Biology Reference
In-Depth Information
delivery of improved varieties. Neutral, 'genetic
background' molecular markers that are well
distributed through the genome are also part
of molecular marker-assisted selection (MAS)
schemes such as marker-assisted backcrossing
(MABC), where the goal is to reassemble the
genetic background of the recurrent parent with
the addition of one to a few target traits. Accord-
ingly, MAS provides a powerful and potentially
cost-saving avenue for increasing the rates of
genetic gain in plant breeding programs (Xu and
Crouch 2008).
In the recent past, cost and technology limi-
tations meant that MAS was restricted to major
crops and to one or a very few high priority traits,
where phenotyping was costly or otherwise prob-
lematic. The development of relatively low cost
high-throughput SNP genotyping platforms in
many crops and the availability of high-density
genetic linkage maps have greatly enhanced what
is now possible with MAS. These capabilities
mean new breeding strategies can now be con-
sidered that utilize molecular marker informa-
tion at tens to thousands of points in the genome,
encompassing selection for multiple traits and/or
multigenic traits simultaneously. Selection tar-
gets include easy-to-phenotype agronomic and
grain quality traits such as grain size, texture,
and color, in addition to multiple biotic stress
tolerance and resistance traits that are more dif-
ficult to phenotype.
The effectiveness of MAS breeding schemes
depends on high quality phenotyping and preci-
sion genotyping for assembly of robust marker-
trait associations and QTL estimates, closeness
of marker-trait determinant linkages, and rel-
ative cost of genotypic selection compared to
traditional selection protocols based on phe-
notypic selection. MAS can increase the effi-
ciency of breeding in several other ways, through
the selection of desirable progeny for crossing
before flowering, providing year-round, environ-
mentally independent selection capability, and
through simultaneous selection of multiple traits
and for a particular genetic background. In addi-
tion, it is possible to deduce useful marker-
trait associations and marker effects in early
generations of breeding cycles that can then
be used for selection in later generations if
genotypic information is available at the later
stage.
Pioneering the Use of SSR
Markers for Introgression
of Striga Resistance
To date only limited deployment of molecu-
lar markers in cowpea breeding programs has
occurred. MAS using simple sequence repeat
(SSR, or microsatellite) markers to develop cow-
pea varieties resistant to the parasitic weed
Striga
gesnerioides
(Striga) was initiated in 2006 by
the national research organizations of Senegal,
Burkina Faso, Nigeria, and Mali, and at IITA
in Nigeria in collaboration with the University
of Virginia (Timko et al. 2007). These projects
have focused on the use of a limited number
of SSR markers with the goal of introgress-
ing Striga resistance into improved or local sus-
ceptible varieties. This approach eliminates the
need for phenotypic evaluation of Striga resis-
tance at each stage of the breeding process.
Also, by incorporating resistance genes effective
against multiple races of Striga prevalent across
national boundaries, MAS eliminates the need
for meeting quarantine requirements associated
with the movement of Striga seeds of different
races across national boundaries that would be
required for centralized phenotypic evaluations
of resistance. However, the approach uses only
trait-linked 'foreground' markers, meaning that
recovery of the recurrent parent 'background'
genotype will occur at nearly the same rate
as with conventional backcross breeding. SSR
genotyping with numerous background markers
on multiple individuals to facilitate recovery of
the recurrent parent would be cost prohibitive.
Another major drawback of MAS using SSR
markers in backcross breeding is that high qual-
ity gels are difficult to produce consistently, thus
Search WWH ::
Custom Search