Abstract
BackgroundAn interesting seed protein family with a role in preventing insect herbivory is the multi-gene, APA family encoding the α-amylase inhibitor, phytohemagglutinin and arcelin proteins of common bean (Phaseolus vulgaris). Variability for this gene family exists and has been exploited to breed for insect resistance. For example, the arcelin locus has been successfully transferred from wild to cultivated common bean genotypes to provide resistance against the bruchid species Zabrotes subfasciatus although the process has been hampered by a lack of genetic tools for and understanding about the locus. In this study, we analyzed linkage disequilibrium (LD) between microsatellite markers at the APA locus and bruchid resistance in a germplasm survey of 105 resistant and susceptible genotypes and compared this with LD in other parts of the genome.ResultsMicrosatellite allele diversity was found to vary with each of the eight APA-linked markers analyzed, and two markers within the APA locus were found to be diagnostic for bruchid resistance or susceptibility and for the different arcelin alleles inherited from the wild accessions. Arc1 was found to provide higher levels of resistance than Arc5 and the markers in the APA locus were highly associated with resistance showing that introgression of this gene-family from wild beans provides resistance in cultivated beans. LD around the APA locus was found to be intermediate compared to other regions of the genome and the highest LD was found within the APA locus itself for example between the markers PV-atct001 and PV-ag004.ConclusionsWe found the APA locus to be an important genetic determinant of bruchid resistance and also found that LD existed mostly within the APA locus but not beyond it. Moderate LD was also found for some other regions of the genome perhaps related to domestication genes. The LD pattern may reflect the introgression of arcelin from the wild into the cultivated background through breeding. LD and association studies for the arcelin gene, linked genes and other members of the APA family are essential for breaking linkage drag while maintaining high levels of bruchid resistance in common bean.
Highlights
An interesting seed protein family with a role in preventing insect herbivory is the multi-gene, APA family encoding the α-amylase inhibitor, phytohemagglutinin and arcelin proteins of common bean (Phaseolus vulgaris)
The objectives of this research were 1) to genetically fingerprint arcelin-containing advanced lines of common bean and their wild progenitors for microsatellites from the APA region of linkage group B4 2) to compare the linkage disequilibrium (LD) around the APA locus in these lines compared to susceptible genotypes from both the Andean and Mesoamerican genepools, and 3) to compare the LD found at the arcelin locus with the LD found across the rest of the genome based on the evaluation of microsatellite markers on all 11 linkage groups of the common bean genome
Germplasm survey showed arcelin specific microsatellite alleles To begin with, this study focused on some previously developed arcelin region markers selected because they targeted sequences of the APA gene family (BMd9, BMd15, BMd16, PV-ag004 and PV-atct001) as shown in Figure 1 or because of their map location and proximity to the arcelin locus (BMd26, BMd8 and BMd30) on linkage group B4 [34,35]
Summary
An interesting seed protein family with a role in preventing insect herbivory is the multi-gene, APA family encoding the α-amylase inhibitor, phytohemagglutinin and arcelin proteins of common bean (Phaseolus vulgaris). Arcelin differs from other APA proteins in size and electrophoresis pattern [10] and to date, there are seven variants: the first four identified by Osborn et al [12], arcelin 5 found by Lioi and Bollini [13], and two additional variants found by Santino et al [14] and by Acosta-Gallegos et al [15] all in wild accessions of common bean These variants are somewhat similar but different gene copy numbers have been discovered for the variants and each is associated with different levels of resistance to the bruchid insect pest, Z. subfasciatus [2,7,16]. Experiments with transgenic transfer of arcelin have raised doubts about the effectiveness of arcelin alone in providing resistance [6,17] the APA locus is associated with resistance in breeding pedigrees and inheritance studies [18,19] this can be lost upon backcrossing as in the case of recently-made near isogenic lines for some arcelin alleles [20]
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