posted 06. February 2003 00:35                   

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The genes controlling the early events in the development of Drosophila can be classified into three broad categories: Gap genes are a set of genes that act to define broad regions of the early embryo; these can regulate the expression and action of Pair rule genes which further define the broad regions into more numerous segments; the pair rule genes can affect the expression and action of Segment polarity genes which will determine the fate of certain structure within each segment. As with the gradients of morphogens described above, one can envision mutations that alter the interactions between these broad classes of genes controlling the developmental fate of parts of the organism which, if established in the population, could lead to the evolution of new morphological "plans" (.g., a new Bauplan).

There is good evidence for such a supposition in another very important set of genes: the homeotic genes. Certain mutations in these genes result in homeotic mutations where one body part is transformed into the structure of another body part. The best examples are the Antennapedia complex and the Bithorax complex which are large regions of the chromosome containing several genes each. The positions of the genes on the chromosome have a remarkable correlation with the segment of the body in which they are active! (see figures below). The genes contain a region of DNA that codes for a highly conserved stretch of amino acids, known as the homeobox that generally are involved in the determination of body segments (but bicoid has a homeobox and it is more involved with anterior-posterior determination). While mutations that move a leg to the position of an antenna (in the Antennapedia complex) or transforms the balancing organs (halteres) into a pair of wings (in the Bithorax complex) is of dubious fitness value to the organism, it does show that modifications of the general body plan be achieved by mutations in one or a few genes, i.e., there is genetic evidence that Hopeful (hopeless?) Monsters could be produced.

These phenomena are compelling in light of the belief that arthropods (insects, crustaceans, etc.) evolved from annelids (segmented worms; see figure below). One can envision that sequential modification of body segments, through mutations such as those described above, might allow for the evolution of insects from a worm-like ancestor. Suggestive of this is the observation that when the Antennapedia complex and the Bithorax complex are mutated the larval stage of the fruit fly is transformed into a larva with many thoracic segments rather than the wild-type pattern of differentiation into maxillary, labial and abdominal segments (see fig.below). This "throw back" to the ancestral form (i.e., the middle segments of worms are relatively undifferentiated) is called an atavism.

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Most species maintain abundant genetic variation and experience a range of environmental conditions, yet phenotypic variation is low. That is, development is robust to changes in genotype and environment. It has been claimed that this robustness, termed canalization, evolves because of long-term natural selection for optimal phenotypes. We show that the developmental process, here modeled as a network of interacting transcriptional regulators, constrains the genetic system to produce canalization, even
without selection toward an optimum. The extent of canalization, measured as the insensitivity to mutation of a network’s equilibrium state, depends on the complexity of the network, such that more highly connected networks evolve to be more canalized.
We argue that canalization may be an inevitable consequence of complex developmental–genetic processes and thus requires no explanation in terms of evolution to suppress phenotypic
variation.

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Scientists used to think that developmental fidelity evolved via natural selection, principally through survival and reproduction of organisms with redundant genetic systems -- that is, ones with copies of important gene sequences. But Siegal and Bergman's results indicate that redundancy may only be one small manifestation of a bigger theme: the complexity of gene networks. In short, more complex systems are more resistant to change in their outputs.

"It is typically assumed that important properties of organisms are crafted by natural selection," says Dmitri Petrov, assistant professor of biological sciences. "What Siegal and Bergman show is that robustness in the face of mutation, or canalization, may be a byproduct of complexity itself and therefore that robustness may be only very indirectly a product of natural selection."

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Here we compare mechanisms of segmentation in different organisms and discuss how the transition between the different types of segmentation can be explained by small and progressive changes in the underlying gene networks.

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Summary
Morphological differences between species, from simple single-character differences to large-scale variation in body plans, can be traced to changes in the timing and location of developmental events. This has led to a growing interest in understanding the genetic basis behind the evolution of developmental systems.
Molecular evolutionary genetics provides one of several approaches to dissecting the evolution of developmental systems, by allowing us to reconstruct the history of developmental genetic pathways, infer the origin and diversification of developmental gene functions, and assess the relative contributions of various evolutionary
forces in shaping regulatory gene evolution. BioEssays20:700–711, 1998. r 1998 John Wiley & Sons, Inc.

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posted 07. February 2003 01:06                   

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In the sea urchin genus Heliocidaris, two modes of development are present.

One species goes through a pluteus (free-living) larval stage. The other, however, develops directly into the adult. These ontogenies are remarkably different from each other, yet the adult sea urchins are more or less indistinguishable.

