How much do we know about the frequency of hybridisation and polyploidy in the Mediterranean region?

Abstract

Natural hybridisation and polyploidy are currently recognised as drivers of biodiversity, despite early scepticism about their importance. The Mediterranean region is a biodiversity hotspot where geological and climatic events have created numerous opportunities for speciation through hybridisation and polyploidy. Still, our knowledge on the frequency of these mechanisms in the region is largely limited, despite both phenomena are frequently cited in studies of Mediterranean plants. We reviewed information available from biodiversity and cytogenetic databases to provide the first estimates of hybridisation and polyploidy frequency in the Mediterranean region. We also inspected the most comprehensive modern Mediterranean Flora (Flora iberica) to survey the frequency and taxonomic distribution of hybrids and polyploids in Iberian Peninsula. We found that <6% of Mediterranean plants were hybrids, although a higher frequency was estimated for the Iberian Peninsula (13%). Hybrids were concentrated in few families and in even fewer genera. The overall frequency of polyploidy (36.5%) was comparable with previous estimates in other regions; however our estimates increased when analysing the Iberian Peninsula (48.8%). A surprisingly high incidence of species harbouring two or more ploidy levels was also observed (21.7%). A review of the available literature also showed that the ecological factors driving emergence and establishment of new entities are still poorly studied in the Mediterranean flora, although geographic barriers seem to play a major role in polyploid complexes. Finally, this study reveals several gaps and limitations in our current knowledge about the frequency of hybridisation and polyploidy in the Mediterranean region. The obtained estimates might change in the future with the increasing number of studies; still, rather than setting the complete reality, we hope that this work triggers future studies on hybridisation and polyploidy in the Mediterranean region.

Different Patterns of Ecological Divergence Between Two Tetraploids and Their Diploid Counterpart in a Parapatric Linear Coastal Distribution Polyploid Complex

“…Whole genome duplication leading to polyploidy is a widespread mechanism of plant evolution and diversification (Soltis and Soltis, 1999;Soltis et al, 2010;Jiao et al, 2011). Estimates of the incidence of polyploidy in angiosperms range from 20 to 40% (e.g., Stebbins, 1938Stebbins, , 1950Wood et al, 2009) and is considerably higher in specific geographical regions (e.g., 69-87% in the Arctic Flora, Brochmann et al, 2004;37-49% in the Mediterranean Basin, Marques et al, 2017). Most studies of the incidence of polyploidy are based on chromosome counts obtained during taxonomic studies; however, due to technical and logistical difficulties, such studies are usually based on a few counts per species, which can limit one's ability to detect multiple cytotypes (Bennett, 1998;Soltis et al, 2007;Marques et al, 2017).…”

“…Estimates of the incidence of polyploidy in angiosperms range from 20 to 40% (e.g., Stebbins, 1938Stebbins, , 1950Wood et al, 2009) and is considerably higher in specific geographical regions (e.g., 69-87% in the Arctic Flora, Brochmann et al, 2004;37-49% in the Mediterranean Basin, Marques et al, 2017). Most studies of the incidence of polyploidy are based on chromosome counts obtained during taxonomic studies; however, due to technical and logistical difficulties, such studies are usually based on a few counts per species, which can limit one's ability to detect multiple cytotypes (Bennett, 1998;Soltis et al, 2007;Marques et al, 2017). With the emergence of high throughput methods such as flow cytometry, the number of studies focused on the incidence of polyploidy has increased Husband et al, 2013) and revealed that the extent of polyploidy is underestimated in many plant species (Marques et al, 2017).…”

“…Most studies of the incidence of polyploidy are based on chromosome counts obtained during taxonomic studies; however, due to technical and logistical difficulties, such studies are usually based on a few counts per species, which can limit one's ability to detect multiple cytotypes (Bennett, 1998;Soltis et al, 2007;Marques et al, 2017). With the emergence of high throughput methods such as flow cytometry, the number of studies focused on the incidence of polyploidy has increased Husband et al, 2013) and revealed that the extent of polyploidy is underestimated in many plant species (Marques et al, 2017).…”

The conservation significance of natural hybridisation in Mediterranean plants: from a case study onCyclamen(Primulaceae) to a general perspective

“…Our objective here was not to have an exhaustive and complete selection of papers on hybridisation in Mediterranean plants (see Marques et al . ) but to have a representative sample of papers on the occurrence of hybridisation (including allopolyploidy) in the Mediterranean flora. Autopolypoidy was not included since it does not involve hybridisation between distinct species.…”

Polyploidy promotes species diversification of Allium through ecological shifts

“…For example, polyploidisation can be temporally associated with speciation events (Smith et al, 2018;Cai et al, 2019), yet with lags of up to millions of years (Schranz et al, 2012). Polyploidisation may also be an important mechanism underlying the formation of biodiversity in geographic areas, such as the Pan-Himalayan region (Wen et al, 2014), the Andes (Luebert & Weigend, 2014) or the Mediterranean Basin (Marques et al, 2018). Therefore, a thorough reevaluation of the evolutionary significance of polyploidy and its impact on diversification is needed (Kellogg, 2016).…”