Hybridization and Introgression in Carduus nutans and C. acanthoides Reexamined

Abstract

The present paper provides a reexamination of an often cited example of hybridization and introgression in plants, that involving two introduced thistle species, Carduus nutans (2n = 16) and C. acanthoides (2n = 22), which occur sympatrically in Grey Co., Ontario, Canada. Evidence gathered from a range of characters (morphology, allozymes, flavonoids, molecular data, chromo- some numbers, and artificial hybridization and backcrossing experiments) revealed that both hy- bridization and backcrossing occur in this region. The two species were well separated morpho- logically, while the hybrid swarms exhibited the complete range of morphological variation evident for both species as well as intermediate types. There was a very close correlation with reduced fruit set and intermediacy in morphology. In contrast, a putative introgressed population, I1, exhibited a distinct shift towards C. acanthoides, with almost all individuals corresponding to an acanthoides or intermediate phenotype. Chromosome data from hybrid swarms and the introgressed population indicated a range of numbers from n = 8 to 11. Genetic identities based on allozyme frequencies was 0.68 between the two species and was 0.81 and 0.87 among the C. acanthoides and C. nutans populations, respectively. The three hybrid swarms showed equal similarity to both parental types, i.e., identity values of 0.82-0.83, while I1 was very similar to C. acanthoides. Allozyme data also confirmed the hybrid nature of the swarms with either additive or intermediate patterns. Flavonoid profiles were very similar in the two species. All individuals of C. acanthoides have the same four compounds, while populations of C. nutans were more variable and distinguished from C. acanthoides by the presence of up to three additional flavonoids. The hybrid swarms exhibited the complete range of flavonoid profiles, including characteristic acanthoides profiles, nutans profiles, and mixtures of the two. In artificial hybridization studies, C. nutans was the most frequent maternal parent, although both species served equally well as backcross parents. The F1 hybrids have 2n = 19 chromosomes and were almost completely sterile. Analyses based on 17 restriction enzymes, showed that the chloroplast genome and the transcribed region of the nuclear ribosomal genome of the two Carduus species are highly conserved; although some variation was detected in the ribosomal DNA spacer region. Although there was evidence of introgression in populations within the hybrid zone, patterns revealed by different characters were not always congruent. The results suggested that introgression may well be bidirectional. The Grey Co. region appears to represent a stable zone of hybridization in which the nature of the Carduus populations has changed little in the past 30 years.