Subgenus Pilosella (sometimes considered as a separate genus) is characterized by the occurrence of several ploidy levels in most species. Diploids are sexual; tetra- and hexaploids are sexual or apomictic; tri-, penta-, hepta-, and octoploids are apomictic. At least three populations with sexual pentaploids have already been found. Tetra- and pentaploids are often predominating in natural populations. Aneuploids are extremely rare in their native distribution area, but frequent in crossing experiments involving odd ploidy level parents. While autogamy can sometimes be stimulated by foreign pollen, the plants are generally self-incompatible. Allopolyploidy is considered to be the usual mechanism for the formation of polyploids. The (facultatively) apomictic plants usually produce pollen and can therefore regularly take part in hybridisations and backcrosses. They show residual sexuality and can therefore also act as seed parents. Hybridization between Pilosella species is common and leads to both sexually or apomictically reproducing plants. High diversity of progeny caused by versatility in reproductive pathways (apospory, haploid parthenogenesis, cross-fertilization of both reduced and unreduced female gametes) was documented in some species. Different hybridogenous taxa originate with different frequency, ranging from multiple recent hybrids to uniform clones which may cover large areas. Individual (hybridogenous) species differ in their size, ability and form of clonal growth (via stolons), and in their preferences with respect to habitat conditions. It seems that they also differ with respect to preferences of individual plant communities.
The combination of these processes has lead to a heavily reticulate taxonomic structure, and the vast number of morphological forms resulting from it makes species delimitation an extremely difficult task.
Pilosella species are mostly weak competitors in their native environment, growing on poor soils, preferring open and disturbed habitats. Many hybridogenous taxa have arisen in man-made habitats. Several species have been introduced to other continents (North and South America, North-East Asia, New Zealand) where they often became noxious weeds.
For a recent review on Pilosella, see Fehrer et al. 2007b