Why Male And Female Salmon Age Differently

A group of small Atlantic salmon swimming in Tana River, northern Finland. Panu Orell/Natural Resources Institute Finland
Janet Fang 05 Nov 2015, 15:55

There’s a worrying trend among Atlantic salmon these days: They’re maturing too quickly. This impacts their size, reproductive success, and survival. But now, researchers studying 1,518 Atlantic salmon (Salmo salar) have discovered the genetic strategy that determines at what point in time the fish mature. The findings, published in Nature this week, could help us manage and protect salmon populations.

Atlantic salmon reproduce in freshwater, and their offspring head out to sea to feed before returning to their home river to spawn. The number of years spent at sea before spawning, or their “sea age,” is also their age at maturity. In one year, an individual can grow up to 65 centimeters (26 inches) and weigh three kilograms (seven pounds); but after three years, growth can be as much as 100 centimeters (39 inches) and 20 kilograms (44 pounds). The age at maturity in Atlantic salmon represents a classic evolutionary trade-off: Bigger, later-maturing fish have higher reproductive success on spawning grounds, but they also have a higher risk of dying before they even get to reproduce for the first time.

Males and females have developed different tactics for dealing with this dilemma. While males mature early and at a small size, females feed for longer and mature later. Females also have a stronger correlation between body size and reproductive success. These contrasting life histories indicate sex-specific selection patterns, though the genetic mechanisms underpinning these sex differences aren’t well understood.

To investigate, Nicola Barson from the Norwegian University of Life Sciences and colleagues looked at genetic sequences from 57 wild populations of Atlantic salmon: 54 Norwegian populations spanning Skagerrak to Barents Sea, scales from two subpopulations harvested by local fisherman in the Tana River in northern Finland, and scales from the Baltic Sea lineage collected by anglers in the Tornio River. 

The team found that variants of a key gene operate differently between the sexes. Like us, Atlantic salmon carry two versions (or alleles) of each gene, one from mom and one from dad. But for the vestigial-like family member 3 (VGLL3) gene, the dominant allele in one sex is the recessive allele in the other. This phenomenon is called sex-dependent dominance, and it causes males to mature at a different rate than females. 

VGLL3 regulates the accumulation of body fat, which helps determine the timing of maturation. This same gene has previously been implicated in human puberty.

Image in the text: A large Atlantic salmon swimming in the Tano River, northern Finland. Panu Orell/Natural Resources Institute Finland

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