Isolation by environment in the highly mobile olive ridley turtle (Lepidochelys olivacea) in the eastern Pacific

Abstract

Spatial and temporal scales at which processes modulate genetic diversity over the landscape are usually overlooked, impacting the design of conservation management practices for widely distributed species. We examine processes shaping population divergence in highly mobile species by re-assessing the case of panmixia in the iconic olive ridley turtle from the eastern Pacific. We implemented a biophysical model of connectivity and a seascape genetic analysis based on nuclear DNA variation of 634 samples collected from 27 nesting areas. Two genetically distinct populations largely isolated during reproductive migrations and mating were detected, each composed of multiple nesting sites linked by high connectivity. This pattern was strongly associated with a steep environmental gradient and also influenced by ocean currents. These findings relate to meso-scale features of a dynamic oceanographic interface in the eastern tropical Pacific (ETP) region, a scenario that possibly provides different cost–benefit solutions and selective pressures for sea turtles during both the mating and migration periods. We reject panmixia and propose a new paradigm for olive ridley turtles where reproductive isolation due to assortative mating is linked to its environment. Our study demonstrates the relevance of integrative approaches for assessing the role of environmental gradients and oceanographic currents as drivers of genetic differentiation in widely distributed marine species. This is relevant for the conservation management of species of highly mobile behaviour, and assists the planning and development of large-scale conservation strategies for the threatened olive ridley turtles in the ETP.

Large‐scale connectivity, cryptic population structure, and relatedness in Eastern Pacific Olive ridley sea turtles (Lepidochelys olivacea)

“… Full sites include two‐letter country codes. Also indicated is whether site was originally included in the Rodríguez‐Zárate et al (2018; R‐Z) data or present study (P), and population inferred by Rodríguez‐Zárate et al (2018) …”

“…Though comprehensive, the study did not survey genetic variability of key population index sites in Costa Rica (but did survey index sites in Mexico and Nicaragua, Table 1; Abreu‐Grobois & Plotkin, 2008; Bernardo & Plotkin, 2007). It is difficult to test for this Mexican–Central American split using mtCR haplotypes because haplotypes are not well reported for Central America, and the MUs (Mexican and Central American populations) proposed by Rodríguez‐Zárate et al (2018) based on nuclear genetic variability are yet to be formally integrated into management frameworks. Further, while Rodríguez‐Zárate et al (2018) found that environmental variables (namely oceanographic features such as the Costa Rica Dome) may play a role in this structuring, other factors such as relatedness and the effects of bottleneck events were not explored (although they were previously reported for the Mexican population; Rodríguez‐Zárate, Rocha‐Olivares, & Beheregaray, 2013).…”

“…We conducted this study to explore the nature and scale of population structuring in Costa Rican and Eastern Tropical Pacific olive ridleys using hierarchical and comparative population inference (i.e., clustering versus ordination population inference) to refine structuring beyond broad Mexican and Central American populations. We generated mtCR haplotype and nuclear microsatellite data from olive ridleys nesting at three sites in Costa Rica and inferred fine‐scale structuring, and then retrieved published microsatellite data from Rodríguez‐Zárate et al (2018) to compare patterns in population structuring at different scales within the Eastern Tropical Pacific. To our knowledge, this is the first study since Bowen et al (1997) to report mtCR haplotype data from olive ridleys south of Mexico.…”

Landscape genetic structure of Scirpus mariqueter reveals a putatively adaptive differentiation under strong gene flow in estuaries

“…However, local adaptation can evolve in the presence of gene flow, and even sometimes, gene flow may promote local adaptation through the introduction of genetic variation, the spread of advantageous alleles, nonrandom dispersal, and demographic benefits (Räsänen & Hendry, 2008). In the last few years, there has been a growing interest in understanding how gene flow and divergent selection interact to generate spatial pattern of genetic variation in heterogeneous habitats (Ferchaud & Hansen, 2016;Stanton, Galen, & Shore, 1997;Tigano & Friesen, 2016), and more recently in the marine environment (Diopere et al, 2018;Rodríguez-Zárate et al, 2018;Sexton et al, 2014). A central challenge has been presented to disentangle the relative contributions of selective and neutral processes (such as gene flow and genetic drift) underlying genetic variation (McCairns & Bernatchez, 2008;Räsänen & Hendry, 2008;Sexton et al, 2014;Tigano & Friesen, 2016).…”

Shared genomic outliers across two divergent population clusters of a highly threatened seagrass

“…However, few studies have addressed spatial variation of outlier loci among populations and how these might contribute towards shaping population divergence in natural systems. In addition, it can be challenging to disentangle the signatures of potential adaptation to different environments from the simple isolating effect of distance, especially as a high degree of collinearity exists between environmental gradients and neutral population structure (Wang & Bradburd 2014;Manthey & Moyle 2015;Prunier et al 2017;Weber et al 2017;Rodríguez-Zárate et al 2018). In broad spatial and environmental contexts, both Isolation By Distance (IBD) and Isolation By Environmental (IBE) will act in differentiating populations.…”