Subject : Speciation in Silene nutans : timing of the emergence of reproductive isolation and involvement of cytonuclear incompatibilities
Keywords : Speciation, post-zygotic reproductive isolation, cytonuclear co-evolution, plastid-nuclear incompatibilities
Supervisor : TOUZET Pascal (E.E.P, Lille)
Date of begining : 1st October 2019
mail: zoe.postel@univ-lille.fr
bureau: SN2, 101
tel: 03.20.33.63.97
Studying the emergence of new species is one of the main question in evolutionary biology. Speciation generally occurs through the emergence of reproductive barriers between populations, leading to reproductive isolation between them. Post-zygotic reproductive barriers affect hybrids fitness and results from various mechanisms, among them the accumulation of genetic incompatibilities (Presgraves, 2010). Following the Bateson-Dobzhansky-Muller model of genetic incompatibilities, where reproductive isolation is due to mismatch between new allelic combinations, a form of genetic incompatibility is of particular interest : the cytonuclear incompatibilities (CNIs). These incompatibilities result from the disruption of the coevolution between cytoplasmic and nuclear genes, whose gene products normally interact (Greiner and Bock, 2013). Due to the endosymbiotic origin of the organellar genomes (Sloan et al., 2018), 90% of the organellar proteins are encoded in the nucleus (Rand et al., 2004), the rest of the essential protein being encoded in the organellar genome. For proper cell function, organellar complexes need gene products from nucleus leading to a tight coordination between organellar and nuclear genomes, both partners imposing selection pressure on one another, leading to coevolution between organellar and nuclear genome (Rand et al., 2004; Greiner and Bock, 2013). In isolated populations, divergence of gene sequences occurs through the independent accumulation of mutations. As coadaptation is lineage specific, each lineage will have unique coadapted nuclear and organellar gene combinations (Sloan et al., 2018). When hybridization occur between isolated lineages, coadaptation between these combinations will be disrupted leading to CNIs, malfunctioning organellar complexes and ultimately hybrid breakdown. Several studies focused on these incompatibilities (reviewed in Postel and Touzet, 2020).
My PhD is focusing on a gynodioecious angiosperm species, Silene nutans, in which previous studies identified four strongly differentiated evolutionary lineages in France (Martin, 2016; Martin et al., 2016; Van Rossum et al., 2018). Also, no hybridization events were detected at secondary contact zones suggesting the absence of gene flow between these lineages and strong and asymmetric reproductive isolation was suggested by the results from diallelic crosses between these four lineages, directly observed in F1s (Van Rossum, 2018 – unpublished results). S. nutans seem to be composed of four cryptic species and CNIs could play an important role in the speciation process at stake.
During my PhD, the idea is to investigate the evolutionary mechanisms and the demographic scenarios leading to this reproductive isolation and the emergence of the four cryptic species of S. nutans. To do so, we focus on studying the impact of CNIs in the reproductive isolation observed by searching for nuclear and cytoplasmic genes likely involved. Working with DNA sequence data for organellar genome, the idea is also to study the evolutionary dynamics and inheritance patterns of organellar genome in S. nutans and its role in the emergence of the four lineages. Using genomic data and DILS platform (Fraisse et al., 2020) we also aim at reconstruct the evo-demographic scenarios of the four lineages.
Papiers