Shedding armour helped stickleback evolve

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Shedding excess armour may have helped fish to make the transition from marine to freshwater environments, a new study suggests.

Researchers from the University of British Columbia have identified a mutant gene in sticklebacks that prohibits the growth of their bony lateral plates.

It is believed that the presence of this mutated gene may have aided sticklebacks to adapt to life in freshwater habitats following the last ice age.

The study, to be published in the journal Science, found that the mutant gene was present in less than one percent of marine sticklebacks, but was very common in their freshwater counterparts.

Authors Sean Rogers (left) and Rowan Barrett (right) seining the experimental ponds to sample.

Researchers suggest that this mutation led to an increased growth rate in the sticklebacks, allowing them to breed earlier in the season, and consequently adapt better to their new freshwater environment.

If the fish aren t expending resources growing bones " which may be significantly more difficult in freshwater due to its lack of ions " they can devote more energy to increasing biomass, said author and PhD candidate Rowan Barrett. This in turn allows them to breed earlier and improves over-winter survival rate.

The researchers transferred 200 marine sticklebacks carrying the rare mutated gene into experimental freshwater pools.

Completely armoured (top) and low armoured (bottom) sticklebacks, sampled from the experimental ponds.

We found a significant increase in the frequency of this allele in their offspring, evidence that natural selection favours reduced armour in freshwater, said Barrett.

This study provides further evidence for Darwin s theory of natural selection by showing that environmental conditions can directly impact genes controlling physical traits that affect the survival of species.

For more information, see the paper: Barrett, R.D.H., Rogers, S.M. and Schluter, D. 2008. ~Natural Selection on a Major Armor Gene in Threespine Stickleback. Science, August 28, 2008 (10.1126/science.1159978).