Postdoc position in the Dunn Lab

posted by Casey Dunn / on November 6th, 2009 / in lab

Leucospermum

December 8th, 2009: The post-doc position has been filled. Thank you for your inquiries.

A joint postdoc position is open in the labs of Casey Dunn and Alexis Stamatakis as part of the iPlant Collaborative. The focus will be on phylogenomics, specifically data-set assembly and analyses. The tools will be developed as part of a large-scale effort to figure out how plants are related to each other, but will of course be relevant to the study of any group of organisms. The postdoc will be expected to spend time in both labs (in Providence and Munich). Please contact Casey or Alexis if you are interested.

Photo of Leucospermum flower by Casey Dunn.

Mating when you are stuck to a rock

posted by Stefan Siebert / on November 6th, 2009 / in Arthropods

If you are stuck to a rock it is tricky to get close enough to a partner to mate. One solution to this problem would be to release eggs or sperm into the open water, which is what many animals in this situation do. Acorn barnacles (Semibalanus balanoides), however, found a different solution. They have evolved the longest penis relative to their body size of any animal. In this video the penises of several barnacles are probing the neighborhood for mates. The penis is re-grown each mating season.

Acorn barnacles are hermaphrodites, each one has both male and female organs.  The specimens in the photos below have been removed from the rock and turned upside down. Two yellow egg clusters can be easily identified (top and middle images). The testis occur as white regions along the body of the animal which is here removed from its calcareous shelter (middle image). The penis is centered in between the feeding structures (bottom image).

Barnacles2

During the mating process, the penis of one barnacle is inserted into the body cavity of another and the sperm are released. Each barnacle can be fertilized by multiple partners, which means that a mothers offspring often have several different fathers. The developing embryos remain within the mother for several weeks before the larvae are eventually released.

Genetic paternity tests on the offspring performed by David Rand indicate that the closest neighbors are often not mates, which suggests that mating could be a selective process influenced by more than just the availability of partners. The barnacle penis shows adaptive plasticity—  J. Matthew Hoch has shown that barnacles growing where waves are stronger have thicker penises. The barnacles shown here were collected by Brown graduate student Patrick Flight who, together with David Rand, uses them as models to study genome evolution. Photos and video by Stefan Siebert.

Deadly bands

posted by Stefan Siebert / on October 28th, 2009 / in Jellies, Siphonophores

091028Tentillum3bright

Cnidaria is a group of animals that includes – among other things – jellyfish, corals and sea anemones. They take their name from the greek word for nettles (knide) because of to the sting and rash that a close encounter with them can cause. They elicit this response with a particular type of stinging cell that only they possess, the cnidocyte, which is arguably the most complex cell possessed by any animal. When triggered, a cnidocyte releases a hollow harpoon that penetrates prey organisms – or a swimmer’s skin- and injects toxins. These harpoons are microscopic, and there are many types of cnidocytes each with a different type of harpoon. Some create a painless sticky sensation, others are so powerful that a sting from just one cell can cause considerable burning.

The siphonophores, a group of colonial cnidarians, have multiple polyps and medusae that are specialized for tasks such as locomotion, feeding or reproduction. The picture on the left shows a feeding polyp (the prominent white structure in the center) of the siphonophore Nanomia bijuga. This feeding polyp is attached to the stem of the colony, which stretches across the top of this photo. Each feeding polyp has a single tentacle, and this tentacle has side branches with dense batteries of cnidocytes. Most of the cnidocytes are densely packed into a fascinating complex structure – the cnidoband. These are the orange spirals in the photos. The cnidoband ends in a filament (lower part of the picture) which contains sticky cnidocytes. The terminal filament makes first contact to the prey and sticks to it, which then tugs the cnidoband as the prey struggles. The cnidoband then stretches out and its cnidocytes fire as a unit, deploying their deadly power. These Nanomia bijuga were collected using the ROV Ventana with the friendly support of MBARI. Photos by Stefan Siebert.

Mitochondrial lenses

posted by Christopher Laumer / on October 21st, 2009 / in convergent evolution, optics

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When making observations of the tiny flatworms I study, I seldom paused to consider that they might also be looking back at me. However, when sampling in estuaries near Woods Hole, MA, I recently encountered several animals whose visual system proved impossible to ignore. On each eye, normally an inconspicuous black spot, I found a tiny spherical lens perfectly situated above their light-sensitive cells! These animals turned out to belong to a group of minuscule predatory flatworms that have an organ on their head (the acorn-looking thing) which they can shoot out rapidly to subdue their Lilliputian prey. The animal you see above is probably Toia ycia, which gets to be about half a millimeter long as an adult. We’re not sure if Toia can see in the same sense that your dog or goldfish can – I’d doubt it personally. But it’s likely that their eyes are more powerful than those of most other flatworms, which at best distinguish light from dark and can approximate the direction of the light source.

I’m apparently not the only person that’s found himself interested in these eyes—zoologists have been using electron microscopes to peer deep inside their cells for years. These investigations revealed quite a surprise. Most of us think of mitochondria as the “powerhouse” of the cell, as we learned in high school; some may remember learning about their origins as symbiotic bacteria. In these flatworms, however, these enslaved microbes serve another purpose: by accumulating refractive proteins, packing together, and becoming enlarged, these mitochondria have become lenses that focus ambient light onto the light-sensitive cells.

Toia and its ilk aren’t the only flatworms that do this: mitochondrial lenses seem to be a feature of quite a few distantly related flatworms. Some of these worms are free living, such as the beach predator Ptychopera westbladi, or the photosynthetic Dalyellia viridis. Others are more or less parasitic, as for example, Urastoma cyprinae, a pest of the commercial oyster, or the fish-gill parasite Entobdella soleae, a distant relative of the too-familiar tapeworm and liver fluke.

What are we to think of this spotty distribution of mitochondrial eyes in the flatworm tree of life? Maybe the most recent common ancestor of these organisms had an eye with a mitochondrial lens, and this feature was lost or unrecognisably modified in most descendant lineages. Another possibility, however, is that this represents a convergence – unrelated lineages of flatworms may have all found a way to build lenses with mitochondria, just as dolphins, sharks, and ichthyosaurs all independently became streamlined for drag reduction in the water. Without knowing exactly how these flatworm eyes develop, and in particular the cellular signals these animals use to guide their mitochondria to differentiate into lenses, it is difficult to distinguish between one or multiple origins of the mitochondrial lens.

Other organisms, though, have clearly discovered their own ways of making lenses with endosymbionts. Some dinoflagellates, single-celled photosynthesizers of the shallow ocean, have found a way to make lenses of their photosynthetic plastids, endosymbionts that were engulfed independently of mitochondria. Yet another clear de novo reinvention of the lens has occurred in the acoel Proporus venosus, a type of animal that was once considered a flatworm, but which has recently been shown to be as distantly related to to Toia it is to you or I. Below is a video I made of a Proporus venosus individual I captured in Sardinia.

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.