Citrullus lanatus (Thunb.) Matsum. & Nakai - Cucurbit Breeding ...

the data from each replication over family and used 9 to 12 data points (F 3 plants) per family to predict the
genotype of each F 2 plant.
Through BSA of the molecular markers polymorphic among the parental lines, we identified five SSR
markers and one RAPD marker as candidates for mapping on the segregating populations (Table 3). Since the
difference between the segregating bands in BSA for the three SSR markers was not always clear, we verified
their polymorphism on the DNA of the individual plants composing the bulks, prior to mapping on the entire
populations. Based on the results of this additional analysis, we discarded four of the five markers, and mapped
the fifth [C.l.1-20-a from Jarret et al. (1997)] on the F 2 population from the cross PI 482283 × 'NH Midget'. We
found that this marker was not linked to resistance to gummy stem blight. The RAPD marker UBC 338
(University of British Columbia, Biotechnology Center) in BSA produced a band (molecular weight ≈ 600 bp)
segregating among the parental lines of the cross PI 189225 × 'NH Midget'. The band was a candidate
dominant marker, linked to the resistant phenotype. Thus, we mapped this marker on the F 2 (Fig. 2).
The segregation of the 600 bp band of the RAPD marker UBC 338 (hereafter referred to as marker
UBC 338-600) in the F 2 population was compared to the F 2 phenotypic values for resistance to gummy stem
blight, as inferred by progeny testing of the F 3 families. Based on our linkage analysis performed with
Mapmaker 2.0 (Kosambi function), the molecular and the phenotypic markers for resistance had a
recombination frequency of ~35%, estimating a map distance of ~42 cM.
We would suggest that the UBC 338 marker might be linked to either a major QTL for resistance or to
the db gene. Under both hypotheses, the high variation of the trait due to its quantitative genetic components
may have contributed to estimate a longer mapping distance between the molecular and phenotypic marker in
our experiment.
When we tested the marker UBC 338-600 on resistant and susceptible PI accessions and cultivars, we
could find a band of similar molecular weight in 21.6% of the resistant PI accessions, compared to 4.8% of the
susceptible PI accessions, and in none of the four cultivars (Fig. 3). For our test, the UBC 338-600 was not
converted to a sequence-characterized amplified region (SCAR) marker. Thus, we cannot confirm that the 600
bp band recovered in some of the PI accessions tested was the same marker previously identified in PI 189225.
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