Abstract
Evolutionary processes fuelling rapid species diversification are not yet fully understood, although their major contribution to overall patterns of plant biodiversity is well established. Hybridisation is among the least understood of these processes, despite its multifaceted role in speciation processes being widely accepted. Species of the large arctic-alpine genus Saxifraga are notorious for their ability to hybridise; however, the overall role of hybridisation and polyploidisation for the diversification of this genus remains unknown. Here, we provide a comprehensive genus-wide review of hybridisation accounts and ploidy levels. We find that the sections of Saxifraga vary greatly in their propensity to hybridise. The majority of natural hybridisation accounts are from recent localised events (n = 71). Hybridisation hotspots were located in the Pyrenees and the European Alps, thus contrasting with the overall distribution of species richness in the genus. Hybrids or hybrid populations are often short-lived in Saxifraga due to a multitude of reproductive barriers, most commonly low F1 hybrid fertility. However, these barriers are not always fully effective, allowing for backcrossing and the formation of hybrid swarms. In addition, we find that the incidence of polyploidy varies widely across different sections of Saxifraga, with species-rich sections Porphyrion and Saxifraga showing divergent polyploidy proportions. Overall, we show that hybridisation and polyploidisation played differential roles in the diversification of this large genus. Nevertheless, a significant proportion of species are yet to be scrutinised, particularly among the Asian Saxifraga species, illustrating the need for systematic further study to fully unravel the role of hybridisation during the evolution of Saxifraga.
Highlights
Hybridisation, defined as the mating of members of genetically distinct populations [1,2], has historically been largely viewed as evolutionary noise (e.g., [3,4]) since F1 hybrids are generally less viable and fertile than their parental species
The vast body of evidence from model systems involving hybridising species pairs clearly points to a multifaceted role of hybridisation in the process of speciation [4,5,6]
When fixed in a hybrid population, these traits may foster reproductive isolation from parental species, and hybrid genotypes may embark on their own evolutionary track, potentially leading up to hybrid speciation [6,12,13]
Summary
Hybridisation, defined as the mating of members of genetically distinct populations [1,2], has historically been largely viewed as evolutionary noise (e.g., [3,4]) since F1 hybrids are generally less viable and fertile than their parental species. In Helianthus L., a classic model system for interspecific hybridisation, two parental species gave rise to a salt-resistant hybrid species, which was able to conquer a new adaptive landscape compared to the parental species [18] This system appears to be a rare instance of homoploid hybrid speciation (parents and hybrid offspring having the same ploidy level), for which only ca. We provide a comprehensive review of the available evidence regarding hybridisation and polyploidy in Saxifraga, framing this information in the context of Saxifraga’s evolutionary history, in order to elucidate the contribution of these processes to the species diversity of this large arctic-alpine genus
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