Please read the abstract in the section 00front of this document / Dissertation (M Inst Agrar (Sustainable Plant-Genetic Resource Management))--University of Pretoria, 2006. / Genetics / unrestricted

no abstract provided by author / Master of Science

Doubled haploidy (DH) is a plant breeding technique that is often utilized by plant breeders to minimize the time required to reach homozygosity in breeding lines. The first objective of this study was to compare two methods of generating DH lines in tobacco (Nicotiana tabacum L.). Inbred burley tobacco varieties TN 90LC and GR 149LC were used to produce both androgenic derived doubled haploids (ADDH) and maternally derived doubled haploids (MDDH). The relative agronomic performance of TN 90LC and GR 149 LC ADDH and MDDH lines was compared when used either as pure-line cultivars or when used for the production of the KT 204LC and TN 97LC hybrid cultivars, respectively. The ADDH method was more efficient than the MDDH method in generating large numbers of haploid plants. On average the ADDH TN 90LC population was statistically inferior to the inbred TN 90LC for several agronomic traits; this inferiority of the ADDH method was not observed in the GR 149LC populations. For both genotypes, the MDDH populations were comparable to the inbred parental genotypes. The ADDH method was inferior for TN 90LC, but several individual TN 90LC ADDH lines were equal or superior to the inbred source. The agronomic variability observed in both ADDH and MDDH lines was decreased when they were used to produce hybrid cultivars. Less variation was observed in the DH-derived hybrids KT 204LC and TN 97LC compared to the ADDH and MDDH TN90LC and GR149LC parental lines, respectively. The significant inferiority of ADDH TN 90 lines in comparison to inbred TN 90LC was not observed in the ADDH derived KT 204 population compared to KT 204LC. The second objective of this study was to compare DH Lines derived from an F1 breeding population versus DH lines derived from a segregating F2 population where plants used for DH were pre-screened for quantitatively inherited resistance to soil-borne diseases black shank (Phytophthora nicotianae) and/or Fusarium wilt (Fusarium oxysporum f. nicotianae). There was a clear difference in susceptibility to black shank between the F1 and F2 derived DH populations, both in terms of average disease incidence, and more importantly, in the percentage of individual lines displaying high disease resistance. For two different burley crosses, DH lines derived from the F1 generation were considerably more susceptible to black shank than DH lines derived from the F2 generation. No differences in the incidence of Fusarium wilt were observed between DH lines of F1 and F2 generations; this was likely due to low overall disease incidence. Although delaying the DH process in tobacco from the F1 to the F2 generation could add time to the development of homozygous breeding lines, the delay may be offset by having to screen fewer finished DH lines to identify superior lines.