This week's paper is in Current Biology, lead authored by Ralph Peters who is a museum curator at the Zoologisches Forschungsmuseum Alexander Koenig in Bonn, Germany. The Hymenoptera is an amazing group. The diversity in diet alone is staggering. Hymenoptera includes sawflies that eat leaves, predacious wasps, parasitoid wasps that lay their eggs in caterpillars, ants that farm fungus gardens, and bees that forage on nectar and pollen. Additionally, the Hymenoptera includes insects that vary from solitary to eusocial (one queen and lots of workers like a honeybee). Not to mention that while many Hymenoptera do not sting at all, some have venomous stings so toxic that they can debilitate a human. This variation makes the Hymenoptera an amazing group for studying evolutionary transitions from one foraging type to another, or from solitary to social lifestyles. The paper reports the most comprehensive phylogeny ( = evolutionary tree) of the Hymenoptera to date.
What they did
This phylogeny is so comprehensive because of the amount of DNA sequencing that they did. They sequenced the entire transcriptome of 167 species of Hymenoptera. Early in evolutionary biology scientists made phylogenetic trees from morphological characters (how many teeth and how long they are are, etc). Species were then clustered based on how similar they were in these different traits because if they are more similar they are more likely to be closely related. With the advent of DNA sequencing scientists began doing the same thing but with gene sequence similarity. But as with morphological characters, the phylogeny is better the more different genes you include (like measuring teeth, and ears, and foot bones, etc). Most current DNA phylogenies are therefore constructed using multiple different genes, each of which evolves at different rates and in different groups. By sequencing the entire transcriptome this paper sequenced every gene that is transcribed (will be made into a protein), a technique that is increasingly being used to construct phylogenies.
What they found
The Hymenoptera began to diversity 281 million years ago (for reference the first mammals were around 224 million years ago, so this is an old, old, group). The Hymenopterans first began when a leaf-eating insect diverged from a predacious insect. This herbivorous diet may have opened up new niches for Hymenoptera, allowing them to greatly diversify. Next Hymenoptera evolved parasitoidism in which females lay their eggs inside the bodies of caterpillars, and their larvae eat the caterpillar from the inside-out. The next big evolutionary step was the evolution of the distinctive "wasp waist" body shape that we associate with most wasps. The narrow waist allows insects more maneuverability of their abdomen independent from their thorax. It may have given parasitoid wasps more maneuverability when laying their eggs in caterpillars. The next transition was the evolution of a stinger. The stinger may have evolved in parasitoids because it allowed them to sting and immobilize the caterpillars that would be prey for their offspring. The stinger may have also enabled the evolution and diversification of many predacious Hymenoptera that could now attack and disable prey that they could not have otherwise subdued. Once the stinger had evolved, eusociality evolved repeatedly in the Hymenoptera. I wonder if the evolution of a stinger as a defense mechanism enabled the group living of large numbers of individuals which might otherwise be susceptible to attack from predators. The evolution of collecting pollen as a source of protein then led to further diversification, especially in the bees. The authors state that "the switch from a predatory to a herbivorous lifestyle was a key to the tremendous diversification of bees".
The take away
This is certainly not the first phylogeny of Hymenoptera, but it is the most extensive. Phylogenies of this group are so important (and interesting!) because it contains so much amazing biological diversity and innovation. The evolution of such a range of foraging strategies, venom, and complex sociality makes the Hymenoptera particularly unique. Having a good phylogeny allows us to speculate and develop hypotheses about the causes of major evolutionary transitions that can be tested empirically.