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Age at maturity in Atlantic salmon: molecular and ecological dissection of an adaptive trait

Periodic Reporting for period 2 - MATURATION (Age at maturity in Atlantic salmon: molecular and ecological dissection of an adaptive trait)

Reporting period: 2019-03-01 to 2020-08-31

In biology, life-history research is a field of study focused on understanding the causes and consequences of variation in reproduction and survival, i.e. fitness. As in many species, age at maturity in Atlantic salmon is tightly linked with size at maturity and thus represents a classic evolutionary trade-off: later maturing individuals spend more time at sea before returning to freshwater to spawn and have higher reproductive success due to their larger size but also have a higher risk of dying prior to first reproduction as they delay their first breeding attempt until a later age. My earlier research discovered a gene with a large effect on this key life history trait. A single gene having such a large effect on a life-history trait is unusual, and this fact, combined with the features of Atlantic salmon as a model system offer an ideal opportunity to better understand the molecular mechanisms and ecological drivers underlying a locally adapted life history trait. In MATURATION I am i) characterizing age at maturity candidate gene functions and allelic effects on phenotypes (WP1) ii) elucidating fitness effects of these phenotypes and the combined effects of genetic and environmental factors (WP3) iii) developing a mechanistic model for the sex-dependent dominance and validate intra-locus sexual conflict resolution (WP3) and assessing the evolutionary conservation of the genetic architecture (WP4). Better understanding the molecular mechanisms and ecological consequences of life-history variation also has a broad range of practical applications, from being a key tool for estimating maximum sustainable yield in commercial fisheries, to explaining why girls are reaching puberty at an increasingly early age.
Overall, progress in the project has been very good: my transition to the University of Helsinki prior to the start of the project gave access to excellent common garden facilities that have been particularly important for the good progress in WP1 and WP2 and parts of WP3. A key achievement has been the establishment of a common garden system for studying salmon maturation within one year, including temperature and feeding treatments. Although this took considerable effort for optimisation, it is now functioning very well and is a key resource for the project.

WP1. Functional characterization of age at maturity candidate genes is proceeding ahead of schedule. We have collected a large developmental time series (samples collected routinely at different developmental time points) of tissues in fish with different vgll3 genotypes. We have submitted the fist study of vgll3 gene expression across the first year of development (up until male maturation) and show that there is differential expression in individuals with different vgll3 genotypes, and that vgll3 expression is lower in maturing males. A study reporting these first results is currently under revision for re-submission (pre-print of the submitted version available here: https://www.biorxiv.org/content/10.1101/777300v1 ). We have also identified correlated gene expression patterns between vgll3 and other genes known to be important in pubertal timing and adiposity, which provides indicators of which other genes are worth of study. This work has been published in Kurko et al. 2020. Similarly, Studies of the effect of genotype on fitness-related traits is also proceeding ahead of schedule. A key finding from our first large-scale common garden study is to not only validate that age at maturity is strongly linked with vgll3 genotype in two populations, but that condition factor (fatness) in late summer is a strong predictor of male maturation probability in the winter. This further strengthens the notion that adiposity level may be an important factor in the maturation process, and that vgll3 may contribute to this. These results are under revision for re-submission and are also reported in the following pre-print: https://www.biorxiv.org/content/10.1101/780437v3

WP2. For better understanind reaction norms related to large-effect maturation genes, we have conducted both temperature and food restriction treatments and found they temperature has a large effect on maturation rate, there is no interaction with the effect of vgll3 (it affects maturation in a similar manner in both temperatures tested (https://www.biorxiv.org/content/10.1101/780437v3). Ongoing experiments are also testing a less stable temperature regime, combined with a low-fat food treatment that is ongoing. Studies of historical reaction norms was completed on schedule. We have identified a strong link between the decline of the large, late maturing phenotype in a wild population, and vgll3 genotype, strongly suggesting that the population has undergone adaptive evolution for increased frequency of early maturation (Czorlich et al. 2019).

WP3. Empirical studies of sexual conflict resolution in a wild population are proceeding as scheduled. We have now collected 10 cohorts of data, and have published several studies outlining the basics or reproductive fitness in the population (Mobley et al. 2019, 2020). At the same time, molecular analyses have characterised the vgll3 genotypes of all individuals, and we are currently analysing the data for publication. Work Identifying interaction partners and molecular mechanisms of dominance in VGLL3 is ongoing, and on schedule with one post-doc focusing on the laboratory work currently.

WP4. Studying the evolutionary conservation of genetic architecture is slightly behind schedule, but we have an manuscript identifying loci linked with age at maturity in 4 Pacific salmon species is expected to be ready for submission soon.
As originally envisioned, I expect to successfully report a detailed dissection of the molecular mechanisms and ecological consequences of the age at maturity trait in Atlantic salmon. This will be one of the most detailed accounts of the molecular basis of a life-history trait ever conducted in a non-model organism, and when also linked with the ecology and evolution of the species will be of high significance in the fields of ecology and evolutionary biology. Better understanding the molecular mechanisms and ecological consequences of life-history variation also has a broad range of practical applications, from being a key tool for estimating maximum sustainable yield in commercial fisheries, to explaining why girls are reaching puberty at an increasingly early age.