Evolution by co-option

posted by S. Zachary Swartz / on August 20th, 2009 / in Development

Onthophagus taurus

In the course of evolution organisms sometimes acquire completely new and sometimes dramatic features, like horns or new appendages.  The evolutionary origins of new structures are much more difficult to study than modifications to existing ones.  One approach, however, is to study the development of newly arisen structures in as many different species as possible.  The genome does not code for a body plan directly; rather, it encodes genes that coordinate the process of development.  Development is a series of events that pattern a fertilized egg into a multicellular organism. The timing and spatial organization of gene function in an embryo is therefore central to creating body structure.  Again and again it has been seen that new structures don’t necessarily mean new genes; the development of many new structures is controlled by previously existing genes that have been deployed in new contexts. The use of existing genes for a new purpose is called co-option.

A recent study by Moczek et al. provides fresh detail on the development of new structures in a particularly interesting group of animals. Many species of beetle possess rather gaudy horns on their heads and thoraxes. Horns are not modified mouthparts or limbs; they exist in addition to a full set of these other structures.  Certain limb genes, though, are turned on in the horns.  These genes, distal-less, dachshund, and homothorax, play central roles in the limb development of other insect species. When the authors disrupted the function of these genes in beetle larvae, the animals grew abnormally short horns and limbs.  Their experiment indicates their dual functions in beetles: an ancestral function for making legs, and the more recently evolved functions in making horns.  You therefore might think of these genes simply as tools for making something that sticks out, be it a limb or horn.

Genetic co-option is not limited to beetles, but by studying creatures like these we can develop a more general picture of how body structure evolves. These studies have made it clear that, just as your brain doesn’t have a neuron that is specific to your grandma, there aren’t new genes that are specific to each new structure.

The photo above, by Alex Wild, is of two Onthophagus taurus males.

Glowing worms in Bermuda

posted by Stefan Siebert / on August 19th, 2009 / in Annelids

Odontosyllis phosphorea

Reproduction is a complex business, and often requires that the partners meet. Polychaete worms belonging to Odontosyllis have developed a highly elaborate mating behavior that includes bioluminescent signals. During a recent stopover on the Bermuda islands, on a sailing trip across the Atlantic, I was able to witness the fascinating mating dance of Odontosyllis enopla. The species normally spends its life in shallow water on rocky or sandy bottoms. Once a month, 2-5 days after full moon and around 55 min after the astronomical sunset, the animals start ascending to the sea surface. Here the circling female tries to attract a male by emitting green light and the repeated release of green glowing clouds. The male signals its presence via bioluminescent flashes. In the course of this dance – which may last from 10 to 30 min – the animals spawn and the sea turns black again as they go dark. Remarkably, the worms undergo severe modifications of their body and behavior when switching from the bottom dwelling mode of living to the free floating form. In the case of the males this means, amongst other things, a considerable enlargement of the eyes. After spawning the worms return to the bottom and can potentially swarm again. The photo above, by Greg Rouse, is of the Californian species Odontosyllis phosphorea in its benthic phase. Data from Dimitri Deheyna and Michael Latz suggest the involvement of a photoprotein in the bioluminiscence of this species.

kahikai

posted by Casey Dunn / on August 16th, 2009 / in Jellies, Science & Art

scypho

Eric Roettinger and Mattias Ormestad have launched kahikai.org to showcase some of their beautiful animal photographs. Both are postdocs in Mark Martindale’s lab at the University of Hawaii, where I also spent a couple years. In addition to presenting their photos, kahikai (which means “one ocean” in Hawaiian) will be serving as a repository for primary developmental biology data, such as in situ hybridization images. Eric also curates a set of photos he has taken of other subjects at livingonabeach.org.

CreatureCast – Squid Iridescence

posted by Casey Dunn / on August 12th, 2009 / in molluscs, Podcast, Science & Art

We are pleased to present Episode 1 of CreatureCast, by Sophia Tintori. In this first video, Alison Sweeney talks about work that has been done in the Morse lab on Squid iridescence. Audio production and animations are by Sophia, who normally studies siphonophores in the Dunn lab. Music by  Lucky Dragons (here, and slowed down versions of this and this) and Sophia on the musical saw.

Creative Commons License
This video is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 3.0 United States License.

(Episode 1 was replaced with a new slightly different cut on August 18, 2009. It is now higher resolution and includes a couple different musical tracks.)