Inheritance of root-knot nematode resistance in cotton as determined by combining ability, generation mean, and mendelian analysis.

The southern root-knot nematode (RKN), Meloidogyne incognita (Kofoid and White) Chitwood, is a serious pest of cotton, Gossypium hirsutum L. Three greenhouse experiments were conducted to examine the inheritance of RKN resistance in M-315 RNR and six day-neutral primitive lines (M19-RNR, M25-RNR, M75-RNR, M78-RNR, M188-RNR, and M487-RNR). The general combining ability (GCA) estimates of M-315 RNR, M25-RNR, M75-RNR, and M487-RNR were negative. Specific combining ability (SCA) effects of M8 with M-315 RNR and M75-RNR were negative, but SCA was positive for M-315 RNR x M75-RNR. These latter two lines may have the same genes for RKN resistance. The SCA effects of M8 with M78-RNR and M188-RNR were positive, but the SCA effect was negative for M78-RNR x M188-RNR. These two lines may have different genes for RKN resistance. In a generation mean analysis experiment, M8 was crossed with M-315 RNR, M19-RNR, M25-RNR, M78-RNR, and M487-RNR. The data for each cross fit an additive/dominance model of a generation mean analysis. Dominance effects for the five lines were negative and were 66 to 87% as large as additive effects. Mendelian analysis of the cross between M-315 RNR and M8 indicated that one dominant and one additive gene controlled RKN resistance in M-315 RNR. The dominant gene was designated Mi\sb1 and the additive gene was designated Mi\sb2. Though at a lower level of resistance, the same model fit the M25-RNR data. A single partially dominant gene model fit the M19-RNR data and a one dominant gene model fit the M78-RNR data. In an allele test experiment, the six primitive resistant lines were each crossed with M-315 RNR. The lack of transgressive segregates in the F\sb2 indicated that the genes in the primitive lines were probably allelic to those in M-315 RNR. The five population means, ranges, and distributions of the cross of M75-RNR with M-315 RNR were nearly identical. This indicated that the resistance genes in M75-RNR and M-315 RNR are in fact the same as suggested from the combining ability analysis. The M78-RNR x M-315 RNR cross segregated for one additive gene while the M78-RNR x M8 cross segregated for one dominant gene. This supports the two gene model for M-315 RNR. (Abstract shortened by UMI.)