Hybridization and evolution in Picradeniopsis (Compositae)

The predominantly allopatric species of the genusPicradeniopsis, P. oppositifolia andP. woodhousei, are distinct in morphological, in phenolic and terpeniod chemical, and in cytological aspects (n = 24 andn = 12, respectively). In an area of sympatry in northeastern New Mexico, interbreeding occurs frequently with the production of morphologically intermediate hybrids. Morphological and phenolic chemical data from 191 plants in 40 isolated parental populations and from 91 plants in four hybrid populations are of limited value in determining the nature of this hybridization, but meiotic configurations of 12_II and 12_I and low pollen viabilities of 1–11% in the hybrids indicate that they are all of the F₁ generation (with one possible backcross). The absence of observed introgression, and therefore the absence of gene flow between the two taxa, strengthens the case for taxonomic recognition of two species in the genus. Comparisons of the morphology, phenolic and terpenoid chemistry, and cytology of parents and F₁ hybrids suggest that the tetraploid,P. oppositifolia, has arisen by allopolyploidy from a cross betweenP. woodhousei and an unknown diploid species. An extrapolated morphological and chemical reconstruction of this putative diploid parent is advanced.