Comparative growth response in Carduus nutans, C. acanthoides, and their F1 hybrids

The growth response of F1 hybrids relative to parental genotypes was compared in a natural hybridization system involving the diploid species Carduus acanthoides L. (2n = 22) and C. nutans L. (2n = 16). The data were used to test models of hybrid intermediacy, superiority, or greater similarity to a single parent in response to variable soil conditions and intra- and inter-genotypic competition. Progeny from paired reciprocal crosses between the two species were used in the experiments. In each of the half-sib families produced, the rapid and early identification of F1 hybrid genotypes and parental genotypes (resulting from selfing) was confirmed using allozyme markers (Pgi-2, Tpi-1, and Tpi-2). Growth of the F1 hybrids was better than C. nutans in poor soil, and both F1 hybrid and C. nutans genotypes produced significantly more dry weight than C. acanthoides in all soil treatments. F1 hybrids grew at least as well as the maternal parental species C. nutans and significantly better than C. acanthoides in the presence of intra- and inter-genotypic competitors. The F1 hybrid genotypes formed a more aggressive competitive environment than C. nutans, as evidenced by the growth of all three genotypes. These data primarily supported the model of greater hybrid similarity to one parental species rather than hybrid intermediacy, with evidence of hybrid superiority under certain extreme conditions.