 50% of the RFLPs examined, and it
has been suggested that gene action in the diploids differs from
that in the tetraploids. I suggest that markers associated with
a desirable trait in a tetraploid population can be identified by
ascertaining which alleles and allele combinations occur with
significantly greater frequency in a population selected for the
trait, relative to the unselected base population. Statistical
evaluations revealed that 48 plants from each population are
sufficient to detect linkage of 20.8% or less, between a marker
or marker combination, and trait expression. About 1% of RAPD
markers and about 3% of marker combinations occurred
significantly more frequently in a tetraploid population
selected for resistance to downy mildew, than in the base
population. Because this kind of selected/not selected
population combination normally is produced as part of the
breeding process, dozens of combinations dealing with numerous
traits are available. This method holds promise of expediting
the application of molecular markers to the plant improvement
process.
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