each susceptible one (χ2 < 0.12). The assumption of a single gene controlling resistance would suggest that the gene effect is additive. This is supported by the observation that the mean of the F1 generation was not significantly different (P < 0.5535) from the average of the two parental means.
Transfer of resistance into Upland cotton was accomplished by crossing resistant G. arboreum (A genome) with a D-genome species to produce an interspecific hybrid that upon doubling the chromosome number gave a synthetic allotetraploid (Avila et al., 2006). The resulting plant was backcrossed with upland cotton (2AD1) and evaluated for resistance. Screenings of the BC2F1 and the BC2F2 for the ability of the nematodes to reproduce on the plants showed plants having resistance with population mean reproduction ratios intermediate between the resistant A-genome donor and the 2AD1 recurrent parent.
An AFLP marker linked to resistance was identified (Avila et al., 2005) and tested in the BC2F1 generation. This resulted in a good correlation of marker presence and resistance. However, in the BC2F2 generation the marker was not highly correlated with the resistance gene. New markers linked to resistance have been found, and their usefulness for marker- assisted selection is currently being evaluated.
Host gene response to reniform nematode infection in G. arboreum was evaluated through cDNA-AFLP analysis 16 days after inoculation using a resistant and susceptible accession. After sequencing of the differentially expressed transcripts, the sequences were used to search (BLAST) GenBank databases for homologous sequences. Transcripts for which homologous sequences were identified, were grouped according to their putative biological function. Transcripts of genes associated with cellular transport, cell cycle and DNA were up-regulated more in the susceptible than in the resistant accession. These investigators hypothesize that those processes are related to plant syncytium formation at the nematode feeding site. Some of the transcripts may be potential targets for iRNA silencing as a mechanism for transgenic resistance. Cellular processes that may be involved in natural resistance mechanisms, such as cellular rescue, defense, and transcription had the greater number of up-regulated transcripts in the resistant accession.
G. longicalyx. Virtual immunity to the reniform nematode in G. longicalyx (Yik and Birchfield, 1984) has been confirmed in various laboratories. In a project conducted by USDA and Texas A&M University scientists at College Station, TX, two tri-species hybrids of G. hirsutum, G. longicalyx, and either G. armourianum or G. herbaceum (Bell and Robinson, 2004; also see Brown and Menzel, 1950, and Konan et al., 2007) were utilized as bridges to introgress resistance to the nematode from G. longicalyx into G. hirsutum. Introgression was accomplished by recurrent backcrosses to agronomic G. hirsutum with cytogenetic analysis of early backcross generations to assess progress toward the euploid state (2n = 52), selection for nematode resistance at each generation, and examination of self progeny at the first, third, sixth, and seventh backcross to identify and eliminate lineages with undesired recessive traits (Robinson et al., 2007). Altogether, 689 BC1 progeny were generated from the two male-sterile hybrids. Introgression was pursued from 28 resistant BC1 plants, each of which was backcrossed four to seven times to G. hirsutum to derive agronomically suitable types. More than 2,500 backcross and selfed progeny were evaluated for nematode resistance. The resistance trait segregated (resistant:susceptible) 1:1 in backcross progeny and 3:1 in self progeny from putatively heterozygous resistant plants. There was no obvious diminution of the resistance across backcross generations. Advanced backcross plants were indistinguishable from agronomic cotton under greenhouse conditions. Comparisons of 240 homozygous resistant BC6S2 plants with heterozygous, susceptible, and recurrent parent plants in field plantings in 2006 showed normal lint quality