Defining heterotic groups in alfalfa: A molecular marker
perspective
E. Charles Brummer
Iowa State University, Ames, IA 50011
Capturing heterosis remains a desirable but elusive goal for alfalfa
(Medicago sativa L.) breeders. Virtually all commercially marketed alfalfa
cultivars are synthetics, developed by successively intermating selected
plants and increasing seed through 2-3 generations. The inbreeding present
in such a system generally counterbalances any heterosis that may have been
expressed in the initial progeny population. Despite repeated efforts along
several lines, hybrid cultivars cannot be produced feasibly with current
technology. However, the potential for capturing a partial heterotic yield
gain has been generally overlooked: some population hybrids have been shown
to express high levels of heterosis and could be developed using current seed
production processes (Busbice and Rawlings, 1974, Sriwatanapongse and Wilsie,
1968). Population hybrids are actually "semi-hybrids;" half of the progeny
will be hybrid (i.e. between populations); the other half will be crosses
within each of the parental populations. Due to competition-induced seedling
death during establishment, semi-hybrids may yield as much as pure hybrids.
In order to express heterosis, two factors are required: dominance at loci
controlling the trait of interest, and differences in allele frequencies
between the parents at those loci. Most data in maize (Hallauer et al.,
1988) and alfalfa (Bingham et al., 1994) show partial to complete dominance
to be the genetic basis for yield, and population allele frequency
differences have been reported (e.g. Brummer et al., 1991; Kidwell et al.,
1994) indicating that alfalfa breeders should be able to realize heterosis.
The key to making successful population crosses is to identify or develop
heterotic groups and to keep them distinct through breeding, only bringing
them together for testing and seed production. The resulting hybrid
populations should not be recycled for further breeding to avoid mixing
alleles (or linkage blocks) and breaking apart beneficial complementarities
between populations. Molecular markers offer several opportunities to
capitalize on heterosis: assigning populations to heterotic groups,
identifying loci important for the expression of heterosis, selecting parents
for multiple or complementary alleles, and identifying particularly useful
alleles. Simulations show striking differences in the proportion of multi
allelic individuals (i.e. having multiple linkage blocks) resulting from
different population matings. Discussions focusing on the inbreeding
coefficient, F, cannot easily compare the number of individuals containing
multiple complementary linkage blocks (e.g. two-allele and four allele
reference populations would each have F=0, even though the former has no
individuals with three or four different linkage blocks at a particular
locus). Best case crosses are those in which the populations contain
contrasting sets of alleles; marker-assisted selection greatly increases the
number of individuals with multiple linkage groups. Finally, a method to
develop pure population hybrids will be discussed.
References
Bingham, E.T. et al. 1994. Crop Sci. 34:823-829.
Brummer, E.C., et al. 1991. Theor. Appl. Gen. 83:89-96.
Busbice, T.H. and J.O. Rawlings. 1974. Euphytica 23:86-94.
Hallauer, A.R. et al. 1988. In: Corn and Corn Improvement, ASA, Madison, WI.
Kidwell, K.K., et al. 1994. Crop Sci. 34:230-236.
Sriwatanapongse, S. and C.P. Wilsie. 1968. Crop Sci. 8:465-466.
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