Linkage Mapping in a Watermelon Population Segregating for Fusarium Wilt Resistance

Abstract

<jats:p>Isozyme, randomly amplified polymorphic DNA (RAPD), and simple sequence repeats (SSR) markers were used to generate a linkage map in an F<jats:sub>2</jats:sub> and F<jats:sub>3</jats:sub> watermelon [<jats:italic>Citrullus lanatus</jats:italic> (Thumb.) Matsum. &amp; Nakai] population derived from a cross between the fusarium wilt (<jats:italic>Fusarium oxysporum</jats:italic> f. sp. <jats:italic>niveum</jats:italic>) susceptible `New Hampshire Midget' and resistant PI 296341-<jats:italic>FR</jats:italic>. A 112.9 cM RAPD-based map consisting of 26 markers spanning two linkage groups was generated with F<jats:sub>2</jats:sub> data. With F<jats:sub>3</jats:sub> data<jats:sub>,</jats:sub> a 139 cM RAPD-based map consisting of 13 markers covering five linkage groups was constructed. Isozyme and SSR markers were unlinked. About 40% to 48% of the RAPD markers were significantly skewed from expected Mendelian segregation ratios in both generations. Bulked segregant analysis and single-factor analysis of variance were employed to identify RAPD markers linked to fusarium wilt caused by races 1 and 2 of <jats:italic>F. oxysporum</jats:italic> f. sp. <jats:italic>niveum</jats:italic>. Current linkage estimates between the resistance trait and the marker loci were too large for effective use in a marker-assisted selection program.</jats:p>

A genetic linkage map for watermelon derived from a testcross population: (Citrullus lanatus var. citroides × C. lanatus var. lanatus) × Citrullus colocynthis

“…The number of markers with skewed segregation in the present testcross population was not as high (18%) as in the maps constructed for watermelon using an F 2 and an F 3 population (47.5% and 48% skewed markers, respectively; Hawkins et al 2001), and as in the map constructed for watermelon using a BC 1 population (25.7% skewed markers; Levi et al 2001b). Of the 205 markers analyzed, 37 showed skewed segregation patterns from the expected 1:1 ratio at P = 0.05.…”

“…These markers will be used in integrating the present map with the other maps recently constructed for watermelon (Xu et al 2000;Hawkins et al 2001;Levi et al 2001b). Also, an F 2 population derived from Griffin 14113 and NHM has been constructed.…”

“…Extensive genetic linkage maps have been constructed for major cucurbit species such as melon (Baudracco-Arnas and Pitrat 1996;Wang et al 1997;Brotman et al 2000;Dogimont et al 2000;Oliver et al 2000;Perin et al 2000) and cucumber (Park et al 2000;Staub and Serquen 2000). However, only a few small linkage maps have been constructed for watermelon (Navot and Zamir 1986;Navot et al 1990;Hashizume et al 1996;Hawkins et al 2001). Many markers are still required for the construction of a dense map that can be used extensively in watermelon breeding programs and to isolate genes that control fruit quality or confer resistance to diseases and pests.…”

“…However, cultivated watermelon had fewer markers that could be mapped with high confidence in such populations. Hawkins et al (2001) constructed two linkage groups (26 RAPD markers covering a mapping distance of 112.9 cM) and five linkage groups (13 RAPD markers covering a mapping distance of 139 cM), using an F 2 and an F 3 population, respectively. These populations are derived from a cross between PI 296341-FR (C. lanatus var.…”

“…Also, a large number of markers in that map were closely linked (0-2.7 cM), indicating the presence of chromosomal regions with low recombination events. To reduce the effect of a common genetic background on marker resolution and segregation as observed by Hawkins et al (2001), and to examine whether chromosomal regions with low recombination events also occur among watermelon genotypes derived from wide crosses, a testcross population was constructed.…”