Discovery reveals earthworms more diverse than first thought


We’re interested in ground beetles. We’re
interested in them because they feed on crop pests like slugs and aphids. But to maintain
high populations of these beetles in the field, you need to find out what else they’re feeding
on, because there aren’t always any slugs or aphids for them to feed on. One of the
main prey of these beetles is earthworms. This is the field where we come to collect
beetles and earthworms for our laboratory experiments. In a field such as this we might
expect to find anywhere between 100-150 earthworms per square metre of land. So in a field of
this size we’re talking several hundred thousand individual earthworms. Earthworms are important
for soil health in general. They help with the aeration of soil. The burrows that they
form help with the drainage of soil, taking water quickly away from the surface of the
land. They also help incorporate organic matter into the soil and improve the overall fertility. You can see on this sod of earth some very
clear earthworm burrows. We have some here… and here…and some smaller ones round here.
And these will criss-cross through the soil, and help with the drainage of the soil, so
when it rains water will run quickly down these burrows and away from the surface. So I’ve just dug up this sod of earth, and
we can see straight away that we’ve got a nice, large individual here (shows worm),
and if we break it open you can see the wall of the burrow that if formed. And there we
have a nice Lumbricus terrestris. If we didn’t have earthworms, then we’d have
very poorly draining soil; soil with very low fertility; we wouldn’t get as good crop
growth, even with the use of inorganic fertilisers; and, we’d just generally have not as good
soil, as we do have in areas such as this. So this led us to try and find out which species
of earthworm the beetles were actually eating. We started to look at the DNA of earthworms
in the guts of the beetles, and what we quickly discovered was that the DNA of what we thought
were single earthworm species was in fact highly variable, and consisted of several
very distinct lineages. And I can show you some of the evidence for that…..here (points
at DNA sequences depicted on printout)….these should be all the same species of earthworm,
but, as you can see, there are two very, very distinct bands. Each earthworm either has
this band or this band, but not both. This indicated that they were very, very different
genotypes. This was the first evidence that there was something very strange going on.
We then looked in much more detail at this earthworm that I’ve just shown you on the
gel. And we collected hundreds of these things
from all over the country. We analysed their DNA, and we found that, instead of just one
species, we appeared to have five separate species. And these weren’t very close genetically.
They were further apart genetically than we are from orang-utans or gibbons, say. So they
were very, very distinct. We looked in more detail at the DNA to see if any of these different
lineages were interbreeding and we found that two of them were, so they should be considered
one species, but the other three were completely distinct. So we end up with four, non-interbreeding
distinct species where we thought there was only one originally. Finding that there were four species where
previously we thought there was only one has enormous implications in many areas. Clearly
we wish to know if these different earthworms have different ecologies. They may be at different
depths in the soil for instance. But if makes a big difference because these beetles we’re
interested in are generally running around on the surface. So they’re only going to encounter
earthworms that come up near the surface. So they may be feeding on some of these cryptic
species more than they are feeding on some of the others. And we’ve got the DNA now that
can tell us what is happening. And this is important in other areas as well.
There are lots of things that feed on earthworms. They are at the base of many food chains.
Things like blackbirds and hedgehogs and badgers; they all feed on earthworms. So we need to
know more about which of these species, new species, they may be feeding on. We need to know how many species of earthworm,
and what species, are present because of the importance of earthworms, especially with
the testing of products such as pesticides for use on farms. Now that we know that some of those key species
of earthworm are actually multiple species, we now need to repeat those experiments to
see if there is any difference between the species in terms of their resistance or susceptibility
to pesticides or other chemicals that are used in agriculture.

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