Hidden hybridization and habitat differentiation ina Mediterranean macrophyte, the euryhaline genus Ruppia

In many aquatic plant taxa, classification and identification based on
morphology has always been difficult. Molecular markers revealed that the
complexity in several of these aquatic taxa could be addressed to
recurrent hybridization events and cryptic species diversity. The
submerged macrophyte genus Ruppia is one of these
aquatic genera with a complex taxonomy due to the absence of clear
distinguishable traits and several hybridization events. Two species
co-exist throughout Europe, R. maritima and R.
spiralis (previously known as R. cirrhosa), but recent
molecular studies also found several indications of hybridization,
introgression and chloroplast capture between these
species. However, the full extent and frequency of hybridization
and introgression in this genus has not been studied so far, nor is it
clear how these hybrid lineages can co-exist locally with their parental
species. In this paper, we wanted to detect whether a single coastal
wetland where both species co-exist can act as
a Ruppia hybrid zone. As a case study, we chose the
Camargue, a Mediterranean coastal wetland that harbours a wide diversity
in aquatic habitats, especially in terms of salinity and hydro regime. We
sampled several Ruppia populations within this wetland.
To identify each sample and reconstruct the local genetic structure of the
two parental species and their hybrids, we used both chloroplast and
nuclear microsatellite markers. Afterwards, we tested whether different
species had different habitat preferences. Our results confirmed
that R. maritima and R. spiralis are
two strongly divergent species with different reproductive ecologies and
different habitat preferences. This prevents frequent hybridization and
consequently we could not detect any trace of a recent hybridization
event. However, we found several populations of later-generation hybrids,
including a population of R. maritima x
hybrid backcrosses. These hybrid populations occupy a different
habitat and are genetically distinct from their parental species, although
they tend to be morphological similar to parental R. maritima.
Although hybridization and introgression in Ruppia is
less frequent than we expected, the taxonomy
of Ruppia is complicated due to ancient hybridizations
and several back-crossings.