Physiology and morphology of clonal Atlantic salmon-influence of incubation temperature, ploidy, and zygosity

Abstract

Isogenic (clonal) fish lines are useful experimental models to study effects of environment versus genetics on phenotypic traits, as they can be maintained for generations without change, providing advantages over outbred groups prone to generational change and higher variation. Here we performed experiments on isogenic Atlantic salmon groups that were either heterozygous diploid, homozygous diploid, triploid, or heterozygous diploid incubated at 4 °C instead of 8 °C. We measured metabolic rates, stress response, and hypoxia tolerance to assess whole-animal performance traits. Then we measured the morphology of hearts and otoliths since both are known to be influenced by environmental history. Isogenic, ploidy, and zygosity statuses were confirmed from microsatellite markers. Embryonic development is affected by temperature, hence the 4 °C incubation group was tested 9 months later when it had reached an equivalent size as the other groups. Curiously, a bimodal size distribution emerged in this group. Physiological traits were similar between groups apart from higher standard metabolic rates in the 4 °C incubated fish. Each group had distinct heart morphologies where fish with a slower growth history resembled wild-phenotypes while homozygous fish had the most deviating hearts. Proportions of vaterite deposition in otoliths showed high individual variation and did not differ between groups. Lower coefficients of variation within groups were found when compared to outbred fish, but this was not consistent for all traits assessed. As such, substantial phenotypic variation in physiology and morphology was still observed in isogenic Atlantic salmon, which can be ascribed to random environmental factors.

Keywords: Bimodal size distribution; Isogenic fish; Phenotypic variation; Respirometry; Vaterite in otoliths; Ventricle roundness.

Fig. 1

Recorded thermal environment of the isogenic Atlantic salmon groups from incubation to experimental trials. The period with ambient freshwater followed seasonal temperatures. Water was maintained at 7 °C during respirometry trials, and for the slow growing 4 °C incubation group, fish were acclimated at 7 °C for a minimum of 2 weeks before testing. Numbers indicate degree days (dd) at first-feeding, and at the mid-point of experimental trials for the 8 °C and 4 °C incubation groups

Fig. 2

Size parameters of isogenic Atlantic salmon groups at the time of experimentation. Panels show boxplots together with individual datapoints (N = 16). Statistical differences between groups are indicated with different letters (one-way ANOVA with Tukey test, P < 0.05)

Fig. 3

Weights versus fork lengths of individuals to illustrate the bimodal size distribution in the slow growing diploid group incubated at 4 °C. Approximately one-third of the group were smaller sized. These fish were sampled in August 2022 while the diploid group incubated at 8 °C was sampled in November 2021. N = 16

Fig. 4

Metabolic rate traits of the isogenic Atlantic salmon groups. Standard metabolic rate (SMR) (A), maximum metabolic rate (MMR) (B), aerobic scope (AS) (C), and factorial AS (D). Different letters indicate statistical differences between groups (one-way ANOVA with Tukey test, P < 0.05). Figure panels show box plots together with individual data points (N = 16)

Fig. 5

Oxygen uptake rates (ṀO₂) over time in normoxia (A), and recovery time to baseline expressed as a percentage towards the standard metabolic rate (B). Data in A shows group mean ± standard error of the mean over time, and data in B are shown as boxplots together with individual data points. Statistical differences in B between groups within recovery thresholds are indicated with different letters (one-way ANOVA with Tukey test, P < 0.05) (N = 16)

Fig. 6

Response and tolerance to progressive hypoxia in the isogenic Atlantic salmon groups. Panel A shows the critical oxygen tension (P_crit) as box plots together with individual data points. Significant differences are indicated with letters (one-way ANOVA with Tukey test, P < 0.05) (N = 16). Panel B shows scatter plots of all data points colored by clonal group along with lines between mean values of MMR and P_crit

Fig. 7

Heart morphology parameters of the isogenic Atlantic salmon groups. Panels show boxplots together with individual datapoints (N = 16). H = height and W = width. Statistical differences between groups are indicated with different letters (ANOVA with Tukey test, P < 0.05)

Fig. 8

Panel A Percentage vaterite otolith deposition between isogenic groups shown as boxplots together with individual datapoints (N = 16). The treatment groups were not significantly different from each other (one-way ANOVA with Tukey test, P > 0.05). Panel B Representative photograph of an otolith showing vaterite (outer) and aragonite (central) depositions (left), and a healthy otolith only with aragonite (right)

Fig. 9

Coefficient of variation for size and metabolic rate parameters measured in the four isogenic Atlantic salmon groups (present study) and in outbred Atlantic salmon groups from other contemporary experiments using similar methods and protocols (Hvas and Bui ; Hvas et al. 2025). Statistical differences between clonal and outbred groups within a parameter is indicated with an asterisk (two-way ANOVA with Tukey test, P < 0.05). For all groups represented by a data point, N = 16