Taxonomic implications of distribution, breeding systems, vestiture type, and pollen morphology in selected Mexican Hieracium species (Compositae)

The taxonomic relationships of the polytypic Mexican Hawkweeds, Hieracium abscissum Less., H. dysonymum Blake, H. mexicanum Less., H. pringlei A. Gray, H. schultzii Fries, and two potentially new taxa, H. 'potosi' and H. 'chipinque' were addressed in this paper. Extensive field work discovered no clearly intermediate or hybrid individuals, indicating that the polytypy observed in herbarium specimens is more likely due to responses to different sites than to hybridization. Breeding system studies which included pollinator exclusion in-situ, emasculation, and artificial selfing indicate that these Hieracium species are outcrossers. A vestiture survey based on scanning electron microscopy (SEM) and analysis by Spearman's Rank Correlation and Average Linkage Cluster Analysis resulted in the taxa sorting into two groups, the Hieracium abscissum-schultzii group to which H. 'chipinque' belongs, and the Hieracium dysonymum/-mexicanum group to which H. 'potosi' belongs. SEM studies of pollen morphology determined two groups which correspond to the plants' growth form. The many-headed species--Hieracium abscissum, H. schultzii, and H. 'chipinque,' have curved spines; the few-headed species--H. dysonymum, H. mexicanum, H. pringlei and H. 'potosi,' have straight spines.