European ecologists uncover biological arms race amongst the willows
One of the key challenges facing biological research is to understand how complex food webs are assembled and maintained. European researchers have recently investigated how sawflies have evolved to escape their parasitoids (i.e. parasites that lead to the host’s death) and achieve for themselves an ‘enemy-free space for millions of years. This fresh look at the evolutionary relationship between plant-dwelling insects and parasites appears in the online open access journal BMC Biology.
|Lathrostizus lugens, a sawfly|
Tommi Nyman from the University of Jensu in Finland, together with fellow scientists from Sweden and Germany, have revealed a complex food web involving willow trees, sawflies and the parasitoids preying on sawfly larvae. Sawflies are a group of mainly plant-eating insects that belong to the order Hymenoptera, which also includes wasps, bees and ants.
Adult sawflies eat little, while their larvae feed on the foliage of trees and shrubs, especially willow, and create galls, which are protruding growths of plant tissue. The researchers used data from galls collected from willows over an 18-year period, classifying 43 sawfly species using genetic analysis and assessing the degree of parasitism to which these insects had been subjected by 72 different parasites.
Statistical analysis showed that parasitism encourages the insects’ ecological divergence in a co-evolutionary arms race. Ecologists believe that the sawflies develop new types of gall, which allows them to escape the attentions of the parasitoids without having to switch to a new host plant. However, some parasitoids have in turn adapted to the divergence in gall-types, which further drives the evolution of new species. Plants, plant-feeding insects and their parasitoids form some of the most complex- and species-rich food webs and these processes may go some way to explaining the unusual diversity found within them.
The study is the first time key co-evolutionary hypotheses, such as the classic Escape-And-Radiate (EAR) hypothesis, have been tested in a framework relating to the evolutionary development of an organism. EAR suggests that extremely diverse communities result from co-evolutionary arms races involving cycles of escape from an enemy, spreading out and then colonisation by new enemy lineages. These results demonstrate that the EAR hypothesis is too simplistic to be applied to highly complex species-rich food webs.
'Our results indicate that niche-dependent parasitism is a major force promoting ecological divergence in herbivorous insects,' says Nyman. 'Diversification seems to be spurred by a continuous stepwise process, in which ecological and phenotypic shifts in prey lineages are followed by a lagged evolutionary response by some of the associated enemies.'
Reciprocal diversification in a complex plant–herbivore–parasitoid food web
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