The Monday morning session was on...evo-devo! What a great way to start a Monday.

BK Hall. Why should devo care about evo-devo? Hall gave a lengthy historical overview of the contributions of developmental biology to evolutionary biology, and vice versa. He began with that obsession of latter-day creationists, the question of why embryos are more similar to one another than adults. Of course, it wasn't in the creationist context of 'here's an uncomfortable fact we're going to deny even exists', but that sensible scientific perspective of 'here's a real and interesting observation, now how do we explain it?'

Hall argues that the 19^th century foundations of evo-devo were linked to four broad concepts that are still valid:

• Knowledge of shared and conserved stages (like von Baer's embryos)
• Universality of germ layers (diploblastic vs. triploblastic and the utility of dividing embryonic tissues into endoderm, mesoderm, and ectoderm derivatives)
• The embryonic criteria of homology rooted in the germ layer concept
• The linkage of embryology to classification

Beginning with Darwin, development was considered the most favorable avenue for studying the problems of evolution. As we all know, that changed around the turn of the last century with the domination of Entwicklungsmechanik, or experimental embryology, that emphasized (very successfully!) an entirely operational view of developmental analysis that divorced it from theory to a large extent.

The figures Hall believes most crucial in leading development back into the mainstream of biological/evolutionary thinking were Balfour (and his ideas about the homology of embryonic characters), Garstang ("ontogeny creates phylogeny", and the argument that evolution works by modifying developmental processes to produce novel adult phenotypes), and DeBeer (emphasis on heterochrony, or timing changes in development).

The grand aims of modern evo-devo are:

• Determine the origins in evolution of development
• Understand how modification of developmental processes yields novel features
• Determine mechanisms of developmental plasticity in life history evolution
• Learn how ecology impacts development to modulate evolutionary change
• Identify the developmental basis of homoplasy and homology

Hall quickly described a few examples of interesting questions and successful application of evo-devo principles. He pointed out that pervasive misconception that early developmental periods are stable, and how common the switch from direct to indirect development is in frogs. He mentioned the search for the origins of that characteristic vertebrate innovation, the neural crest, and how molecular evidence is revealing a 'proto neural crest' in Amphioxus, and that neural crest were an excellent example of plasticity, with cells that migrate and and have vast potential for creating new functions with small changes. He illustrated the importance of heterochrony with chipmunks, that have an internal cheek pouch, and pocket gophers, that have an external pouch, and how their development reveals that this dramatic difference is a result of a relatively simple transformation of the rate of growth of buccal tissues.

NH Patel. The evolution of arthropod segmentation. Patel is well known as a guy who has been working on the problem of pattern specification in segmenting animals for a long time. He's been studying the different levels of gene activity in early arthropod development in a variety of organisms, trying to work out how, for instance, short germ band development that we see in the Orthoptera could evolve into the long germ band mechanisms of the Diptera. In this talk, he focused on crustaceans, in particular a new model system he has been studying, the amphipod crustacean Parhyale hawaiensis.

The familiar sequence of activity in flies is
maternal genes→gap genes→pair-rule genes→segment polarity genes

The segment polarity rules are the most highly conserved, and ultimately what they must do is be expressed in a very regular segmental pattern, where one row of cells out of every four will have it turned on.

The bulk of Patel's talk was taken up with introducing us to the spectacular Parhyale. He showed several time-lapse movies of fluorescently labeled cells doing amazing things. I mentioned before how it was stunning to see straight lines of gene expression in organic material: Patel showed a movie of cells in the embryo as they were organizing themselves into tidy rows and columns, and it was like watching a marching band—a milling mass of disorganized cells just jostled and shifted, and presto, it looked like a checkerboard. Another movie showed the origins of the Parhyale mesoderm. It starts as just a single row of cells called the mesenteloblasts, which then carry out asymmetric divisions, budding off tiny mesodermal precursors one after the other as they migrate, leaving a chain of cells behind them. Patel had a gorgeous movie of this, with the mesenteloblasts ponderously cruising anteriorly like a wing of dirigibles, gently blurping out small progeny in parallel lines behind them.

