Embracing discordance: Phylogenomic analyses provide evidence for allopolyploidy leading to cryptic diversity in a Mediterranean Campanula (Campanulaceae) clade

Abstract

The Mediterranean Basin harbors a remarkable amount of biodiversity, a high proportion of which is endemic to this region. Here, we present an in-depth study of an angiosperm species complex, in which cryptic taxonomic diversity has been hypothesized. Specifically, we focus on four currently recognized species in the Roucela complex, a well-supported clade in the Campanulaceae/Campanuloideae: Campanula creutzburgii, C. drabifolia, C. erinus, and C. simulans. This study takes a phylogenomic approach, utilizing near-complete plastomes and 130 nuclear loci, to uncover cryptic diversity and test hypotheses regarding hybridization and polyploidy within this clade. Genome size estimates recovered tetraploid and octoploid lineages within the currently recognized, widespread species C. erinus, showing an east-west geographic pattern. Though genomic data clearly differentiate these two cytotypes, we failed to discern morphological differences. The formation of a cryptic octoploid lineage, distributed across the eastern Mediterranean, is hypothesized to be the result of an allopolyploid event in which one parental morphology is retained. The tetraploid C. erinus and C. creutzburgii (also a tetraploid) are implicated as parental lineages. Our results highlight the utility of target-enrichment approaches for obtaining genomic datasets for thorough assessments of species diversity and the importance of carefully considering gene-tree discordance within such datasets.