The bean pod weevil (Apion godmani) is a serious pest of bean in Mexico and Central America. Resistance exists in Mexican landraces selected in past years. However, attack will vary from year to year, making breeding for resistance unpredictable. Efforts to identify molecular markers for resistance were initiated as long as ten years ago, but were hindered by small population size, erratic phenotypic data and lack of adequate markers.

Continuing efforts to overcome these limitations, in collaboration with the Mexican national program, have resulted in the first reliable markers. A total of nine RAPD bands were significantly associated with resistance were selected for cloning. Fifteen primer sets were designed for the nine RAPD bands and were tested on the population parents and on the bulks. Most of the PCR products were monomorphic as SCARs but a single SCAR (W6800R) showed a polymorphic fragment with clear positive and negative signals in PCR amplification. All monomorphic SCARs were tested with frequently-cutting (4 bp recognition sites) restriction enzymes, revealing CAPs polymorphisms for four of the PCR fragments. The molecular markers were mapped to loci on chromosomes 2, 3, 4 and 6 (linkage groups b01, b08, b07and b11, respectively).

In single point regression analysis, individual markers explained from 3.5 to 22.5% of the variance for the resistance trait with the most significant markers mapping to chromosome 2 (b01). Two additional significant markers were mapped to chromosome 6 (b11) and explained from 4.3 to 10.2% of variance depending on the season. There may be additional resistance genes on chromosome 4 (b07) and chromosome 3 (b08).

These are among the first specific markers developed for tagging insect resistance in common bean and are expected to facilitate breeding for resistance to A. godmani.

Contact: Matthew Blair