Resumen de Taraxacum: ploidy levels, hybridization and speciation. The advantage and consequence of combining reproductive systems
The genus Taraxacurn• comprises many hundreds of species, grouped into c. 40 sections. Approximately 21 sections are recorded from the Mediterranean area. Some are modern, derived (sections like Ruderalia, Erythrosperrna and Alpestria), others primitive (among others, Piesis, Glacialia and Rhodotricha). The genus forms a polyploid complex with diploid up to octoploid representatives, triploids form the large majority. It was assumed that apomictic seed production prevailed in the large majority of the derived taxa, whereas sexual reproduction was said to be restricted to the primitive ones. Apomixis leads to fixed patterns of morphological traits within clonal families. The consequent, apparently consistent morphological differences between clones provided the basis for the extensive (micro-)taxonomy. However, sexual reproduction is more common than assumed: sexuality occurs in many sections. Ploidy level and breeding system are correlated: sexuality is strongly linked to diploids, and polyploids are usually apomictic. In the Mediterranean, diploid sexuals occur in 14 out of 21 sections. Purely diploid sections are the relatively primitive ones like Piesis, Glacialia, and Oligantha. In 10, possibly 11, more (species rich) sections, both sexual and asexual reproduction occur. In Ruderalia and Erythrosperrna, co-occurrence of sexual diploids and asexual triploids in mixed populations is very common in central and southern Europe. The following has been found: 1, apomixis is not obligate, polyploids show a variable (low) degree of sexuality by means of several mechanisms; 2, in mixed populations, hybridization, and introgression between ploidy levels takes place; this also leads to 3), a di-/ triploid cycle, that may bring extra potential for evolutionary response to environmental change. These processes may lead to: 1, the continuous production of new apomictic lineages (microspecies), 2, the fade-away of others (due to introgressive hybridization and selection), and 3, the generation of advanced diploids as well. It has been found that self-incompatibility of diploids may be broken down by pollen from polyploids. This gives diploid plants the possibility to reproduce in fully polyploid neighbourhoods, and favours the spread of diploids. Sections in which these phenomenona are present, are to characterise as evolutionary very dynamic, but at the same time as taxonomically very difficult to treat, among other things, because the "micro-"species concept is of limited applicability. Given the presence of diploids and polyploids in many of the sections in the Mediterranean, this may well hold for some of these too.