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A continuum of modifications of the pluteus has evolved, ranging from obligately feeding pluteus to loss of all larval features. These modifications include facultative planktotrophy (Emlet 1986), nonfeeding plutei (Okazaki 1975; Olsen et al., accepted), armless nonfeeding larvae

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The present study demonstrates that radical changes in development, oogenesis, and spermatogenesis can occur in relatively brief spans of evolutionary time. A thorough comparative analysis of the coding sequences in nuclear DNA of these species would probably display little difference. The kinds of differences that result between these species more likely arise from patterns of control and timing of expression of specific gene sets.

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To investigate the bases for evolutionary changes in developmental mode, we fertilized eggs of a direct developing sea urchin, Heliocidaris erythrogramma, with sperm from a closely related species, H. tuberculata, that undergoes indirect development via a feeding larva. The resulting hybrids completed development to form juvenile
adult sea urchins. Hybrids exhibited restoration of feeding larval structures and paternal gene expression that have been lost in the evolution of the direct-developing maternal species. However, the developmental outcome of the hybrids was not a simple reversion to the paternal pluteus
larval form. An unexpected result was that the ontogeny of the hybrids was distinct from either parental species. Early hybrid larvae exhibited a novel morphology similar to that of the dipleurula-type larva typical of other classes of
echinoderms and considered to represent the ancestral echinoderm larval form. In the hybrid developmental program, therefore, both recent and ancient ancestral features were restored. That is, the hybrids exhibited features of the pluteus larval form that is present in both the paternal species and in the immediate common ancestor
of the two species, but they also exhibited general developmental features of very distantly related echinoderms. Thus in the hybrids, the interaction of two genomes that normally encode two disparate developmental modes produces a novel but harmonious ontongeny.

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Conclusion
We presently view development as a concatenation of discrete operations, each of which results from the action of a small set of interacting genes called a syntagm. The operations themselves
are largely invariant, due to the high inertia of the underlying syntagms. The connections between operations, however, are more flexible since changes at this level do not impair the workings
of each operation but rather create more possibilities and serve as a source of species diversity. Thus we expect evolution to be
essentially as discontinuous as the developmental program itself. Major aspects of the evolutionary process, such as its saltatory nature and the highly discontinuous morphologies of the major
phyla, are consistent with this view.
The idea that evolution and the creation of new forms stem from within and not from without, and that the changes reflect a continuous reassortment of existing, highly invariant operations, makes it necessary to reconsider the belief that natural selection and adaptation is the driving force for species diversity. Darwin himself was well aware that the driving force of evolution was variation: "...[Natural selection] implies only the preservation of such variations
as arise and are beneficial to the being under its conditions of life..." (Darwin, 1859). What we have now discovered is that the syntagmatic structure of the developmental program imposes
strong constraints on the type of variation that can arise and be viable.

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posted 07. February 2003 11:33                    

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I guess my main point is that there are probably hundreds of biological solutions between unicellularity and a multicellular-multilineage organism, ranging from bacterial quorum sensing, to biofilms, to colonial ciliates, to dictyostelium, to volvox, to sponges etc etc. Somewhere in there is where the answer to the evolution of "ontogenetic depth" ultimately lies, not in drosophilas and nematodes.

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posted 07. February 2003 13:14                    

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While heritability is important, I believe the experiment showed how the developmental networks are far more open to change than perhaps implied in your paper...While the hybrids may not have been viable by themselves they show that novel developmental changes are not beyond reach of the genome.

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posted 07. February 2003 18:36                   

• Surely cells are more plastic during their development than this? I'm thinking, offhand, of how the brain's structures work to compensate when a person suffers a cognitive defecit from a trauma or disease. Then there's the recent experiment where they switched quail bills with duck bills. Just how brittle are you claiming the development process is?
• I thought that the cells in a developing embryo were more like a swarm than a marching band. Don't the cells differentiate in response to signals provided by their immediate environment, mostly meaning their immediate neighbors? If your marching band's current show causes the tuba player to start at one goal line & end up at the other, he'll trace out the same path if you tell him to play his part alone on the field as he would when he's part of the performance. But if you isolate an embryonic stem cell that has started to differentiate, it's not going to continue developing on its own exactly like it would have in vivo, is it? (IOW, isn't the developmental pathway somewhat contextual?)
• I can see how the average change you make to a cell's developmental trajectory would be deleterious to the resulting organism, but it's already well known that a population undergoing selection pressure is a population that's experiencing more premature deaths than one that isn't undergoing pressure. I'm not sure what the marching band problem brings to the table here, except to make the developmental process look more brittle.