Sea snails break the law

Lizards gave rise to legless snakes. Cave fishes don’t have eyeballs. In evolution, complicated structures often get lost. Dollo’s Law states that complicated structures can't be re-evolved because the genes that code for them were lost or have mutated. A group of sea snails breaks Dollo’s law, Rachel Collin, Staff Scientist at the Smithsonian Tropical Research Institute and colleagues from two Chilean universities announce in the April, 2007, Biological Bulletin.

"This is important because it shows that animals may carry the potential for evolutionary change around with them. When the environment changes, new life forms may be able to regain abilities that were lost earlier in evolutionary history," Collin explains.

Most species of sea snail go through several life stages on the way to becoming reproductive adults. The early stages, or larvae, usually live in the water column eating microscopic algae and swimming with a specialized structure called the velum. This stage has been lost in many species, where development happens in immobile capsules protected by the mother. In these species, small bottom-dwelling juvenile snails (miniature adults) hatch out of eggs and crawl away. Thus, a whole life stage, the motile larva, is lost and thought to never been re-gained.

But how can you tell what happened in the past to bring this about? Collaborators from Chile, Argentina and the Smithsonian in Panama, using embryological observations and DNA sequencing, show that the larval stage can be reacquired.

The group collected 6 species of the genus Crepipatella from the shorelines of Argentina, Chile, Panama, Peru, South Africa and the United States. They observed the developmental stages of each species and sequenced a gene called mitochondrial cytochrome oxidase I. Then, based on the differences in gene sequences, they used several different techniques to reconstruct family trees.

Indeed, they found that motile, feeding larvae had been lost and re-gained in the same family group, which breaks Dollo’s law. Collin sums this up: "The embryos of limpets in a group called Crepipatella seem to retain some of the apparatus they would need for larval feeding and swimming, even though they do not produce larvae. Then, from DNA data we see that one species with larvae has re-evolved in the middle of a group that doesn't have them. It does go both ways! There’s more flexibility in animal evolution than people thought."

Source: Smithsonian Tropical Research Institute

Related stories:

Parasite morphs ant into ripe red berry
A newly discovered parasite so dramatically transforms its host, an ant, that the ant comes to resemble a juicy red berry, ripe for picking, according to a report accepted for publication in The American Naturalist. This is the first example of fruit mimicry caused by a parasite, the co-authors say.
Beetle dung helps forests recover from fire
Armed with a pair of tweezers and a handful of beetle droppings, University of Alberta forestry graduate Tyler Cobb has discovered why the bug-sized dung is so important to areas ravaged by fire.
Chemical exchanges show wasps are bad losers
Wasps have more than just a sting in their tail according to new research published this week in the Proceedings of the Royal Society B, they also carry the insect version of pepper spray in their heads, which they can release when fighting other wasps. The research not only gives us a fascinating insight into insect behaviour but could also help us to use wasps to kill crop destroying pests.
Molecular evolution is echoed in bat ears
Echolocation may have evolved more than once in bats, according to new research from the University of Bristol published this week in Proceedings of the National Academy of Sciences (PNAS).
New research challenges long-held assumptions of flightless bird evolution
Large flightless birds of the southern continents – African ostriches, Australian emus and cassowaries, South American rheas and the New Zealand kiwi – do not share a common flightless ancestor as once believed.
Battle-scarred birds are more co-operative
Just as football fans commiserate with their friends in the pub when their team loses a match, research from the University of Bristol demonstrates that birds support one another following contests with their rivals.
'Armored' fish study helps strengthen Darwin's natural selection theory
Shedding some genetically induced excess baggage may have helped a tiny fish thrive in freshwater and outsize its marine ancestors, according to a UBC study published today in Science Express.
Exploring the function of sleep
Is sleep essential? Ask that question to a sleep-deprived new parent or a student who has just pulled an "all-nighter," and the answer will be a grouchy, "Of course!"

News discussion:

General Science news