An inheritance model for heptachlor exo-epoxide transport in summer squash (Cucurbita pepo L.)

Abstract

We aimed to characterize the inheritance of HEPX (heptachlor exo-epoxide) uptake ability in summer squash (Cucurbita pepo L.). Crosses between ‘Patty Green’, a cultivar that cannot take up HEPX, and ‘Toyohira 2’, a cultivar that can take up high levels of HEPX, were evaluated in this study. The pattern of inheritance for F1 progeny indicated partial dominance since the measured amount of accumulated HEPX was close to that in ‘Toyohira 2’. In the F2 generation, plants segregated into those that did not take up HEPX and those that did take up HEPX at approximately a ratio of 1:5. This segregation pattern was similar to that for the inhibiting gene (dominant suppression of a dominant allele) in the dihybrid; the expected segregation ratio of 3:13 was supported by a chi-square test. Indeed, the I gene suppresses the N gene (non-transporting gene), but the i gene cannot suppress N (II or Ii suppression of NN or Nn). In this case, the genotype of ‘Patty Green’ is proposed to be iiNN and that of ‘Toyohira 2’ to be IInn. Additionally, we proposed three gene models to explain quantitative variation in HEPX transport. The genotypes of ‘Patty Green’ and ‘Toyohira 2’ are presumed to be ABC and abc, respectively. HEPX cannot be taken up unless two or more different dominant genes are present in a plant. Thus, the genotypes can be divided into HEPX non-transporting (Abc:aBc:abC:abc) and HEPX transporting (ABC:ABc:AbC:aBC) classes. Two or three different dominant genes, irrespective of the gene combination, work together to take up HEPX. In this model, the expected segregation ratio of 10 HEPX non-transporting:54 HEPX transporting was supported by a chi-square test. This pattern of inheritance was also supported by the segregation ratio of self-propagated plants (BC1-s) derived from a backcross. Although both of these inheritance models were correct phenotypically, the function of these genes should be clarified to explain the quantitative differences in HEPX uptake.