Biology Reference
In-Depth Information
included in genetic maps, offers the possibility
of locating new genes.
dently in two labs using RAPD markers (Adam-
Blondon et al. 1994b; Young and Kelly 1994,
1996a). Since that date, more refined SCAR,
AFLP, and SSR markers linked to at least 9 of
the 14 major Co-genes (Table 9.3) have been
reported (reviewed by Kelly and Vallejo 2004).
In the absence of a saturated map of P. vul-
garis , researchers first screened segregating pop-
ulations with RAPD markers to detect linkage
with many of the major Co-genes (Young and
Kelly 1997a). Markers that proved the most use-
ful were converted to more robust SCAR markers
to facilitate their utilization across breeding pro-
grams (Young et al. 1998). In some instances,
the markers were used in fine-mapping regions
around the gene (Melotto and Kelly 2001), but
in most cases they were used in marker assisted
selection (MAS) or marker-assisted backcross-
ing (Garzon et al. 2008; Ferreira et al. 2012).
Many markers were linked to the Co-genes at
distances of less than 5cM, which is sufficient for
MAS, but a few were more tightly linked at less
than 1 cM (Kelly and Vallejo 2004). When RAPD
markers failed to produce useful polymorphisms
between parents, researchers turned to AFLP and
eventually to SSR markers to find useful linkages
with new anthracnose genes. Markers linked to
many of the major Co-resistance genes are shown
in Table 9.3. More recently sequence-based STS
markers from the bean genome are being used
to tag resistance genes (Gon¸alves-Vidigal et al.
2011) and these will continue to become more
important in future studies.
Molecular Markers
Molecular markers detect DNA polymorphism
both at the level of specific loci and at
the whole genome level. Different methods
allow the analysis of DNA sequence variation.
Currently, PCR-based molecular markers such
as simple sequence repeat or microsatellites
(SSR), sequence characterized amplified regions
(SCAR), or single nucleotide polymorphism
(SNP) are the most commonly used in plant
breeding and genetics. Numerous SSR markers
have been described in common bean (Blair et al.
2003; Grisi et al. 2007), although not all have
been mapped. SCAR markers linked to specific
genes have also been described and many of them
mapped in common bean ( http://www.css.msu
.edu/bic/Genetics.cfm). Se quencing projects of
different bean genotypes are in progress. The
number of markers is expected to expand
dramatically with the recent publication of
the common bean genome sequence ( http://
www.phytozome.net/commonbean_er.php). I n
conclusion, numerous molecular markers are
available in common bean and they offer the pos-
sibility to carry out studies such as genetic anal-
ysis and development of genetic linkage maps to
saturate specific regions, as well as to locate new
loci or to investigate interactions between loci.
Tagging of Co-Genes with Molecular
Markers
Genetic Linkage Map of Common Bean
Two linked loci are transferred together to the
progenies, in a proportion that depends on the
genetic distance between them. Genetic distance
between two loci is estimated from the frequency
of recombination between both loci. The major
Co-genes conditioning resistance to anthracnose
in common bean were among the first traits to be
used in the discovery of the molecular markers
linked to resistance genes. The first anthracnose
resistance gene, Co-2 (Are) , was tagged indepen-
Genetic linkage maps show the relative position
of loci established from recombination fraction.
Linkage maps are important tools for geneticists
and breeders because they show the linkage rela-
tionship among loci.
In recent years, building saturated genetic
maps has been possible due to the availability
of techniques exploring the size or sequence
polymorphism
at
the
DNA
level
(molecular
markers).
Different
genetic
maps
have
been
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