The mesenteloblasts also express snail rhythmically (see Pourquié's talk from the other day for why this is significant). They can also be deleted, and the animal forms absolutely no mesodermal derivates at all, while the ectoderm continues to segment normally.

One last bit of razzle-dazzle technology. These are tiny embryos, but Patel is using microfluidic techniques to raise different parts of the embryo at different temperatures—he can stream warm water past the anterior half so that it develops at 28°C, and simultaneously stream cooler water past the posterior half so it grows at 24°C. That skews the order of appearance of segmental stripes, but doesn't affect the location of their appearance.

DK Jacobs. Evidence from sponges and jellyfish of early evolution of sensory gene families—data from Six genes. Jacobs is looking for genes associated with stereotypical basal characters in most animals—eyes and ears—and has found a suite of Pou/Six homeodomain genes in cnidarians that are homologous to those found in us vertebrates and other animal groups, which suggests that functional groups of ciliated cells, the precursors to our eyes and ears, were set aside and organized very early in metazoan evolution. In addition, they found Pit1, a pituitary-specific gene, in their animals. They pointed out that the pituitary homolog in Amphioxus is a simple ciliated pit, Hatschek's pit, and predicted that the origins of the pituitary gland were in a sensory organs specialized for reproduction: "sexual sniffing" as a basal metazoan character.

PM Burton. Nematostella: a model outgroup for bilaterian evolution. I've written about Nematostella before, so I won't say much here. Burton made the case for Nematostella as a model organism, as it is easy to culture, prolific, develops rapidly, can be raised in dense cultures, and can be used for regeneration studies as well as normal development. They are bilaterally symmetric, and express Hox genes in staggered columns, with an oral-aboral axis defined by Hox genes and a dorsal-ventral axis by decapentaplegic, so they share those characreristics with bilaterians. They are also diploblasts, and one other thing they addressed was the origin of that unique triploblastic germ layer, the mesoderm: it turns out that common mesodermal genes, twist, forkhead, GATA, snail and lim are all expressed in the Nematostella endoderm, suggesting an endodermal origin of mesoderm.

N King. The choanoflagellate transcriptome: insights into animal origin and evolution. This was, I thought, one of the standout presentations of the meeting. The question is about the evolution of multicellularity in metazoans. The transition from unicellular organisms to colonial forms to true multicellularity has occurred multiple times in evolution; our animal multicellularity is of independent origin from plant multicellularity, for instance. King has been using a comparative genomics approach to determine what the genetic and developmental complexity of the first animal might have been, and to identify the common elements of the molecular toolkit that first animal inherited from its ancestors. Her method is to examine the closest modern group to the metazoa, the choanaflagellates, and ask what characters they share with us.

Choanoflagellates are curious little unicellular (primarily; there are some colonial forms) organisms that have long been recognized as possessing some features that group them with the metazoa. They are only about 10µm in diameter, and have a flagellar ring around one end of their blobby little spherical bodies. The flagellar collar is something we also see in animal cells, and only animal cells. Analysis of mtDNA shows that they also possess cassettes of conserve genes also shared with us animals.

Comparing the transcribed genes in metazoa and the choanoflagellates reveals that choanoflagellates have many unique genes, so it's safe to conclude that they are not simple degenerate multicellular animals. The really interesting point, though, is that they also have many genes shared with us, and these seem to be the key genes that form the foundation of multicellularity: diverse signalling and cell adhesion genes.

The choanoflagellates have cadherins, C-type lectins, receptor tyrosine kinases, etc., all parts of pathways that we've thought special to the development of multicellular animals. These little guys have complete animal signalling pathways!

