Abstract
Grape phylloxera is an insect pest of grapevines and is controlled by rootstocks bred from resistant North American <i>Vitis</i> species. However, rootstocks with <i>V. vinifera</i> in their parentage, such as the <i>V. vinifera x V. rupestris</i> hybrid AXR#1, have failed. This study is the first to examine the inheritance of resistance to tuberosity and nodosity formation separately. F<sub>2</sub> progeny from a remake of AXR#1 were screened with B type phylloxera originally collected from AXR#1 roots. Transgressive segregation was observed in the progeny for all measures of resistance and its inverse, susceptibility: number of nodosities, number of tuberosities, number of adults on tuberosities, and age to first reproduction. These metrics were used to define resistance and susceptibility and to separate progeny into the two phenotypic classes for chi-square analyses of segregation ratios based on one and two locus models. Segregation of nodosity formation using all definitions fit a 1:7 phenotypic distribution; thus, at least two loci are involved. Segregation of tuberosity formation and numbers of adults varied depending on the resistance definition that was applied; it is unclear whether one or two genes control tuberosity formation. In each case the resistant class was recessive. Due to the different distributions best fit by the two phenotypes, these results suggest that separate mechanisms control nodosity and tuberosity formation.
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