FURTHER STUDIES ON NATURAL SELECTION AMONG HYBRIDS OF CARDUUS ACANTHOIDES AND CARDUUS NUTANS

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Abstract
A third 5-year survey made in 1962 of Carduus acanthoides, C. nutans, and their hybrids in Grey Co., Ontario, revealed that a great decrease in these populations had occurred. C. acanthoides and hybrids similar to this species had survived better than C. nutans but very little spread of either species seemed to have occurred in 1957–1962. In experimental plots the hybrid has been made and backcrossed to the parental species. The species differ in chromosome number (C. acanthoides, 2n = 22; C. nutans, 2n = 16) and hybrids have intermediate numbers. Evidence was found from field and experimental studies that the progeny of the F1 hybrid included a greater proportion of seedlings with the higher chromosome numbers than with the lower and intermediate numbers. It is suggested that this selection may operate through the rejection of the longer chromosomes received from C. nutans, which, in certain zygotic combinations may constitute an excess of chromatin lethal to the zygote.