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19 SSR markers in 18 linkage groups. Langar et al. (2003) used both AFLP
and DALP for constructing a linkage map for an RIL population derived
from a cross of HA 89 x LR4. The map contained seven allele-specific PCR
markers, 64 DALP, and 301 AFLP markers, and covered 2,168.6 cM in 18
linkage groups. This RIL population displayed the lowest percentage (about
4%) of markers showing distorted segregation.
3.5.3 SSR Maps
The publication of an SSR map (Tang et al. 2002) marked the most significant
milestone in sunflower genome mapping. The first sunflower SSR map was
constructed from a mapping population of 94 RILs derived from a cross
between two public inbred lines: a confectionery, RHA 280 (Fick et al. 1974),
and a single-headed oilseed restorer line, RHA 801 (Roath et al. 1981). In the
map, the 459 loci were amplified with 408 SSR primer pairs, each of which
produced one to three loci, and coalesced into 17 linkage groups with an
accumulated length of 1,368 cM. Although the mean density was 3.1 cM per
locus, there were four large gaps longer than 30 cM in four linkage groups
and two gaps longer than 20 cM in two linkage groups. Since there were
three polymorphic loci unlinked, the completeness of this map remained
questionable. Nevertheless, these mapped single or low-copy, co-dominant
DNA markers became the markers of choice for establishing a genome-wide
framework to anchor and cross reference genetic linkage maps constructed
from different mapping populations with a universal linkage group
nomenclature.
This public SSR map was soon expanded to 1,432 cM adding 118 new
SSR and INDEL markers and was aligned with other proprietary maps
constructed from the PHA × PHB RIL population and from the HA 370 × HA
372 F 2 map with 80 RFLP marker loci (Yu et al. 2003). Lai et al. (2005) further
advanced the map by adding SNP markers identified from sunflower ESTs
to this map. Of the 273 amplified polymorphic loci, 243 were mapped to the
17 established linkage groups. One observation was that several markers
derived from ESTs with putatively related functions were co-located with
previously mapped QTLs for traits such as salt tolerance, stem diameter,
shattering, flowering time, and achene size. The mapped SSR markers have
been used by numerous researchers in their mapping projects and made it
possible for all published maps to have the same linkage group numbers.
For example, Kusterer et al. (2004) used 19 mapped SSRs in their AFLP map
and Yue et al. (2008a) used 44 mapped SSRs in their TRAP marker map
constructed for locating Sclerotinia head rot resistance QTL. Recently, the
original RHA 280
RHA 801 RIL map has been expanded to 1,161 SSR loci
(Steven Knapp, personal communication).
 
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