Historical and ongoing hybridisation in Southern South American grassland species

Abstract

Natural hybridisation in plants can impact genetic and morphological diversity, including the emergence of better-adapted new populations and the potential extinction of some lineages involved in this intricate process. Under progressive global warming, species often need to migrate to newly suitable areas, which may be an additional challenge for species with low dispersal ability. Throughout the search for new environments, previously allopatric lineages can come into secondary contact and eventually hybridise if reproductive isolation is incomplete. Here, we focused on two taxa representing the natural herbaceous component of southern South American lowland grasslands. We aimed to evaluate the impact of contact zones and potential hybridisation on the evolutionary relationships and population dynamics. We used single nucleotide polymorphisms and morphological data of multiple individuals from allopatric and contact zones between taxa to shed light on hybridisation patterns and demographic scenarios. Our results indicated that the contact zones impact taxa's genetic and morphological diversity, and each contact zone had different patterns of genetic diversity and morphology, constituting stable populations that potentially reflect hybridisation events occurring at distinct evolutionary times.Hybridisation plays a significant role in plant diversification, and the evolutionary outcomes are influenced by several aspects, such as genetic diversity, population structure, geographical context in which species are inserted, ploidy level, and ecological factors 1 . On one side, hybridisation contributes to evolutionary diversification by introducing new allelic combinations, which may have a higher adaptive potential 2 . Conversely, hybridisation can increase reproductive isolation through reinforcement, in which natural selection acts against maladapted hybrids. In extreme cases, hybridisation can be responsible for lineage extinction 3 .Young and recently diverged species or subspecies often keep the interbreed potential 4 , usually because the genomic regions that differentiate the taxa correspond to loci that confer local adaptation or are under divergent selection 5 . The outcomes of hybridisation are further influenced by geographic context (reviewed in 3,6 ). A "mosaic hybrid zone" may form in regions with different environments where parentals occupy contrasting habitats and hybrids occur in sites where the parentals occur close or even overlap 7 . A flexible and inclusive definition of the hybrid zone 8 includes "situations ranging from sporadic or occasional hybridisation between species that are broadly sympatric (perhaps associated with different habitats or resources) to narrow zones of hybridisation between taxa with effectively parapatric distributions".The genus Petunia (Solanaceae) is a young group encompassing annual herbs endemic to southern South American grassland domains 9 . Among these species, P. axillaris is an ideal model for investigating the speciation process and hybridisation driving dive...