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 »  Home  »  Science  »  Tomislav Domazet-Loso Croatian biologist in cover article of Nature
 »  Home  »  Education  »  Tomislav Domazet-Loso Croatian biologist in cover article of Nature
Tomislav Domazet-Loso Croatian biologist in cover article of Nature
By Prof.Dr. Darko Zubrinic | Published  01/14/2011 | Science , Education | Unrated
Development of embryos mirrors evolutionary history

Dr. Tomislav Domazet-Ločo, Ruđer Bočkoviæ Institute, Zagreb

Zagreb, 9 Dec 2010 - Ideas that development of embryos mirrors evolutionary history are well known even outside biological circles. However, due to the lack of tools that would allow explicit quantitative testing, the scientific status of this elusive concept has been unclear. Evolutionary geneticists, Tomislav Domazet-Ločo and Diethard Tautz, from Ruđer Bočkoviæ Institute in Zagreb (Croatia) and Max Planck Institute for the Evolutionary Biology in Plön (Germany) devised a new approach that measures phylogenetic age of the expressed genes across ontogeny and showed that phylogeny-ontogeny correlation is real. The results are published in the 9th December issue of Nature as a cover story.

Short history of the concept

Prominent biologist of 19th century like Karl von Baer, Charles Darwin and Ernst Heackel strived to link morphological changes that could be observed in development of an individual (ontogeny) with morphological diversity seen across divergent species of animals (phylogeny). It was well established at that time that ontogeny mimics some aspects of historical expansion of animal diversity, however it was a matter of vivid debate what is the cause of these parallelisms and how strong and general they are.

Astonishingly, controversy did not diminish in the next century despite the tremendous advances in genetics, developmental and evolutionary biology. Illusive nature of parallelisms between ontogeny and phylogeny irritated some researchers to the extent that they dismissed their existence altogether, and, as Stephen J. Gould elaborated in his book "Ontogeny and Phylogeny", the subject almost became taboo.

The interest for this problem has been revived by the synthesis of developmental and evolutionary biology (Evo-Devo) in the last decades of 20th century. Some Evo-Devo researchers suggested that understanding of ontogeny-phylogeny relationship requires a clear genetically based quantitative approach to effectively tackle the problem. However, effective quantitative analysis has been precluded so far by a lack of straightforward approaches for simultaneous mapping of genes on the ontogenetic and phylogenetic hierarchy.

Phylogenetic age of the transcriptome

As a result of joint project between Ruđer Bočkoviæ Institute in Zagreb (Croatia) and Max Planck Institute for the Evolutionary Biology in Plön (Germany), which was in part supported by the Croatian Unity Through Knowledge Fund, evolutionary biologists Tomislav Domazet-Ločo and Diethard Tautz, in the latest issue of Nature, have shown how to overcome this problem.

Building on their previously developed "phylostratigraphic" method that establishes phylogenetic scale in the genome, researchers devised a measure that successfully ties phylogenetic and ontogenetic hierarchy. This measure called transcriptome age index (TAI), in a very intuitive way summarizes the phylogenetic age of all active genes at any stage of ontogeny. The higher the TAI, the younger the set of expressed genes.

Armed with this new tool, scientist from Zagreb and Plön generated a fine-grained series of gene expression data that covers zebrafish ontogeny from the fertilized egg until the aged adults. Obtained TAI profiles in zebrafish, a model fish species, showed in a surprisingly clear way that correlation between phylogeny and ontogeny on the molecular level indeed exists. "This immediately warned us that morphological pattern recognition capabilities of 19th century pioneers are not to be underestimated" – explained dr. Domazet-Ločo.

Hourglass model of development

The phylotypic stage, a mid-embryonic stage that shows the lowest morphological variability across vertebrates, has been especially elusive and in the focus of the phylogeny-ontogeny debate. According to a popular "hourglass" model of development before and after this stage, increased morphological variability is expected. "The recovered TAI profile is faithfully mirroring the hourglass model of development and clearly marks the phylotypic stage where the oldest set of genes is expressed. This is the first clear support for the model and the phylotypic stage at the molecular level" - confirm the researchers.

