A survey was made of the occurrence of natural hybrids between two species of introduced thistles,Carduus acanthoidesL. var.acanthoidesandC.nutansL. var.nutans, in Grey County, Ontario. The population of one field was studied in detail. Specimens were taken and their morphological variation was evaluated by means of a hybrid index. These species have different chromosome numbers:C.acanthoidcsL., 2nensp;=ensp;22;C.nutansL., 2nensp;=ensp;16. All the intervening numbers were found in the hybrids. There is a clear correlation between chromosome number and hybrid index: plants with the chromosome number of a first generation hybrid have an index intermediate between those of the parental species. The chromosome numbers of seedlings raised from seed collected from hybrid plants were determined. It was found that seedlings with the number 2nensp;=ensp;22 were much more frequent than those with other numbers. It is suggested that both ecological and gametic selection are acting in this area to favor production and survival of plants with the chromosome number and morphology ofC.acanthoides.
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