Evolution and biogeography of the endemic Roucela complex (Campanulaceae: Campanula) in the Eastern Mediterranean

Abstract

At the intersection of geological activity, climatic fluctuations, and human pressure, the Mediterranean Basin – a hotspot of biodiversity – provides an ideal setting for studying endemism, evolution, and biogeography. Here, we focus on the Roucela complex (Campanula subgenus Roucela), a group of 13 bellflower species found primarily in the eastern Mediterranean Basin. Plastid and low‐copy nuclear markers were employed to reconstruct evolutionary relationships and estimate divergence times within the Roucela complex using both concatenation and species tree analyses. Niche modeling, ancestral range estimation, and diversification analyses were conducted to provide further insights into patterns of endemism and diversification through time. Diversification of the Roucela clade appears to have been primarily the result of vicariance driven by the breakup of an ancient landmass. We found geologic events such as the formation of the mid‐Aegean trench and the Messinian Salinity Crisis to be historically important in the evolutionary history of this group. Contrary to numerous past studies, the onset of the Mediterranean climate has not promoted diversification in the Roucela complex and, in fact, may be negatively affecting these species. This study highlights the diversity and complexity of historical processes driving plant evolution in the Mediterranean Basin.

Hybridization‐facilitated genome merger and repeated chromosome fusion after 8 million years

“…In the Early to Middle Miocene, Asia Minor and the present-day Aegean Islands, Crete and the Peloponnese represented a continuous stretch of land, the € Ag€ ais (e.g. Cellinese et al, 2009;Crowl et al, 2015;Sfenthourakis and Triantis, 2017), and this landmass existed at the time of the purported origin of Ricotia, dated between 20.7 and 14.6 Ma. Approximately 12 Ma, as the waters of the Mediterranean innundated € Ag€ ais, today's Crete was separated from Karpathos' island complex, and by 8 Ma the Cretan group of islands was isolated by the sea (Kougioumoutzis et al, 2018;Sfenthourakis and Triantis, 2017).…”

Naming diversity in an evolutionary context: Phylogenetic definitions of the Roucela clade (Campanulaceae/Campanuloideae) and the cryptic taxa within

“…Damboldt, or the Roucela complex. More recently, Crowl et al (2015) elucidated the systematics and historical biogeography of this clade (Figure 1), motivating the establishment of a formal phylogenetic definition for this group.…”

“…We prefer to select the name Roucela because it is the oldest name by which this group of close relatives has been consistently referred to. Crowl et al (2015, figure 2, page 4).…”

“…Two recent phylogenetic studies have focused on the evolution of the Roucela complex (Crowl, Myers, & Cellinese, 2017;Crowl et al, 2015). These groups include small, herbaceous, annual Campanula plant species restricted to the Mediterranean Basin and characterized by a dichotomous branching habit and unappendaged calyx lobes (Carlström, 1986;Lammers, 2007).…”

“…Our recent study used this group as a model for understanding historical drivers of speciation and endemism in the Mediterranean Basin because of its narrow distribution and high level of endemism in this region (Crowl et al, 2015). Most species are narrow endemics to one or few islands primarily in the eastern Mediterranean Basin, with the notable exception of C. erinus, which is widespread across the Mediterranean climate zone, from the Arabian Peninsula to Macaronesia, including some coastal regions of North Africa.…”