Genomic Adaptations to Salinity Resist Gene Flow in the Evolution of Floridian Watersnakes

Abstract

The migration-selection balance often governs the evolution of lineages, and speciation with gene flow is now considered common across the tree of life. Ecological speciation is a process that can facilitate divergence despite gene flow due to strong selective pressures caused by ecological differences; however, the exact traits under selection are often unknown. The transition from freshwater to saltwater habitats provides strong selection targeting traits with osmoregulatory function. Several lineages of North American watersnakes (Nerodia spp.) are known to occur in saltwater habitat and represent a useful system for studying speciation by providing an opportunity to investigate gene flow and evaluate how species boundaries are maintained or degraded. We use ddRADseq to characterize the migration-selection balance and test for evidence of ecological divergence within the N. fasciata-clarkii complex in Florida. We find evidence of high intraspecific gene flow with a pattern of isolation-by-distance underlying subspecific lineages. However, we identify genetic structure indicative of reduced gene flow between inland and coastal lineages suggesting divergence due to isolation-by-environment. This pattern is consistent with observed environmental differences where the amount of admixture decreases with increased salinity. Furthermore, we identify significantly enriched terms related to osmoregulatory function among a set of candidate loci, including several genes that have been previously implicated in adaptation to salinity-stress. Collectively, our results demonstrate that ecological differences, likely driven by salinity, cause strong divergent selection which promotes divergence in the N. fasciata-clarkii complex despite significant gene flow.

Proteomic Response to Environmental Stresses in the Stolon of a Highly Invasive Fouling Ascidian

“…In aquatic species, salinity is a common environmental stressor that can remarkably affect metabolism and osmotic regulation of various organisms (Vargas-Chacoff et al, 2015). Marine animals can maintain their most critical physiological functions through multiple molecular mechanisms when exposed to salinity stresses, despite that there are alterations in osmotic pressure and cytoplasmic composition in their cells (Rautsaw et al, 2020). In our study, the identified DAPs in C. robusta exposed to both low and high salinity stresses were significantly enriched in the pathways "posttranslational modifications (PTMs)," "cytoskeleton, " and "signal transduction, " suggesting the involvements of these pathways in the responses of stolon to salinity stress.…”

Contemporary Philosophy and Methods for Studying Speciation and Delimiting Species

“…It is possible now to determine which traits are involved in reproductive isolation by examining significant differences between taxa across their genomes and then relate those differing alleles to morphology (Han et al, 2017). For example, an intriguing recent study (Rautsaw et al, 2020) looked at genomic divergence between the Florida watersnakes Nerodia clarkii, occurring in high-salinity environments, and N. fasciata, in freshwater environments. They demonstrated that despite recurring historical and contemporary gene flow between these taxa, several key loci were responsible for ecological divergence in these environments.…”

Right out of the gate: the genomics of Lessepsian invaders in the vicinity of the Suez Canal

“…Dispersal (and its genetic consequence, gene flow) counter the potential for local adaptation. However, when selection is strong, it may overcome the homogenizing effects of gene flow, to the point of creating population structure and even occasionally resulting in speciation (Nosil 2008;Savolainen et al 2013;Arick et al 2020). Considering the strong environmental gradient encountered by Lessepsian migrants in the Suez Canal, genes under selection involved with osmoregulation have been found in the bluespotted cornetfish, a very successful Lessepsian migrant (Bernardi et al 2016).…”