Heterosis and the inheritance of paraquat resistance in Canada fleabane

Heterosis, or hybrid vigour, refers to the phenomenon whereby the progeny of a diverse cross exhibits enhanced growth, development or fertility compared to either parent. While heterosis as a research subject is common to much of the plant breeding literature, there are few weed science studies that discuss heterosis and, in particular, its potential impact on the inheritance of herbicide resistance. The objective of this research was to observe the inheritance of paraquat resistance in C. canadensis in three reciprocal crosses that were comprised of two resistant biotypes from unique evolutionary origins and a susceptible standard. Previously described SSR markers were used to genotype the parental biotypes and to ensure the successful production of the F1 generation. Segregating F2 generations were screened at two doses of paraquat: i) a discriminating dose (i.e., the lowest dose that discriminated the parents based on their dose responses) and ii) a parental eliminating dose (i.e., the dose at which both parental biotypes were controlled). Results of this research indicated that F2 families deviated from the expected 3:1 ratio for single gene inheritance. Evaluated digenic models suggested an 11:5 ratio may best describe paraquat resistance in C. canadensis. At both the discriminating and parental eliminating doses, F2 progeny from the cross of a paraquat resistant and susceptible parent displayed heterosis in survivorship and biomass accumulation whereas the F2 progeny of two resistant biotypes did not. In fact, the survivorship and biomass accumulation of F2 progeny from resistant parents declined, indicating that they were less fit than their parental biotypes. This suggest that resistant biotypes from unique evolutionary origins may have similarly fixed regions in their genomes due to the selection pressure exerted by the herbicide and that this then limits the potential for heterosis.