Chloroplast DNA diversity of Hieracium pilosella (Asteraceae) introduced to New Zealand: Reticulation, hybridization, and invasion

Abstract

The European hawkweed Hieracium pilosella is a successful invader and a troublesome weed in New Zealand. The systematics of the genus Hieracium is extremely complex and contentious, probably due to recent speciation, hybridization, polyploidy, and diverse reproductive strategies. In the first chloroplast DNA survey of the group, we sequenced 285 plants (including H. pilosella and 12 other species of subgenus Pilosella) from New Zealand and Europe for 900 bp of trnL-trnF. Eleven haplotypes were identified with much sharing among species. Three haplotypes (A, D, G) were found in seven, three, and four species, respectively, but two species (H. lactucella and H. auricula) had single, private haplotypes. Our cpDNA data for subgenus Pilosella are consistent with the group's having incomplete lineage sorting and/or recent reticulate evolution. Six haplotypes were identified in H. pilosella, four of these unique to this taxon in our sample. In New Zealand, haplotype A was common and occurred in plants of different ploidy (i.e., 4X, 5X, 6X), whereas haplotypes C, B, and M were restricted to 4X, 5X. and 6X plants, respectively. The distribution of haplotype variation suggests that some or all of the H. pilosella seeds accidentally introduced into New Zealand probably came from cast Europe rather than the United Kingdom and that a minimum of four lineages were introduced. Within New Zealand, hybridization of H. pilosella with a related taxon (probably H. praealtum) has occurred at least three times, involving both obligate sexual tetraploids and facultative apomictic pentaploids of H. pilosella.