Contents - Faperta

Physico-Chemical and Molecular Markers for Resistance to Insect Pests 177
S. graminum-resistant sorghum germplasm are associated with RFLP loci in regions
syntenic to locations of the wheat Dn genes controlling resistance to Russian wheat aphid,
D. noxia (Katsar et al., 2002). Nagaraj et al. (2005) identifi ed three QTLs associated with
resistance to S. graminum biotype I, and fi ve QTLs associated with resistance to biotype K.
The amount of phenotypic variation explained by these QTLs ranged from 9% to 19.6%.
Tao et al. (2003) identifi ed two different mechanisms for resistance to sorghum midge, S.
sorghicola, through QTL mapping. Genetic regions located on two separate linkage groups
were associated with the antixenosis mechanism of resistance, and explained 12% and 15%
of the total variation in egg laying, respectively. One region was associated with antibiosis
resistance to midge, and explained 34.5% of the variation in egg and pupal counts. Two RIL
mapping populations (BT 623 IS 15881-3 and 296B IS 18551) have been phenotyped
and genotyped for resistance to sorghum shoot fl y, A. soccata, and one population (ICSV
745 PB 15001-3) for resistance to spotted stem borer, C. partellus and sorghum midge, S.
sorghicola SSR-based genetic linkage maps for these populations have been constructed to
identify QTLs associated with resistance to these insects. Polymorphic SSRs associated
with resistance to shoot fl y and/or phenotypic traits asso ciated with resistance to this
insect have been identifi ed (Folkertsma et al., 2003; Hash et al., 2003). Markers Xtxp258 (bp
190/230) and Xtxp289 (bp 270/294) are linked to trichome density; Xgap1 (bp 180/254) and
Xtxp141 (bp 154/169) to deadheart incidence, leaf glossiness, and trichome density; Xisp328
(bp 144/166) and Xisp264 (bp 153/207) to leaf glossiness; and Xisp258 (bp 170/193) and
Xtxp65 (bp 125/134) to deadheart incidence and leaf glossiness. These QTLs are now being
transferred into locally adapted hybrid parental lines via SSR-based MAS.
Potato
Bonierbale et al. (1994) mapped RFLP loci on progeny of crosses between cultivated and
wild potato, Solanum berthaultii Hawkes, for genes controlling glandular trichome-based
resistance to Colorado potato beetle, L. decemlineata. Several QTLs explained various components
of resistance based on potato leaf trichome type, density, and exudates. Yencho et
al. (1996) mapped L. decemlineata resistance genes in reciprocal backcrosses between
S. tuberosum and S. berthaultii using RFLP markers to identify QTLs linked to reduced oviposition
or feeding. Three QTLs on three chromosomes in BCB (backcross to S. berthaultii),
and two QTLs on two chromosomes in BCT (backcross to S. tuberosum) infl uenced resistance
to L. decemlineata involving S. berthaulti as a source of resistance. The QTLs were
additive, but one instance of epistasis was also observed. Each QTL accounted for 4% to
12% of the phenotypic variation. In the more resistant BCB population, a three-QTL model
explained nearly 20% of the variation in oviposition. When alleles at the three QTLs were
from S. berthaultii, the egg laying was reduced by 60% compared to that on the heterozygotes.
The QTLs for resistance to L. decemlineata coincided with loci associated with the
glandular trichomes, confi rming the importance of the glandular trichomes in mediating
resistance to this insect. However, a relatively strong and consistent QTL for insect resistance
in both BCB and BCT (backcross to S. tuberosum) on chromosome 1 was also observed,
but was not associated with any trichome traits, suggesting that the trichomes may not
account for all of the resistance observed in these progenies. Chen et al. (2004) constructed
two BAC libraries for wild Mexican diploid potato, Solanum pinnatisectum Dunal. Fifteen
BAC clones harbored PPO (polyphenol oxidase) loci for L. decemlineata resistance.
Development of these BAC libraries will be useful for BAC contig construction and mapbased
cloning of insect resistance genes.