Resumen
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Flue-cured tobacco (Nicotiana tabacum L.) cultivar Coker 371-Gold (C371-G) possesses a dominant gene, Ph o f unknown origin, that confers high
resistance to race 0 of the black shank fungus Phytophthora parasitica var nicotianae (Breda de Haan) Tucker (Ppn). Sixty RAPD markers linked to the Ph locus in BCjF, ((K 326 X C 371-G) X K 326)) doubled haploid (DH) lines were identified by bulked segregant and random amplified polymorphic DNA (RAPD) analyses. BC2Fi families totaling 164 DH lines derived from BC[F[ DH lines DH92-2770-40, DH92-3397-2 and DH92-3397-13 homozygous for Ph and a fourth BCiFt family of 50 DH lines derived from NCTG 61 (Ph/Ph) were generated. The 214 DH lines were screened with 30 of the RAPD markers and 149 of the BC2F [ DH lines and their testcross hybrids evaluated in black shank nurseries. No recombination between the RAPDs and Ph was observed. RAPDs O P Z - 5 77o and OPZ-7370 in coupling- and repulsion-phase linkage to Ph, respectively, were used to further backcross the gene to K 326. Complete linkage was observed between the 30 RAPDs and Ph in the K 326 BC5 generation. Black shank nursery evaluations o f 11 BC4S[ lines and their testcross hybrids confirmed the efficiency of marker-assisted selection for Ph. The origin of Ph was determined using Ph/Phl and Ph/Php testcross populations produced from F i hybrids of DH burley lines possessing Ph and L8, known to contain a gene designated Phi from N. longiflora Cav., and flue-cured DH92-2770-40 and NC 1071, possessing Php from N. plumbaginifolia Viv. Disease trials of the Ph/Phl and Ph/Php testcross populations in field black shank nurseries and greenhouse inoculations o f 200 Ph/Phl progenies with Ppn race 0 showed that Ph is allelic to
Php, while Phi and Php integrated 3 cM apart in the same tobacco chromosome.
Recombination o f RAPD markers in resistant Ph/Phl testcross progenies indicated that crossing over occurs between the Ph and Phi introgressed segments. N. plumbaginifolia and N. longiflora accessions, L8, NC 1071, C 371-G, DH92- 2770-40, K 326, Hicks and Coker 139 were screened with 48 o f the RAPDs and cluster analyses performed. A cladogram confirmed that Ph and Php are indistinguishable, but different from Phi. Analyses o f variance were conducted on yield in kg ha'1, weighted grade index, maturity and percent nicotine and reducing sugar levels in 92 BC2F! and 23 B C ^ D H lines grown in disease free nurseries at four locations in NC. A 9% yield penalty and a corresponding 10% increase in nicotine content were associated with the introgressed block surrounding Ph in the homozygous condition. The peak force required to dislodge 2000 stalks of 23 BC2Fi DH lines from one family was measured using a dynamometer attached to a metal collar and a fork-lift to provide a slow uniform pulling force. Ph had no significant effect on root strength or did not affect lodging ability in these lines.