Demonstrating Adaptive Evolution in Parasites

Host defense triggers rapid adaptive radiation in experimentally evolving parasites (2019) Bush et al., Evolution Letters, p. 1-9

The Crux

Adaptive radiation is a fascinating ecological concept, one with which anyone who knows the tale of Darwin’s finches will be familiar with. The basic premise is that an organism may evolve different forms (and ultimately become different species) in response to pressures exerted upon them.

But whilst this may have been observed in many vertebrates, it’s often overlooked in parasites, whereby host defenses can prompt divergence in parasite morphology. Today’s paper wanted to test the two basic concepts of evolution. 1) Can host defenses prompt physical changes in parasites? 2) Are these changes heritable?

What They Did

The researchers used feather lice, a parasite that (in this study) attaches itself to pigeon feathers, which the pigeons are thought to fend off through preening. Whilst it has been previously theorised that lice whose colour matches the pigeon’s feathers (ie. that are ‘cryptic’), this hasn’t been shown before. So the researchers needed to establish 2 things.

1. Preening favours cryptic lice

The researchers used white, grey and black varieties of rock pigeons (Columba livia), which were isolated over a four year period, roughly equivalent to 60 generations of lice. In that time, lice were sampled every six months and tested for ‘luminosity’ (lighter-coloured lice had high luminosity, darker-coloured lice had lower luminosity).

To ensure preening was the mechanism responsible, there was a shorter experiment at the start which placed lice either painted white or black on either white or black pigeons, half of which were rendered incapable of preening. After 48 hours, they tested for numbers of both types of lice on the pigeons.

2. Crypsis is heritable

For a trait to lead to evolution, it has to be heritable. After two years, the relative luminosity of lice offspring was compared to that of their parents, to ensure the trait was being passed on.

Did You Know: Parasite Defense

Parasites engage in a variety of strategies to effectively parasitize their hosts. This study focused on a visual defense of the parasite (crypsis), but other feather lice utilize behavioral adaptations to escape host preening. Some lice species will selectively attach to bird wings¬†under the feather. Because birds preen in the same direction that the feathers grow, the bird’s beak simply presses down on top of the feather, resulting in slight pressure on the louse, but it is not dislodged from the wing itself.

19178013139_d375ab2dd5_o.jpg

Feather lice like the one seen here can be a major problem for birds, and are generally cleansed through preening

What They Found

The shorter experiment showed that lice which did not match the colour of the pigeons’ feathers were found in much lower numbers on pigeons which could preen, proving that preening favours cryptic lice. Over a four year period, lice which lived on white varieties of the pigeon increased dramatically in luminosity, whilst those on black varieties decreased in luminosity, though not as markedly. Offspring colour on lice closely matched their parents’ colour, showing that colour is a heritable trait.

Problems?

This study is quite elegant, and I think the only problem it really suffers from is that it is very specific – one species of parasite on one host. More studies could build on the example given here to show a larger trend, including parasites which use other defense mechanisms, like those mentioned in the Did You Know section above.

So What?

Understanding the mechanisms behind adaptive radiation and speciation is crucial in judging how species may respond to fluctuations in their immediate environment. Parasites play a huge role in our lives, and being able to predict their adaptive responses to defense mechanisms can play a huge role in not only countering their potential negative effects, but conserving them where we need to.

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