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

Abstract

Interspecific hybridization has been fundamental in plant evolution. Nevertheless, the fate of hybrid zones throughout the generations remains poorly addressed. We analyzed a pair of recently diverged, interfertile, and sympatric Petunia species to ask what fate the interspecific hybrid population has met over time. We analyzed the genetic diversity in two generations from two contact sites and evaluated the effect of introgression. To do this, we collected all adult plants from the contact zones, including canonicals and intermediary colored individuals, and compared them with purebred representatives of both species based on seven highly informative microsatellite loci. We compared the genetic diversity observed in the contact zones with what is seen in isolated populations of each species, considering two generations of these annual species. Our results have confirmed the genetic differentiation between the species and the hybrid origin of the majority of the intermediary colored individuals. We also observed a differentiation related to genetic variability and inbreeding levels among the populations. Over time, there were no significant differences per site related to genetic diversity or phenotype composition. We found two stable populations kept by high inbreeding and backcross rates that influence the genetic diversity of their parental species through introgression.

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).…”

“…In the contact zone between P. exserta and P. axillaris, we have followed two sites encompassing canonical individuals as well as interspecific hybrids that have several changes in morphology (Teixeira et al 2020) and genetic profiles (Turchetto et al 2019b) comprising stable populations, despite being reduced in size (Lorenz-Lemke et al 2006;Segatto et al 2014;Turchetto et al 2015b). Recent analyses based on these sites' genetic composition showed that individuals are mainly F 2 hybrids or products from backcrosses with canonical individuals of both species across different flowering seasons (Schnitzler et al 2020). Hybrids were always found inside shelters (Turchetto et al 2019b;Schnitzler et al 2020), close to P. exserta plants, and F 1 hybrids were rarely observed.…”

“…Recent analyses based on these sites' genetic composition showed that individuals are mainly F 2 hybrids or products from backcrosses with canonical individuals of both species across different flowering seasons (Schnitzler et al 2020). Hybrids were always found inside shelters (Turchetto et al 2019b;Schnitzler et al 2020), close to P. exserta plants, and F 1 hybrids were rarely observed. Our current results concerning the compatibility among hybrids and backcrosses support the hypothesis that this contact zone represents a stable population, promoting introgression (Turchetto et al 2019a;b) and increasing morphological variability (Teixeira et al 2020) in both P. exserta and P. axillaris.…”

“…We analysed three plants per colour class in each experiment and performed all bidirectional crosses as each colour class provided to and received pollen from each other phenotype (seven colour classes). We used only three plants because the wild Petunia populations usually encompassed just a few individuals, especially in the contact zone between P. axillaris and P. exserta (Schnitzler et al 2020). Moreover, each fruit produces hundreds of small seeds, which display high viability (Rodrigues et al 2018a; also see Results section).…”

“…To accomplish this goal, we assessed differences in fruit production, seed quantity (number of seeds per fruit), and seed germination (number of seedlings that reached true-leaves stage) derived from controlled pollination experiments in a greenhouse; we also evaluated the association of these results with the corolla tube length and the distance between the stigma and dorsal anthers. We are motivated by the fact that these hybrid populations are observed for a long time with no changes in size or genetic polymorphism (Schnitzler et al 2020), even with most individuals displaying characteristics that are not related to the canonical species (Teixeira et al 2020) and, therefore, not included in parental floral syndromes.…”