MAPPING HOST-PLANT RESISTANCE QTL FOR DOWNY MILDEW PATHOGEN IN PEARL MILLET [Pennisetum glaucum (L.) R. Br.]
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Date
2014
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ACHARYA N. G. RANGA AGRICULTURAL UNIVERSITY, RAJENDRANAGAR, HYDERABAD
Abstract
The present study entitled “MAPPING HOST-PLANT RESISTANCE QTL FOR
DOWNY MILDEW PATHOGEN IN PEARL MILLET (Pennisetum glaucum [L.] R.Br.)”
undertaken at ICRISAT - Patancheru was carried out for mapping the chromosomal regions
harbouring QTL for downy mildew resistance by utilizing the F8 RIL population generated
from a cross between ICMB 89111-P6 and ICMB 90111-P6.
Pearl millet downy mildew, caused by Sclerospora graminicola, is the most
devastating disease of pearl millet causing huge losses to grain and straw in India.
Sclerospora graminicola is known to be a highly variable pathogen because existence of
sexual stages in its life cycle helps the pathogen to undergo rapid genetic recombination
leading to the emergence of new pathotypes and races with high degree of spatial and
temporal variation for virulence. Evolution of more virulent populations of S. graminicola
in the recent past has resulted in the susceptibility of pearl millet accessions hitherto
resistant to earlier pathotypes. As the host is a crop of poor and marginal areas, the use of
resistant cultivars is the most appropriate, efficient, environmental friendly and economical
means to control pearl millet downy mildew. However, the conventional breeding alone is
tedious, time consuming and the progress is very low for understanding and manipulating
quantitative traits, molecular markers are used as highly effective research tools to uncover
the genetic basis of complex traits such as pest/disease resistance.
Keeping in view the seriousness of the downy mildew problem, the present
investigation was proposed to map resistance QTLs for diverse virulent isolates of
Sclerospora graminicola [(Sg445 (Gujarat), Sg519 (Haryana), Sg526 (Rajasthan)] in a RIL
mapping population of F8 generation of 188 genotypes from the cross ICMB 89111-P6 ×
ICMB 90111-P6.
For the QTL mapping available 20 pairs of mapping population parental lines were
screened against three above new isolates of downy mildew pathogen and ICMB 89111-P6,
ICMB 90111-P6 which are recorded high contrast for downy mildew reaction in screening
were selected for mapping of downy mildew resistance QTLs.
Selected mapping population parents are screened for polymorphism with 468 SSR
primers and 118 SSR markers recorded polymorphism. From this 118, 88 polymorphic
SSR marker loci were used for genotyping the 188 RILs. In genotyping about 62.6% (21)
of polymorphic marker loci showed mendelian segregation ratio of 1:1, while 37.4% (13)
of polymorphic marker loci showed segregation distortion.
A skeleton linkage map of seven linkage groups with a total map length of 725.5
cM (Haldane units) was constructed using data from 74 marker loci for 188 RILs using
JoinMap at LOD threshold value of 5.0 and MapMaker/Exp version 3.0b and map was
drawn using Map Chart 2.2. The map length of individual linkage groups ranged from a
minimum of 32.1cM (LG3) to a maximum of 140.2 cM (LG1). The average inter marker
distance was 9.8 cM, with an average density of 0.102 markers/cM. The total number of
mapped loci per linkage group (LG) ranged from 5 on LG3 to 23 on LG1.
For QTL mapping the primary data of downy mildew incidence percentage was
converted later into resistance percentage and used for QTL analysis which was performed
by Composite Interval Mapping with Windows QTL Cartographer ver2.5 using a LOD of
2.5 as the threshold value at 0.05 significance levels and walk speed of 1cM with 1000
permutations for QTL significance. A total of seven different major disease resistant QTLs
were identified from the F8 RIL mapping population against the three isolates. Two QTLs
each were identified on LG4 against Sg445, Sg519 and Sg526 and one QTL on LG6
against Sg519. The inheritance of these QTLs showed that male parent ICMB 90111-P6
was contributing the resistance alleles. The highest LOD score (30.86) for the QTL
identified on LG4 against Sg445 and largest amount of observed phenotypic variation
(78.59) was contributed by the QTL on LG4 against Sg519. These identified DMR QTLs
can be transferred to genetic backgrounds of elite pearl millet hybrid lines through markerassisted
backcrossing programs. Flanking markers of the identified QTLs can facilitate
foreground selection of resistant progenies during the backcrossing process, where as other
marker loci can be used in reducing the length of the donor segments carried along with the
introgressed downy mildew resistance genes through background selection.
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Keywords
MAPPING, HOST-PLANT, RESISTANCE, DOWNY, MILDEW, PATHOGEN, PEARL MILLET