Marnix Everaert

posted by Erwin Keustermans / on October 15th, 2009 / in Science & Art
Ciliata_gr

Many people are familiar with the dazzling plates of Haeckel’s “Kunstformen der Natur” (http://en.wikipedia.org/wiki/Kunstformen_der_Natur). Haeckel set a standard for further similar undertakings, and at the same time stood firmly in a long tradition of documenting the abundance of strange creatures in the natural world. From a spectator’s point of view, it was and still is not easy to know or to see which creatures are real and which are imaginary, for the layman to decide which details are observed and which are made up from stereotypes, preconceptions or simply for reasons of symmetry or convenience.

Marnix Everaert’s (http://www.marnix-everaert.be) is a Belgian artist, a European expert on non-toxic printing techniques. His drawings remind the viewer of Haeckels pages.  Of course this is not the same encyclopaedic undertaking. There are differences of composition, for instance Everaert’s creatures are sometimes drawn on a common backdrop in a way that suggests that they share an imaginary space, while Haeckel’s items are often laid out on an empty page. Obviously Everaert is a contemporary artist and his style is looser than the standards that were set for 19th century illustrations, scientific or otherwise. Also, there seems to be more attention to structure than to detail, as if Everaert is taking elements from a repertory of geometric shapes that together constitute a generic type of creature. But for the viewer the question can be raised again: without more investigation or sound prior knowledge it is not possible to know what is real and what is imagined.

CreatureCast – Multicellularity

posted by Casey Dunn / on October 14th, 2009 / in Podcast

In Episode 2 of CreatureCast, Sophia Tintori and Cassandra Extavour talk about the evolution and development of multicellular organisms, and in particular the specialization of reproductive cells. Audio production and animations are by Sophia, who normally studies siphonophores in the Dunn lab. Music by Cryptacize.

With Episode 2, we are also happy to announce that you can now subscribe to the CreatureCast video podcast through Brown University at  iTunes U.

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

A tale of two holes

posted by S. Zachary Swartz / on October 9th, 2009 / in Development

two_holes

I recently attended a meeting between the Dunn and Wessel labs about the evolution of the mouth and anus. A new paper by Mark Martindale and Andreas Hejnol offers a hypothesis on the origin of these very important holes. Many animals, including jellyfish and their relatives (e.g. sea anemones and Hydra), have a single opening to their gut, so they eat food and release waste from the same opening. Most bilaterian animals (e.g. humans, fish, snails, and so on), which diverged from jellyfish a long time ago in the course of evolution, have two openings. These two holes create a through gut: a tube that takes in food at one end (the mouth) and releases waste at the other end (the anus). This raises a couple straightforward questions. Some animals have one hole, others have two—how did this happen? Does the single hole in one-holed animals correspond to the mouth or anus of animals with two holes?

There are a few hypotheses. Most invoke a hypothetical ancestor called the “Gastrea” which was postulated to have a single opening to the gut at the tail end of the animal and a sensory organ on the head end. This hypothesis relies largely on observations of jellyfish embryos. A single hole forms in the embryo, which then grows into a swimming larva. The “head” and “tail” ends were assumed to correspond to the swimming direction of the larva. There is a sensory organ is on the leading end, which was interpreted as the “head”, and a single orifice on the the trailing end, which was interpreted as the “tail”. This single hole was ascribed to be the anus since it was on the trailing end.  This hypothesis therefore implies that the one hole in one-holed animals corresponds to the anus in two holed animals.

Molecular analysis, however, suggests otherwise. All animals start out in development with one hole, the blastopore. If there are two holes, the second hole forms later. The blastopore can arise at the top or the bottom of the embryo. In the jellyfish and their relatives the blastopore forms at the top of the embryo and becomes the dual-functioning hole of the adult. Blastopore formation is started by a protein called disheveled, which gets stuck at the top of the egg and then activates a specific set of genes. In the same location of jellyfish embryos, however, there are genes strikingly similar to the mouth genes of bilaterians. In the sea urchin, a bilaterian, these same mouth genes are also on the top of the embryo. However, disheveled has moved to the bottom. The blastopore forms at this new site of disheveled accumulation, rather than at the mouth. The mouth genes that remain on top still direct the formation of the mouth there. Martindale and Hejnol posit that moving disheveled from the top to the bottom of the embryo in some animals moved the location of blastopore, but that the mouth stayed put. In some bilaterians, like urchins and humans, the blastopore then became the anus. In this scenario all mouths are homologous to each other, whether the animal has one or two holes. The site of gastrulation, however, can move from the mouth to the anus and therefore can’t be used to identify which hole is which as it had been in the Gastrea hypothesis. It also indicates that jellyfish larva swim backwards, with their mouth on the trailing end.

By changing the location where a genetic program is activated, a huge and incredibly important change in body plan occurred. The same sets of genes appear in many different contexts within and across species. But the relationships between those genes are often consistent, as a sort of molecular module. In this case there are two distinct modules for mouth and blastopore, and they can be decoupled. Learning that genes evolve in modules was somewhat of an epiphany for me as a new student. Even if one does not care about the evolution of the mouth and anus, this story demonstrate the power of comparative evolutionary biology. Using model organisms (like fruit flies, mice, yeast, etc) to understand human biology is commonplace, but the study of evolution across a broader diversity of species can give us far more detail about what specific changes occurred to create the differences we see.

