Reticulate evolution in the Mediterranean species complex of Senecio sect Senecio: Uniting phylogenetic and population-level approaches

Studies of reticulate evolution among plant taxa continue to be phylogenetically oriented, with an overall emphasis on pattern rather than process. Recently, however, the number of analyses has increased that are using phylogenies as a framework to derive microevolutionary hypotheses about natural hybridization and introgression in various species complexes. These hypotheses can then be tested by using data from extant populations. Here, this dual approach is illustrated by phylogenetic and population-level analyses involving the Mediterranean species complex of Senecio sect. Senecio (Asteraceae), using chloroplast (cp) DNA and rDNA (ITS) sequence information, allozymes, and RAPDs as nuclear data sets. The phylogenetic approach has allowed us to evaluate the extent and overall pattern of reticulation within the species complex. In at least two instances cpDNA and ITS capture, accompanied by differing amounts of introgression of the rest of the nuclear genome, has resulted in the phylogenetic alignment of taxonomically disparate species. In addition, a striking example of differential introgression of cytoplasmic and nuclear (allozymic) elements has been identified by detailed population-level examinations of two parapatric species of Senecio from the Near East allowing inferences to be made about the microevolutionary processes that may have led to this phenomenon. We conclude that an integration of phylogenetic and population-level approaches is critical to understanding the role of hybridization and introgression in the plant kingdom, and especially among species at an early stage of divergence from each other.