Variation in Hieracium subgen. Pilosella (Asteraceae): What do we know about its sources?

The present paper reviews mechanisms producing complicated patterns of vari ation within Hieracium subgen. Pilosella. The taxonomic complexity of this subgenus is due to highly variable basic species and intermediate (hybridog enous) species. The most important sources of variation are polyproidy, hyb ridization and (mostly) facultative apomixis of the aposporous type. The co mbination of hybridization, apomixis and clonal growth leads to the mainten ance of various hybrids having originated from backcrossing and hybridizati on among more than two species, which is possible because of the fertile po llen of apomictic hybrids. Ever since Mendel's experiments, some of F-1 hyb rids have been found to be highly variable, probably reflecting the high he terozygosity of some of the basic species. Variable progeny can also result from unreduced gametes, or the rare parthenogenetic development of reduced gametes. While these processes were detected in experiments, their role wi thin field populations remains unknown. However, multiple origins of interm ediate species, and introgression within basic species are highly likely to result in high levels of variation. While few population level studies hav e been undertaken in Europe, several such studies have been carried out on adventive populations in New Zealand, and these show a different pattern. A neuploid plants, rare in Europe, are common in New Zealand, and there is fr equently more than one ploidy level within a population.