Contents - Faperta

Physico-Chemical and Molecular Markers for Resistance to Insect Pests 171
2006; Yang et al., 2006). DArT markers are typed in parallel, using high throughput platforms,
with a low cost. DArT fi ngerprints will be useful for accelerating plant breeding, and
for the characterization and management of genetic diversity (Wittenberg et al., 2005).
Molecular Markers Linked to Insect Resistance in Different Crops
The linkage between quantitative trait loci (QTL) and molecular marker loci is determined
the same way as phenotypic resistance is linked with the segregation of genes for resistance
to insects. The QTL analyses allow the researcher to identify the loci from a group of
polymorphic segregating molecular makers that contribute most signifi cantly to explain
the phenotypic variation for biochemical or biophysical characters mediating insect resistance.
A key component of QTL analysis is the calculation of a LOD (logarithm of the odds
to base 10) score, a statistical estimate of the likelihood of recombination between two loci
due to chance alone. The LOD scores indicate whether the two loci are likely to be near one
another on a chromosome (at least in terms of recombination based genetic map, if not the
physical map of that character) and are therefore likely to be inherited together. LOD scores
of three or more indicate that the two loci are close to one another on a chromosome, that
is, there is 1 in 1,000 probability of allelic association in the segregating population that
occurs due to chance alone. The progress in identifying genomic regions associated with
resistance to insects in different crops is discussed below.
Cotton
A molecular linkage map of Gossypium hirsutum L. has been developed with 58 doubled
and haploid plants based on 624 marker loci (510 SSRs and 114 RAPDs). The 489 loci assembled
into 43 linkage groups and covered 3,314.5 cM (Zhang et al., 2005). Another map has
been developed based on RFLP markers in four mapping populations of G. hirsutum (Ulloa
et al., 2002), which comprised of 284 loci mapped to 47 linkage groups with an average
distance of 5.3 cM, covering 1,502.6 cM (approximately 31% of the total recombinational
length of the cotton genome). The linkage groups contained 2 to 54 loci each, and ranged
in distance from 1.0 to 142.6 cM. A total of 53 polymorphic fragments and 32 polymorphic
loci, representing fi ve linkage groups, have been identifi ed in two F 2 populations of
G. hirsutum [HS 46 MARCABUCAG8US-1-88 (MAR) and HS 46 Pee Dee 5363
(PD 5363)] by using RFLPs (Shappley et al., 1996).
The prominent effects of “Okra-leaf” locus (which is associated with resistance to
bollworms) on chromosome 15 are modifi ed by QTLs on several other chromosomes
(C. Jiang et al., 2000). Among the 62 possible QTLs (at LOD 2.0) for 14 morphological
traits, 38 (61.3%) mapped to D-subgenome chromosomes, suggesting that D-subgenome
of tetraploid cotton has been subjected to relatively greater evolution than the A-subgenome.
Expression of the dense glanding (dg) (which contributes to bollworm, H. armigera and
H. virescens resistance) mutant is confi ned to expanding leaves, internodes, and bracteoles
(Vroh Bi et al., 1999). The ability to trace DNA segments of known chromosomal locations
from the donor G. sturtianum Willis through segregating generations can be a starting
point to map low- gossypol seed and high-gossypol traits. However, there is no evidence
of association of dense glanding with 13 other mutant markers in G. barbadense L.
(Percy, 1999).