Abstract The genetic control of the synthesis of stearic acid (C18:0) and oleic acid (C18:1) in the seed oil of sunflower was studied through candidate-gene and QTL analysis. Two F 2 mapping populations were developed using the high C18:0 mutant CAS-3 crossed to either HA-89 (standard, high linoleic fatty acid profile), or HAOL-9 (high C18:1 version of HA-89). A stearoyl- ACP desaturase locus (SAD17A), and an oleoyl-PC de- saturase locus (OLD7) were found to cosegregate with the previously described Es1 and Ol genes controlling the high C18:0 and the high C18:1 traits, respectively. Using linkage maps constructed from AFLP and RFLP markers, these loci mapped to LG1 (SAD17A) and to LG14 (OLD7) and were found to underlie the major QTLs affecting the concentrations of C18:0 and C18:1, explaining around 80% and 56% of the phenotypic vari- ance of these fatty acids, respectively. These QTLs pleio- tropically affected the levels of other primary fatty acids in the seed storage lipids. A minor QTL affecting both C18:0 and C18:1 levels was identified on LG8 in the HAOL-9×CAS-3 F 2 . This QTL showed a significant epi- static interaction for C18:1 with the QTL at the OLD7 locus, and was hypothesized to be a modifier of Ol. Two additional minor C18:0 QTLs were also detected on LG7 and LG3 in the HA-89×CAS-3 and the HAOL-9×CAS-3 F 2 populations, respectively. No association between a mapped FatB thioesterase locus and fatty acid concentra- tion was found. These results provide strong support about the role of fatty acid desaturase genes in determin- ing fatty acid composition in the seed oil of sunflower. Keywords Quantitative trait loci · Candidate genes · Stearic acid · Oleic acid · Sunflower · Stearate de-saturase · Oleate de-saturase Introduction The qualitative manipulation of seed oils involves the modification of their fatty acid composition, which in turn determines the chemical properties of the oil and, ul- timately, its end use. Sunflower mutants with increased levels of oleic acid (C18:1>75% compared with 20% to 50% in commonly grown cultivars; Soldatov 1976) and stearic acid (C18:0>22% compared with 5% in standard sunflower seed oil; Osorio et al. 1995) have been devel- oped. The seed-oil phenotypes of both these mutants have important advantages for the food industry. High C18:1 sunflower oil has a higher oxidative stability (Purdy 1986) and heart-healthy properties (Wardlaw and Snook 1990). An increased C18:0 content would produce solid- or semi-solid fats, avoiding the use of chemical trans- formations such as hydrogenation or transesterification, which have been linked to cardiovascular disease (Kritchevsky et al. 1995; Ascherio and Willet 1997). Independent, classical genetic studies have been car- ried out in the high C18:0 mutant line CAS-3 (Osorio et al. 1995), and in high C18:1 lines, as well as in crosses between them. To study the inheritance of the high C18:0 trait, CAS-3 was reciprocally crossed with its original parental line RDF-1–532 (C18:0=8%), and with a sunflower inbred line with a normal high linoleic fatty acid composition, HA-89 (C18:0=5%) (Pérez-Vich et al. 1999). The C18:0 inheritance pattern in crosses between Communicated by B.S. Gill B. Pérez-Vich ( ✉ ) · S.T. Berry Advanta Biotechnology Laboratory, SES-Europe, B-3300 Tienen, Belgium e-mail: bperez@cica.es Tel.: +34-957-499233, Fax: +34-957-499252 J.M. Fernández-Martínez Instituto de Agricultura sostenible (CSIC), Apartado 4084, E-14080 Córdoba, Spain M. Grondona Advanta Semillas, Balcarce, Buenos Aires, Argentina S.J. Knapp Department of Crop and Soil Science, Oregon State University, Corvallis, Oregon 97331, USA Present address: B. Pérez-Vich, Instituto de Agricultura Sostenible (CSIC), Apartado 4084, E-14080 Córdoba, Spain Theor Appl Genet (2002) 104:338–349 © Springer-Verlag 2002 B. Pérez-Vich · J.M. Fernández-Martínez M. Grondona · S.J. Knapp · S.T. Berry Stearoyl-ACP and oleoyl-PC desaturase genes cosegregate with quantitative trait loci underlying high stearic and high oleic acid mutant phenotypes in sunflower Received: 7 December 2000 / Accepted: 21 May 2001