Photos by Casey Dunn. The sea anemone Nematostella vectensis, a cnidarian that has a single hole for eating, excreting, and shedding eggs and sperm, is on the left. This opening is at the top of the photo, between the tentacles. The annelid worm Buskiella vitjasi, whose through-gut can be seen through its transparent body, is on the right. Like other bilaterians, one end of its gut terminates in a mouth (at the top of the photo) and the other at the anus (at the bottom).

Fall in Rhode Island

posted by Casey Dunn / on September 30th, 2009 / in Arthropods, Parasites

Manduca

The leaves are starting to turn and the garden is getting thin as most fruits and vegetables are harvested. There are some fun surprises among the plants that remain, including this tobacco hornworm (Manduca sexta) above that was chewing on our tomatoes. It stayed in one spot, and over the course of two days more and more parasitoid wasp larvae, probably Cotesia congregata, emerged to spin their cocoons. When the mother wasp injected her eggs into the young caterpillar, she also injected a virus that had been multiplying in her ovaries. This virus continued to reproduce in its new host, castrating the caterpillar and preventing it from metamorphosing. This trick provides the the perfect feeding ground for the wasp babies.

Other organisms are also at their peak, and the woods are full of beautiful and delicious fungi. The specimen below is Laetiporus, also known as chicken of the woods because it is so common and quite eatable.

Chicken

Photos by Casey Dunn. Thanks to Doug Morse, Alan Bergland, and Erika Edwards.

Glowing worms in the deep sea

posted by Orla O'Brien / on September 14th, 2009 / in Annelids

Swima

Bioluminescence can be used for myriad purposes in different species—this recently discovered species of annelid, Swima bombaviridis, probably uses bioluminescence to escape from predators. It was described by Karen Osborn and friends. The worm carries eight fluid-filled packets near its head that it can release at will. When these packets are released, they bioluminesce a bright green for several seconds. Since the worms live in the deep sea, these flashes are a contrast to the dark environment and may distract predators—instead of getting a bite of worm, they are left with nothing. The mechanism for releasing these bioluminescent bombs is unclear—in addition to the lack of light at the depths the worms live at they are without eyes—but the release is probably related to a tactile sensory system, as they release their bioluminescent organs when touched.

Photo by Casey Dunn. The head is to the left, and the green bioluminescent packets can be seen attached to the body just behind it.

Hiding submarines beneath jellyfish

posted by Casey Dunn / on September 9th, 2009 / in Jellies, Siphonophores

Nanomia

With the advent of submarine warfare, the ability to locate large underwater objects with SONAR became of prime strategic importance. Active SONAR detects objects by listening for echos from pulses of sound. As SONAR became more widely used, though, some very strange things were seen in the open ocean. At times, the SONAR suggested that the ocean floor was much shallower than maps and direct depth measurements indicated. Ships sitting in one place would also find that the depth of the ocean would appear to change through the course of the day, as if the sea floor were heaving beneath them.

Something was creating a false bottom that the SONAR couldn’t see through. Submarines found that they could dive right through this layer, hiding beneath it and rendering the SONAR above useless. Details about these false bottoms in the open ocean were closely guarded military secrets during World War II.

It had been suspected that the false bottom was made of large groups of animals, but nets sent to this region usually came up empty. Then, in 1963, Eric Barham, a scientist at the US Navy Electronics Laboratory, reported his first-hand observations form aboard the research submarine Trieste. His dives were coordinated with ships above that monitored the position of the false bottom with SONAR. When Trieste arrived at the false bottom it did find animals, and lots of them. They were siphonophores, extremely fragile colonial jellyfish that are notoriously difficult to collect. They are so fragile that they usually turn to slime in nets and pass right through the mesh.

How could something so delicate and gelatinous have such a strong signature on the SONAR, powerful enough to hide entire submarines? Many species of siphonophores have a gas filled float that serves to regulate buoyancy, and possibly to sense which way is up. The siphonophore that was found in the false bottom, Nanomia bijuga (see photo above), has a float that is about a milimeter in diamater, which is predicted to resonate at a frequency very close to the sound pulse used by SONAR. This resonance scatters the sound, and when there are lots of siphonophores the scattering is so thorough that the SONAR can’t penetrate through the swimming jellyfish.

Besides revealing the important impacts of a poorly-known colonial jellyfish on military technology, these findings also indicate how difficult it can be to measure the abundance of jellyfish. They weren’t detected in nets sent to the false bottom, but there were enough of them to hide entire warships. This measurement problem is compounded when we try to establish whether jellyfish are rising or falling in abundance through time. Because they are so difficult to observe, their abundance has likely been dramatically underestimated in the historical record. As we see more jellyfish with improved sampling methods, it is hard to know if they are more numerous than they used to be.

The Nanomia bijuga photo above was taken by Claude Carré at Villefranche. The float is at the upper right of the image. More information on siphonophores can be found at siphonophores.org.

Retractable spots

posted by Casey Dunn / on September 7th, 2009 / in molluscs

cyphoma

The marine snail Cyphoma gibbosum browses on the polyps of soft corals (top). It appears to have a brightly spotted shell, but when disturbed the spots begin to move (middle) and then retract within the white shell, along with the rest of the animal (bottom). This is possible because the spots are not part of the shell at all. They are patterns on the thin mantle tissue that extends out of the shell opening and up around the snail’s back. Photos by Casey Dunn.

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.