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mechanisms underlying
S. sclerotiorum
resistance are complex. Genetic
studies demonstrated a polygenic inheritance of resistance for all three forms
of infection (root, stalk, and head; Robert et al. 1987; Tourvieille and Vear
1990) and no race specificity (Thuault and Tourvieille 1988). Earlier studies
suggested additive gene action to be more important than dominance (Robert
et al. 1987). Several studies have been undertaken for genetic analysis of
resistance to
S. sclerotiorum
using molecular markers (Mestries et al. 1998;
Bert et al. 2002, 2004; Micic et al. 2004, 2005) and important sources of
resistance have been identified (Micic et al. 2004, 2005).
Mestries et al. (1998) used three generations (F
2
, F
3
and F
4
) derived from
a cross between two inbred lines GH and PAC2, for mycelium test on leaves
and capitulum in Clermont-Ferrand in central France. The linkage map
was constructed using 82 isoenzymes and RFLP markers, which covered
760 cM of the sunflower genome and the average distance between markers
was 12.6 cM. For lesion length on leaves, one (LG A), two (LG G and P) and
two (LG I) QTLs were found in F
2
, F
3
and F
4
generations, with R
2
of 10.5%,
18.8% and 20.5%, respectively. For capitulum index, as another resistant
test, one (LG A), two (LG A and M) and two (LG A and M) QTLs were
identified in F
2
, F
3
and F
4
generations, with R
2
of 11.3%, 20.8% and 30.6%,
respectively. For eight QTLs of the 10 QTLs detected for the two traits in
three generations, the PAC2 alleles contributed to positive alleles. Linkage
group A and M contained four and two QTLs, respectively controlling either
both traits (LG A) or one trait in two generations (LG M).
Bert et al. (2002) identified the QTLs controlling resistance to mycelium
extension on leaves and capitulum. An F
2
:F
3
mapping population from the
cross XRQ×PSC8 was used for this study and the experiment was conducted
in two years 1997 and 1999. Mycelium extension on leaves and capitulum
were studied in 1997 and 1999, respectively, but percentage of pathogen
attack and latency index were studied in both the years in F
3
population.
Three and two QTLs were identified for mycelium extension on leave and
capitulum in 1997, with total R
2
of 56.1% and 25.6%, respectively. Five (total
R
2
of 43.3%) and one (R
2
of 9.9%) QTLs were identified for percentage of
attack latency index in 1997, but three (total R
2
of 31.8%) and one (R
2
of
10.4%) QTLs were found for the same traits in 1999 respectively. Both the
parental lines contributed the positive alleles to the QTLs, but PSC8
contributed solely the positive alleles for latency index in both the years.
QTL co-location was observed for some traits on LG 7, 8 and 13. The position
of QTLs from this study when compared with those observed previously by
Mestries et al. (1998) showed no co-location of QTLs for the same traits. The
authors found that sunflower resistance to
S. sclerotiorum
is controlled by
several resistant factors, which differ according to the genotypes tested.
In a later study, Bert et al. (2004) used a population of 150 F
2
:F
3
plants
from the cross between FU (an unbranched maintainer line) and PAZ2 (a
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