New findings on the fore-leg evolution of assassin bugs

Gardena_thumbnailAssassin bugs (Reduviidae) have evolved a rich arsenal of weaponry for prey capture in their 178 million years of diversification. With about 7,000 known species worldwide, the corresponding variety of strategies to take down their next meal consist of lethal combinations of deceit and different ways to incapacitate their prey.

A team of researchers, including our museum entomologists, Hwang Wei Song and Rudolf Meier, published their latest findings on how the assassin bugs’ fore-leg evolved to the diversity we see today.

Scadra costalis_thumbnailThe fore-legs of the assassin bugs are often involved in prey capture, and have undergone remarkable modifications, presumably as an adaptation to a range of hunting techniques. Some assassin bugs possess a pair of enlarged fore-legs, frequently armed with spines or stiff bristles that aid in grasping prey. However, there are also fore-legs that look unmodified, in some cases coupled with the ability to produce or obtain sticky secretions as an alternative method to trap prey. Attempts to explain for the observed variety in leg modifications across the entire group were not formally tested until now.

In the study, specialized leg structures that are hypothesized to be involved in prey capture were tested to see if the loss of one can be explained by the replacement with another. To trace the evolution of the fore-leg structures, the phylogeny (evolutionary relationships) of assassin bugs was first reconstructed using a novel method combining transcriptomic RNA-derived data (all expressed genes of an individual) and a conventional DNA dataset (Sanger sequencing-derived). This results in the establishment of deep phylogenetic relationships that proved elusive previously.

forelegs

Fossula spongiosa (fs) in different assassin bug species.

With this latest phylogeny, a specialized leg structure called the “fossula spongiosa”, a spongy pad thought to improve the grip on prey, is shown to be most primitive and already present in the last common ancestor of all assassin bugs. This structure was then lost multiple times throughout the history of assassin bug diversification. Surprisingly, this is not necessarily replaced by other leg modifications. Our results indicate other behavioural and structural adaptations may have a stronger influence shaping the fore-legs. This finding now shifts the attention towards testing the role of other predatory adaptations such as the toxicity of the saliva injected to immobilize prey on the raptorial leg evolution of assassin bugs.

This study was funded by Singapore’s Ministry of Education AcRF Tier 1 grant, US National Science Foundation’s “Partnership in Enhancement of Expertise in Taxonomy” and Assembling Tree-of-Life grants.

 

Original paper:
Zhang, J. et al. Evolution of the assassin’s arms: insights from a phylogeny of combined transcriptomic and ribosomal DNA data (Heteroptera: Reduvioidea). Sci. Rep. 6, 22177; doi: 10.1038/srep22177 (2016)

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