Species boundary and extensive hybridization and introgression in Petunia

Abstract

Studying the role of hybridization in the speciation of plants is one of the most thrilling areas of evolutionary biology. Hybridization in natural populations can act in opposition to divergence, contribute to adaptation through introgression or foster the emergence of new lineages via hybrid speciation. Species of the plant genus Petunia grow in open areas in southern South America. Some natural interspecific hybrid events have been described for the genus, such as between the endemic P. exserta and the widespread P. axillaris. Both species occur in sympatry in Serra do Sudeste (Brazil), where they occur in diverse habitats and exhibit floral divergence, which has been related to the attraction of different primary pollinators. The present study evaluates the maintenance of the species boundaries front of hybridization and introgression. Direct and indirect methods of estimating gene exchange employed genotyping 720 reproductive plants and 611 progenies of both species with eight microsatellite loci. Gene exchange was found to be frequent and bidirectional between the species, indicating that introgression changes their genetic constitution in areas of sympatry. Limits of the studied species are being maintained because of the high level of inbreeding and backcrosses that are habitat-dependent.

Could the reproductive system explain the stability and long-term persistence in a natural hybrid zone of Petunia (Solanaceae)?

“…Such mechanisms would protect P. axillaris species integrity more than P. exserta, which can be expected considering the evolutionary history of these two taxa (Turchetto et al 2019b) as P. exserta has been originated from P. axillaris. In nature, just a few hybrid morphotypes were found outside shelters together with canonical P. axillaris individuals (Turchetto et al 2019a;b;Schnitzler et al 2020;Teixeira et al 2020), which reinforces the idea of a well-established system protecting P. axillaris from gene exchanging with a sympatric congener. Moreover, the same gene set that allowed P. exserta divergence and adaptation to a particular environment could restrict hybrids' development outside the shelters (Caballero-Villalobos et al 2021).…”

“…Petunia exserta and P. axillaris exhibit perceived contrasting red and white corolla colours by the human eyes, respectively, and individuals with intermediate corolla colour are found in a sympatric zone between them. The intermediate-coloured individuals were considered hybrids based on genetic data (Lorenz-Lemke et al 2006;Segatto et al 2014;Turchetto et al 2015b;2019a;b;Teixeira et al 2019) and they also display different corolla shape compared to the species, not necessarily intermediate between the canonical morphologies of the two species (Teixeira et al 2020). As the canonical type, we considered plants displaying all traits according to the species description, respectively (Stehmann et al 2009).…”

Hybridization: a ‘double-edged sword’ for Neotropical plant diversity

“…For example, recent work on a species pair in Petunia Juss. (Solanaceae) from the grasslands of the pampas found moderate bidirectional gene exchange between ecologically divergent species using microsatellite loci, leading to introgression in sympatry and frequent backcrossing (Turchetto, Schnitzler & Freitas, 2019). This study builds on the previous finding of a novel hybrid lineage, coupled with an increase in floral polymorphism, resulting from hybridization among two other Petunia spp.…”

What could be the fate of secondary contact zones between closely related plant species?

“…Here, as in other species (i.e., Mota et al, 2019), P. exserta is kept as a unique evolutionary unit, independently from P. axillaris, but contrarily to those Bromeliads, the gene exchange is not low between Petunia species in Serra do Sudeste (see Turchetto et al, 2019b). P. exserta can be considered narrowly endemic compared to P. axillaris and, in general, has lower diversity indices than its sister species even accounting only sympatric populations (Lorenz-Lemke et al, 2006;Segatto et al, 2014).…”

“…The species P. axillaris ( Figure 1A) and P. exserta ( Figure 1B) are sympatric in part of the P. axillaris distribution (Lorenz-Lemke et al, 2006;Segatto et al, 2014;Turchetto et al, 2014). They are interfertile sister species (Turchetto et al, 2019b) of annual herbs (Reck-Kortmann et al, 2014), with P. axillaris widely distributed in the Pampas in southern Brazil, Uruguay, and Argentina and meeting P. exserta in the eastern part of its distribution in Serra do Sudeste (Brazil), where this last species is endemic. Sandstone towers with 300-500 m in elevation interspacing mosaics of open fields in middle of the Pampas biome characterize the Serra do Sudeste region ( Figure 1C).…”