What could these creatures be doing with that? King has also looked at choanoflagellate behavior, and they are relatively sophisticated little guys. They have a variety of strategies: some individuals are sedentary, and stick to surfaces and capture bacteria as it passes by; some swim actively, scooping up bacteria as they go; others form loose temporary associations, sticking together to form a bacteria harvesting collective. It's that flexible and interactive behavior that evolved into the more tightly regulated cooperative activity of true multicellularity—but the seeds of it are right there in this obscure group of protozoa.

B Jeffery. Evolution of development: a view from the cavefish eye. Creationists won't like this one. You've probably heard the argument that changes documented by evolutionists are all examples of degeneration, and that something like the cavefish, which "merely" loses an eye, are not examples of the creative power of evolution. Jeffery turns that all on its head; he has analyzed the changes in blind populations of Astyanax mexicanus against the eyed population, and has surprisingly discovered that most of the changes are progressive.

Regressive changes	Progressive changes
Eye loss Reduction of optic tectum Reduction of pigmentation Reduction of schooling and aggression	Enlarged larval jaw More maxillary teeth More cranial neuromasts More taste buds Telencephalon changes Increased fat content

It's a little bit of a mystery why the blind cavefish is blind—it could be a consequence of the accumulation of mutations that are not selected agains, but otherwise, what advantage is there to losing an eye? One hypothesis is that it conserves energy, since the fish doesn't need to build this complex structure. Jeffery found that isn't tenable, though. The reason the eye doesn't develop is that the lens, which is an inductive and organizing structure, degenerates in early development. However, the retina keeps growing, but as soon as new retinal cells appear, they die apoptotically. Basically, the fish keeps building eyes and tearing them down as quickly as they form, hardly an energy-efficient strategy.

Jeffery can rescue the blind cavefish's eye by transplanting lenses from the eyed varieties, and they then develop functional eyeballs. Conversely, transplanting the blind cavefish lens to a normally eyed embryo leads to the degeneration of its eye. In both cases, the changes lead to characteristic distortions of the the orbital bones that can give the fish the morphological appearance of the other population.

Furthermore, Jeffery has done detailed, quantitative assays of gene expression in the two forms of Astyanax, and found that while there are few changes, almost all of them are upregulations in the cavefish (with one exception: Pax6 is downregulated). The molecular mechanism of eye degeneration is based on an expansion of the regions of expression of the gene sonic hedgehog (ssh), which suppresses Pax6. The increased levels of ssh affect the oral area and tastebuds, leading to their increased growth and proliferation. Better jaws and gustatory sensation are more important to a cavefish in the dark, so what we're apparently seeing here is an advantageous expansion of oral tissues that has as a pleiotropic side effect a reduction in the eyes.

TJ Bailey. Regulation of divergent RX genes in vertebrate eyes. This talk was about the evolution of regulatory elements in a small gene family, the RX homeodomain paired-class transcription factors. Frogs and mice have two copies of this gene, each with apparently identical patterns of expression in the eye and hypothalamus. The zebrafish (which, as we all know, is the most highly evolved chordate in the universe) has three RX genes, and each is specialized to be expressed in different parts of the canonical RX pattern. One cool trick: when the zebrafish zRX1 gene, which is expressed only in the eyes of the fish, is extracted, put into a GFP construct, and put into frogs, the gene is also only expressed in the eyes of the frog.

AB Ward. Axial patterning in fishes. A feature that has evolved multiple times in different lineages of fish is the eel-like body plan: elongated body, with reduced fins. Ward has examined the morphology of eel-like fishes, in 54 species from 7 different groups, to determine what morphological parameters have changed. One way to elongate is by keeping the same number of bones, but making each one longer; another is to increase the total number of vertebrae (which ranges from 16 to a thousand in different species); and if there is a numerical increase, it could be within either the caudal or abdominal compartment, or both. She found that 4 of the 7 groups were caudal elongators, that added vertebrae to the tail; 1 group (Polyperus) was an abdominal elongator; and 2 groups had numerical increases in both. Only one group exhibited elongation by increase in the aspect ratio of individual vertebrae, and that increase was uniform across both compartments.