However, it still remains unclear why this across species pattern exists after all. Currently there are two explanations. According to an adaptive scenario the phylotypic period represents ecologically the most protected period during development where there is reduced need for adaptive change. In contrast, the constraint hypothesis holds that the phylotypic stage is the least variable one due to the internal developmental mechanics. In the same issue of Nature an independent group from Max Planck Institute in Dresden published a story that goes in favor of this later hypothesis.

The old get older

Nevertheless, surprises did not stop there, as the TAI showed additional regularities that were completely unexpected. Probably the most peculiar finding reported by Domazet-Loso and Tautz is that adult aging animals are expressing increasingly older genes as the aging advances. A possible interpretation for this pattern is that animals beyond reproductive age are not visible to natural selection and can not therefore be subject to specific adaptations any more. To evaluate these results the team applied the same approach on available data from distant animal species like flies and nematodes where they found the same trends.

"In a broader perspective, this study shows that correlate between phlogeny and ontogeny is not a myth, and that once disputed ideas of von Baer, Darwin and Haeckel have to be revisited. It will be highly exiting to apply our approach in other animals with quite diverse body plan organization and life history strategy", explained dr. Domazet-Ločo.

Sources Ruđer Bočkoviæ Institute in Zagreb, Tomislav Domazet-Ločo

A Genetic Trigger For The Cambrian Explosion Unraveled?

ScienceDaily (Sep. 3, 2007) — A team of scientists led by young Croatian evolutionary geneticist Tomislav Domazet-Ločo from Ruder Bočkovic Institute (RBI) in Zagreb, Croatia, developed a novel methodological approach in evolutionary studies. Using the method they named 'genomic phylostratigraphy', its authors shed new and unexpected light on some of the long standing macroevolutionary issues, which have been puzzling evolutionary biologists since Darwin.

The only direct method of research in evolutionary history involves analyzing the fossil remains of once living organisms, excavated in various localities throughout of the world. However, that approach often cannot provide the full evolutionary pathway of some species, as it requires uncovering of many fossils from various stages of its evolutionary history. As the fossil record is imperfect, the evolution research fundamentally hinges on luck factor in discovering the adequate paleontological sites.

However, the RBI team proposed a novel and interesting approach to bypass this obstacle. Namely, they suggested that the genome of every extant species carries the ‘snapshots’ of evolutionary epochs that species went trough. What's even more important, they also developed the method which enables evolution researchers to readily convert those individual 'snapshots’ into the full-length 'evolutionary movie' of a species.

Applying their new methodology on the fruit fly genomic data they tackled some of the most intriguing evolutionary puzzles - some of which distressed even Darwin himself. First, they demonstrated that parts of the living organism exposed to the environment – so called ‘ectoderm’ - are more prone to evolutionary changes. Further, they explained the evolutionary origin of the ‘germ layers’, the primary tissue forms that form during the first days after the conception of a new animal, and from which subsequently all other tissues are developed. Finally, they discovered the potential genetic trigger for the 'Cambrian explosion', a major global evolutionary event on the planet, when some 540 million years ago almost all animal forms known today suddenly 'appeared'.

The first public lecture on these findings will be given by dr. Domazet-Ločo on September 4th at 5. ISABS Conference in Forensic Genetics and Molecular Anthropology, held in Split, Croatia. The groundbreaking paper fully presenting the theory of genomic phylostratigraphy will appear in the November issue of 'Trends in genetics', the most established monthly journal in Genetics.

Reference: Domazet-Ločo, T. et al. (2007) 'A phylostratigraphy approach to uncover the genomic history of major adaptations in metazoan lineages'. Trends in Genetics (to appear in the November 2007 issue of the journal)


Dr. Tomislav Domazet-Ločo is a son of distinguished Croatian admiral mr. Davor Domazet-Ločo, on the photo.


