Earthworms show evolution in action

28 June 2010 by Adele Walker

Earthworms are clever creatures. Any gardener will tell you they are vital for the health of our soils. But scientists can also tell you they have evolved to live with pollution levels that would give you or me a serious headache.

Earthworm Lumbricus rubellus

Lumbricus rubellus

Earthworm populations in polluted soils can tolerate 'phenomenally high internal body loads' of certain metal contaminants, like lead and zinc. In fact they thrive in levels of contamination that would eventually kill off earthworms that are used to clean soils.

What scientists didn't know for sure was whether the worms had actually evolved in response to this environmental stress, or whether they had learned to 'switch on' different genes that would help them to survive.

In a new study, published in Soil Biology and Biochemistry, researchers studied the worms' genes as well as their coping strategies.

They took samples of the earthworm Lumbricus rubellus from two disused lead and zinc-mine sites in Wales, together with samples of a 'control' population of worms from clean soil. The worms from the mine sites were kept in their own soils, and the control worms were exposed to the contaminated soils.

After 10 weeks the researchers used special chemical techniques to determine where the metals were occurring within the worms' cells.

It's a great example of evolution in action.

- Professor Mark Hodson, Reading University

They found distinct differences in the abilities of the two populations to deal with the contamination. Not only did the worms native to the mine sites accumulate more metal in their tissues, but they were able to convert a much higher proportion into an insoluble form which is no longer toxic.

They do this by combining the metals with phosphorus (and, for zinc, with sulfur as well) in special 'compartments' in their cells. This locks the metals into tiny pellets that come out in the worms' poo.

Next came the genetics. "The romantic days of determining species from bristles and segments are long gone," says Professor Mark Hodson of Reading University. "These days it's all about numbers." The analysis indeed revealed the worms came from two distinct genetic lineages, with the metal-adapted ones almost exclusively belonging to a different group from the control sample. The differences were distinct - a 13 per cent variation - which was enough to tell the scientists they were looking at two different species.

All this suggests that we're looking at evolution in action. When the worms first encountered the contaminated soils most would have died, but those that could tolerate the harsh conditions would have had less competition and their particular qualities would have been favoured through natural selection over successive generations. The result? A new species.

"Though there's no definitive link between their genes and their ability to partition the metals,' says Hodson, 'it looks like the proteins are modified in the adapted population to bind more effectively with the lead. It's a great example of evolution in action."

The discovery of these distinct genetic types is important because worms are important biomarkers - indicators of the health of an ecosystem. But unless you are sure what species you're looking at you can't be sure how it's going to behave. "Comparing the health of Lumbricus rubellus between clean and polluted soils could be like comparing apples and pears," says Hodson.

The research was carried out as part of Jane Andre's NERC-funded PhD work with Mark Hodson together with Professor John Morgan and Dr Pete Kile of Cardiff University and Dr Stephen Stürzenbaum of King's College London.

'Metal bioaccumulation and cellular fractionation in an epigeic earthworm (Lumbricus rubellus): The interactive influences of population exposure histories, site-specific geochemistry and mitochondrial genotype' - J Andre et al. Soil Biology & Biochemistry 2010, doi:10.1016/j.soilbio.2010.05.029