Evolution of Species Diversity through Hybridization

Spectacular examples for rapid speciation and radiation in the evolution of vascular plants are found among polymorphic and expanding species aggregates. Our working hypothesis is that not only differentiation but also hybridization on diploid and polyploid levels within such aggregates are essential and promote hybrid speciation, eco-geographical radiation, and biodiversity. Our model genus to verify this working hypothesis is Achillea (yarrows, Compositae-Anthemideae) with approximately 120 species. During a first phase of radiation in western Eurasia the genus has expanded into many ecological niches from deserts to high mountains with complex, polymorphic, diploid/polyploid, hybridizing, and widespread species groups (e.g., A. millefolium agg., A.nobilis agg. or A. ptarmica agg.), but also with uniform, isolated, and fragmenting diploid species (e.g., A. grandifolia, A.ligustica or A. ageratum). Already available morphological, phytochemical, and cytogenetical information as well as new preliminary data from nuclear and plastid DNA suggest phylogenetic relationships. These studies will be extended and supplemented by more detailed DNA and genome analyses (e.g., AFLP, microsatellites, FISH; in the future also QTL) and by other multidisciplinary approaches (e.g., PC-based morphometry, phytochemical bioessays, eco-indication). In this way we expect to demonstrate the postulated origin of hybrid species as well as their improved adaptive traits responsible for fitness and expansion. Already now preliminary multidisciplinary results (AFLP etc.) from Central European 2x and 4x species of A. millefolium agg. suggest that from two eco-geographically quite "narrow", regressive, and partially isolated parental species three much more "successful" and aggressive species have originated by hybridization (Figs. 5-7). Comparable processes apparently are involved in the expansion of A. millefolium agg. from Eurasia to N. America. There, in a second phase of radiation, the aggregate has occupied many new habitats from the sea shore to timberline. We propose to initiate studies on these 4x and 6x N. American populations and on the probably also hybrid 6x species A. millefolium s.str. This taxon has become an aggressive world-wide weed with the help of man in a third phase of generic expansion. Thus, the innovative aspect of our project concerned with evolutionary mechanisms in Higher Plants is the use of all methods available today to test the importance of hybridization for the origin of new species and their adaptive characters, eco-geographical radiation, expansion and phylogeography.

Does hybridization between different taxa increase species diversity and contribute to the eco-geographical radiation of clades? This central question of evolutionary biology is still much disputed. As a relevant model genus we have chosen the very polymorphic and pharmaceutically important milfoils (Achillea) from the large Compositae family, tribe Anthemideae. With about 140 species centred in SE Europe and SW Asia Achillea has radiated over the whole N Hemisphere, occupying numerous habitats from semideserts and maritime coasts to high alpine levels and from rock fissures and forests to meadows and ruderals. To clarify its relationships and differentiation by multidisciplinary and PC-supported methods, characters from roots to flowers, phytochemistry, chromosomes (common polyploidy: 2x, 4x, 6x and 8x), and particularly DNA (nuclear ITS, plastid trnL-F and AFLP fingerprinting) were analysed. Results show that the genus Achillea is monophyletic after inclusion of two small genera, can be differentiated into 5 sections and has evolved from ancestors in dry and warm habitats. Hybridization processes have been important during the evolution of Achillea and often can be traced reliably by comparing postulated parental and hybrid taxa. This is possible because taxon specific DNA-markers from the nucleus are transferred to the progeny by both parents, whereas plastid markers are handed on by the mother only. Relevant cases have been documented from recent hybrid swarms between otherwise clearly separated 2x and 4x species in the Balkans (A. clypeolata x A. collina) via the hybrid origin of more ancient 4x species from 2x parents belonging to different sections (A. acuminata: sect. Ptarmica x A. asiatica: sect. Achillea in China A. alpina agg., extending into N America) or to different species groups within a section (2x A. nobilis x 4x A. collina 4x A. virescens, W Balkans and Italy) and finally to most complex, reticulate and old species alliances as the A. millefolium aggregate in Eurasia. Here, hybrid and poly- ploid speciation has led to several, intimately connected polyploid series, ranging from 2x via 4x and 6x to 8x, and with hybrid contacts within and across the different ploidy levels. From NE Asia 4x-taxa have migrated relative recently across the Bering Straits and radiated with 4x and 6x ecotypes dramatically into the most diverse habitats of N America. The important advantages of hybrid and polyploid evolutionary processes are that they not only allow the combination of advantageous genetic traits and stimulate mutation rates, but also support the origin of crossing barriers. As such these processes are essential components of the differentiation-hybridization-cycles of evolution. Thus, the question formulated at the outset, should be answered in a positive way.