Spatially Varying Selection Amplifies Intrapopulation Differentiation Among Phenotypic Traits in the Rocky-Shore Mussel, Mytilus californianus

Abstract

Strong ecological gradients along heterogeneous environments play an important role in shaping population differentiation across species ranges. Thus, the selective pressure of environmental variation on phenotypic variation strongly affects an organism's ability to persist across diverse or new environments. We investigated the spatial variability of biological responses in the intertidal bivalve Mytilus californianus to highlight the costs and trade-offs of local adaptation and phenotypic plasticity across various functional traits in a dynamic environment, the marine intertidal. To test this, we performed a reciprocal transplant experiment with M. californianus individuals originating from the upper and lower intertidal measuring relevant phenotypic traits, followed by whole genome sequencing (WGS). We determined that morphological traits in individuals demonstrated phenotypic plasticity when moved to new environments, whereas physiological traits such as metabolism exhibited constraints in plasticity. Additionally, mussels from high intertidal zones, which experience greater heat and aerial exposure stress, maintained lower metabolic rates and showed increased frequencies of non-synonymous mutations in functionally relevant heat shock proteins when compared to low intertidal mussels. These results suggest that morphological and physiological traits responded differently to spatially varying selection within the marine intertidal.

Keywords: divergent selection; environmental gradients; local adaptation; phenotypic plasticity.

FIGURE 1

(A) Map of Strait of Juan de Fuca region. (B) Map of Tatoosh Island, Makah Indian Reservation, WA. Purple dots indicate location of Tatoosh Island (A) and site of experimental population and reciprocal transplant experiment (B).

FIGURE 2

Graphical summary of the results from a reciprocal transplant experiment between Mytilus californianus individuals indigenous to the upper (gray mussels) and lower (black mussels) intertidal. Yellow arrows denote the reciprocal transplant experiment, with arrow thickness representing the degree of plasticity. Gray shells of mussels indigenous to the high intertidal represent loss of periostracum from increased incidence of endolithic phototrophs.

FIGURE 3

Relationship between maximum shell length (mm) and (A) shell height (mm), (B) shell weight estimated as the buoyant weight (g), (C) total weight measured as weight in air (g), and (D) shell width (mm) of low and high intertidal adult (38–51 mm) mussels at the beginning of a reciprocal transplant experiment between the two locales. Shaded red and blue areas represent the 95% CI.

FIGURE 4

(A) Shell height change (mm day⁻¹), (B) shell weight change (g day⁻¹), (C) total weight change measured as weight in air (g day⁻¹), (D) shell width change (mm day⁻¹), (E) growth rate measured as change in shell length (mm day⁻¹), (F) respiration rate measured as oxygen consumption (mg O₂ g⁻¹ h⁻¹), (G) net calcification rate measured as change in buoyancy (g CaCO₃ day⁻¹), and (H) tissue weight (g) of in situ and transplanted Mytilus californianus individuals positioned in the low or high intertidal throughout the reciprocal transplant experiment. Individual data points and means ± SE are shown. Different letters beside each symbol indicate significant differences between experimental treatments evaluated using the Tukey HSD test as a post hoc comparison.

FIGURE 5

(A) Juvenile respiration rate measured as oxygen consumption (m O₂ g⁻¹ day⁻¹) and (B) net calcification measured as total change in buoyancy (g CaCO₃) of in‐situ and transplanted juvenile Mytilus californianus individuals positioned in the low or high intertidal throughout the reciprocal transplant experiment. Individual data points and means ± SE are shown. Different letters beside each symbol indicate significant differences between experimental treatments evaluated using the Tukey HSD test as a post hoc comparison.

FIGURE 6

Biplot of principal component analysis (PCA) showing changes in phenotypic traits based on treatment for adult Mytilus californianus individuals throughout a reciprocal transplant experiment.

FIGURE A1

Site and design of reciprocal transplant experiment. (A) Individual mussels marked using glued bee tags and EZ Poxy paint on umbo of left and right shell valve (not all individuals depicted). (B) Cages with vexar mesh to house individual mussels in experimental plots. (C) Site of experimental intertidal plots for in situ and transplanted mussels.

FIGURE A2

Regression lines of (A) maximum daily temperatures and (B) daily temperature ranges of low and high intertidal positions recorded in 5‐min intervals with Onset HOBO data loggers. Red and blue regression lines are from actual data using locally estimated scatterplot smoothing (LOESS) line where shaded gray areas represent the 95% CI.

FIGURE A3

Locations of sampling sites 148 km along the Strait of Juan de Fuca where 54 Mytilus californianus individuals were collected for whole genome sequencing. Locations from left to right: Tatoosh Is., WA 48.394667, −124.736233; Snow Creek Boat Launch, WA: 48.355883, −124.547683; Slip Point, WA: 48.263817, −124.251717; Freshwater Bay, WA: 48.150733, −123.637883; Port Townsend, WA: 48.14415, −122.77735.

FIGURE A4

(A) Principal component analysis (PCA) of variant frequency for 54 Mytilus californianus individuals and (B) five populations using 133,617 single nucleotide variants that had coverage in all samples and were present in at least one population at a frequency of 50% or higher. Numbers s14 (low intertidal) and s15 (high intertidal) in (A) correspond to Tatoosh Is. individuals. P14 (low intertidal) and P15 (high intertidal) in (B) correspond to the Tatoosh Is. population. (C) PCA using 19,000 single nucleotide variants that had an absolute difference of proportion larger or equal to 0.0075 between 10 low (red dots) and high (blue dots) intertidal M. californianus individuals used in a reciprocal transplant experiment.