Jamniczky, H. | bioRxiv Science

bioRxiv2026

Disentangling shape and size in a population of unusually large Threespine Stickleback (Gasterosteus aculeatus) from Vancouver Island, British Columbia

Sarita Lake, British Columbia houses a distinctive population of threespine stickleback (Gastrosteus aculeatus L.) with a phenotype characterized by unusually large individuals relative to nearby conspecifics. We tested the hypothesis that members of this population are not isometrically larger but rather exhibit variation in allometric trajectories that reflect changes in developmental timing impacting the developmental-genetic architecture of the phenotype. We used 3D geometric morphometrics to characterize the size and shape of skulls, pectoral girdles and pelvic girdles from a sample of individuals from nearby freshwater and marine populations and compare them to a sample from Sarita Lake. We showed that individuals from the Sarita Lake population are larger in each body region compared to most other populations examined. Further, these individuals have dorsally expanded skulls and relatively robust pelvic armour. We also showed that the relationship between size and shape is differently structured among body regions and is heavily influenced by non-uniform sexually-mediated variation across populations sampled. Our results reflect complex underlying developmental trajectories, and we suggest that the large phenotype observed may be driven by fecundity selection on female size in combination with a limnetic trophic niche and relatively increased predation pressure in Sarita Lake.

evolutionary biology↗

bioRxiv2025

Ontogeny of escape response and body shape in Threespine Stickleback (Gasterosteus aculeatus L.)

Escape responses in fishes provide insight into accelerative motions and behavioral response times of these animals, linking numerous fitness-related traits. We sought to connect escape response performance to genotype and phenotype across ontogenetic stages within a single population of Threespine Stickleback (Gasterosteus aculeatus L.), to determine if a gene of major effect (Ectodysplasin; Eda) on adaptive lateral armour plate phenotype and fitness conveys a performance advantage in brackish and saltwater environments. We predicted that possession of one or more low-plated alleles would result in better escape performance in brackish environments across ontogeny, and that Eda genotype would result in variation in shape phenotype that would be predictive of escape performance before the completion of lateral plate development. We assessed three kinematic variables derived from C-start escape responses in wild-caught, F1 juvenile, and F1 adults, and found that Eda genotypes did not directly impact maximum velocity or curvature coefficient, but linked phenotypes contributed to an increase in total distance traveled in F1 adults. We further showed that Eda genotype impacted growth, producing variation in size which in turn influenced escape performance variables across age groups, with a particular focus on total distance traveled. The influence of Eda genotype on shape was confounded by sexual dimorphism, indicating that further sex-specific studies on shape and performance are needed. The presence of at least one low-plated allele conferred a growth advantage, particularly in a freshwater rearing environment, consistent with known pleiotropic effects of Eda, including on skeletal development, that may be modulating anti-predator responses before plate phenotype is fully expressed. Altogether, more ontogenic studies are needed to ensure the influence of development on adaptive traits is more clearly understood.

evolutionary biology↗