Differential introgression and reorganization of retrotransposons in hybrid zones between wild wheats

Abstract

The maintenance of species integrity despite pervasive hybridization is ruled by the interplay between reproductive barriers. Endogenous postzygotic isolation will shape the patterns of introgression in hybrid zones, leading to variable outcomes depending on the genetic mechanism involved. Here, we analysed experimental and natural hybrid populations of Aegilops geniculata and Aegilops triuncialis to examine the genetics of species boundaries in the face of gene flow. Because long-terminal repeat retrotransposons (LTR-RTs) showing differential evolutionary trajectories are probably to affect hybrid dysgenesis and reproductive isolation between these wild wheat species, we addressed the impact of LTR-RTs in shaping introgression between them. Experimental settings involving artificial sympatry and enforced crossings quantified strong, but incomplete reproductive isolation, and highlighted asymmetrical endogenous postzygotic isolation between the two species. Natural hybrid zones located in the northern Golan Heights were analysed using plastid DNA, amplified fragment length polymorphisms (AFLP) marking random sequences, and sequence-specific amplified polymorphisms (SSAP) tracking insertions from six LTR-RT families. This analysis demonstrated asymmetrical introgression and genome reorganization. In comparison with random sequences and quiescent LTR-RTs, those LTR-RTs predicted to be activated following conflicting interactions in hybrids revealed differential introgression across the hybrid zones. As also reported for synthetic F1 hybrids, such LTR-RTs were specifically reorganized in the genomes of viable hybrids, confirming that conflicts between selfish LTR-RTs may represent key incompatibilities shaping species boundaries and fostering long-term species integrity in the face of gene flow.

Jumping genes: Genomic ballast or powerhouse of biological diversification

“…Such hybridizationinduced activation of TEs does not necessarily translate into an immediate TE proliferation that would be deleterious in early hybrids, but a prediction is that TEs having differentially accumulated and diverged among progenitor species should be preferentially activated when merged. As expected, both F1 and established wild wheat hybrids (Senerchia et al, 2016) showed increased dynamics (i.e., both proliferation and deletion). Genome changes mostly affecting specific TE populations in the Cottus hybrids are also consistent with such a prediction, but it remains unknown to what extent TE dynamics matches the "conflict hypothesis" outside plants.…”

Eco‐genetic additivity of diploids in allopolyploid wild wheats

“…geniculata significantly spread into locally semi-arid environments of Palestine, where local hybridisation with Ae. triuncialis (Senerchia et al 2016) may have promoted adaptive introgression, as shown in other species (Arnold et al 2016). In contrast to repetitive elements showing large genomic changes in allopolyploid wheats (e.g.…”

Copy number increases of transposable elements and protein‐coding genes in an invasive fish of hybrid origin

“…This has led to argue that a combination of hybridization and environmental stress during colonization of extreme habitats (deserts, salt marshes) could have initiated bursts of transposition in hybrid sunflower species (Kawakami et al, 2011), although subsequent experimental tests failed to induce TE proliferation under various stress conditions (Ungerer & Kawakami, 2013). The absence of transposition bursts has also been reported from retrotransposons in polyploid plant hybrids, which instead emphasized the importance of TE silencing through reorganization of methylation states and deletions of TE loci (e.g., Senerchia et al, 2015Senerchia et al, , 2016.…”