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

convenient than progeny selection, which requires an additional self-pollination and testing of the next
generation. We were interested to determine whether molecular markers could be found for resistance to
gummy stem blight.
Molecular markers linked with resistance to gummy stem blight are not currently available.
Nevertheless, linkage maps of watermelon have been constructed and several molecular markers are available
(Hashizume et al., 2003; Hawkins et al., 2001; Levi et al., 2002; Zhang et al., 2004). Hawkins et al. (2001)
constructed two partial maps from an F 2 and F 3 population, respectively, of the cross 'NH Midget' × PI 294361-
FR. The two short maps were 112.9 centimorgans (cM) and 139 cM long and included 26 and 13 RAPD
(random amplified polymorphic DNA) markers, respectively. In 2002, Levi et al. published a linkage map
where 171 RAPD markers, 27 ISSR (inter-simple sequence repeats) markers, and one SCAR (sequence-
characterized amplified region) marker were assigned to 24 linkage groups, the haploid number for watermelon
being 11. The linkage map by Hashizume et al. (2003) included RAPD, RFLP (restriction fragment length
polymorphism), and ISSR markers that mapped 554 loci in 11 linkage groups, for a total length of 2,384 cM.
Finally in 2004, Zhang et al. constructed a linkage map from a population of 117 recombinant inbred lines
(RILs) using a total of 104 RAPD, SCAR, and ISSR markers. The map covered a total distance of 1,027 cM
and included 15 linkage groups.
Additional molecular markers have been made available, but not mapped, for use in watermelon
(Guerra-Sanz, 2002; Jarret et al., 1997; Katzir et al., 1996; Poleg et al., 2001). Katzir et al. (1996) identified
SSR (simple sequence repeat) markers from DNA sequences of melon (Cucumis melo L.), watermelon, and
cucumber (Cucumis sativus L.) that amplified DNA fragments also in cucurbit species different from the one of
origin. In 1997, Jarret et al. used seven SSR markers to measure diversity among 32 watermelon lines. Poleg et
al. (2001) designed 24 SSR markers from a genomic library of melon DNA, four from a cucumber cDNA
library, and six from database sequences of cucumber and melon. Guerra–Sanz (2002) designed 19 primer pairs
from watermelon ESTs (expressed sequence tag) containing SSR markers.
Currently, the database of the National Center for Biotechnology Information (NCBI) contains 722
nucleotidic sequences (mostly ESTs) for watermelon (NCBI, 2004). These sequences can be also used to
design additional primers for amplification and screening of candidate sequence-based markers. A similar
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