Shedding new light on embryonic development

Similarities in the embryonic development of various animal species are also found at molecular level

December 15, 2010

The astonishing similarity in the appearance of embryos from different animal species was observed as far back as the 19th century by scientists such as Karl von Baer, Charles Darwin and Ernst Haeckel. Such observations prompted the hypothesis that the individual development of an organism reflects its evolutionary history or phylogeny. Two groups of scientists, including researchers at the Max Planck Institute of Molecular Genetics in Dresden and the Max Planck Institute for Evolutionary Biology in Plön, have now succeeded in demonstrating, for the first time, that parallels exist between individual development and phylogeny on the level of gene expression. (Nature, December 9, 2010)

Ernst Haeckel’s famous comparative analysis of vertebrate development, formed from images of Drosophila embryos showing gene expression in the course of embryonic development. The mosaic comprises around 4,000 components taken from a database of 38,000 images.
Ernst Haeckel’s famous comparative analysis of vertebrate development, formed from images of Drosophila embryos showing gene expression in the course of embryonic development. The mosaic comprises around 4,000 components taken from a database of 38,000 images.
Pavel Tomancak / MPI of Molecular Cell Biology and Genetics

Whether fish or flies - at a certain stage in their development, the embryos of different animal species within a phylum are almost impossible to distinguish on the basis of their appearance. The greatest similarity arises in the middle of embryonic development, during the "phylotypic stage"; species-specific differences predominate before and after this stage. This observation is illustrated by the hourglass model. The question as to how this extensive morphological similarity - the "waist" of the hourglass - arises is one that has long preoccupied researchers. The extent to which the individual development of an organism (ontogeny) and that of a phylum (phylogeny) are linked was also previously unclear.

For the first time, scientists have now demonstrated that the hourglass motif arises in organisms as diverse as the fruit fly and zebrafish, not only at morphological level but also at molecular level - a finding that suggests that parallels do, indeed, exist between ontogeny and phylogeny. In a study carried out on six fruit fly species (Drosophila sp.), the research group working with Pavel Tomancak at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden discovered that the similarities not only in morphology, but also in the expression pattern of the genes are greatest during the phylotypic stage; before and after this phase, the differences between the species are greater. Moreover, the scientists also observed that the expression pattern of key genes reflects the hourglass model most faithfully. Meanwhile, Tomislav Domazet-Ločo and Diethard Tautz, researchers at the Max Planck Institute for Evolutionary Biology in Plön, demonstrated with zebrafish (Danio rerio) that the phylogenetically oldest genes are active during the phylotypic stage and that, before and after this stage, the most active genes are those that arose later in evolutionary history. The Plön-based evolutionary biologists also made another astonishing discovery: they observed that in adult zebrafish progressively older genes are also activated with the increasing age of the animals. The same conclusion was reached in comparative analyses carried out on Drosphila , mosquitoes of the genus Anopheles and threadworms.

These two studies throw new light on an age-old biological conundrum: that of the link between ontogeny and phylogeny. "Our discovery confirms the earlier anatomical studies and broadens our understanding of how development and evolution are linked at molecular level," explains Alex T. Kalinka, a researcher from the Dresden group. "The results show that the similarity between different animal species in the middle of their embryonic development is shaped by selection," adds Casey Bergmann, a co-author from the University of Manchester. Their findings explain how the "waist" in the hourglass arises.

Fruit flies are one of the most thoroughly researched model organisms and offer unique possibilities for the study of the molecular mechanisms that underlie embryonic development. The discovery of the hourglass pattern in different species makes it possible for evolutionary biologists to travel back in time to the earliest days of evolution when the differences between organisms arose. "We hope to gain insight into the processes that led to the variety of forms in the animal kingdom," explains Pavel Tomancak.

For their study on zebrafish, another model organism widely used in evolutionary biology, the researchers from Plön also developed a new method: the transcriptome age index (TAI). This method enables the measurement of the phylogenetic age of active genes. Domazet-Ločo and Tautz used this new tool to trace the development of the zebrafish from the fertilised egg to the adult organism. "The TAI profile faithfully reproduces the hourglass model and therefore demonstrates that parallels exist between ontogeny and phylogeny," reports Diethard Tautz. The scientists explain the observation that the phlogenetically oldest genes are active in older zebrafish with the fact that animals which have passed reproductive age are "overlooked" by selection.

These studies show that naturalists like Karl von Baer, Charles Darwin and Ernst Haeckel were basically correct in their hypothesis that embryonic development is a reflection of phylogeny. "It will be very exciting to extend our approach to other species with different blueprints and life-cycle strategies," says Domazet-Ločo.

Source Max Planck Gesellschaft

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