Embryonic development

What do a fish and a mouse have in common? Not much, most people would say.

Yet, if you look at them at a certain stage in their embryonic development, you can hardly distinguish the two. But this is hardly a new idea - the founder of embryology Karl Ernst von Baer observed this in the 19th century.

"It's a question scientists have tried to explain ever since these similarities were first observed", said Pavel Tomancak, a researcher at the Max Planck Institute of Molecular Cell Biology and Genetics in Dresden, in an interview with Deutsche Welle.

In a new paper published last week in the journal Nature, scientists in Germany have proven that embryos from different species don't just look the same - there are striking parallels between individual development and the development of a species on the level of gene expression, too.

21st century science confirms 19th century hunch

German biologist Ernst Haeckel in 1866 took it a step further by postulating that at some point human embryos look like the embroys of fish species, he noticed, and since fish were seen as an evolutionary more primitive animal form than humans he proposed that they must all have the same ancestral features of some 600 million years ago.

His theory: the development of an organism mirrors the evolutionary development of the species, or as evolutionary biologists put it: "ontogeny recapitulating phylogeny."

The findings were of a mere qualitative nature, of course: the scientists in the 19th century were drawing their conclusions from what they saw. In the 20th century, this qualitiative method was largely abandoned, most scientists stopped looking at the morphology and tried to find quantitative proof for their theories.

The second half of the 20th century were an important time for molecular science, with remarkable progress being made in the field of gene and DNA analysis. Scientists were able to look at embryos before their phylotypic stage, which is the middle of the embryonic development.

The "waist" of the hourglass

They noticed that there were differences before and after the phylotypic stage and came up with what came to be known as the "hourglass model."

Species-specific differences predominate before and after the phylotypic stage, so the phylotypic stage represents the "waist" of the hourglass.

The question as to how this extensive morphological similarity - the "waist of the hourglass" - arises is one that preoccupied Pavel Tomancak and his colleagues from the Max-Planck-Institute in Dresden.

In a process of one and a half years his team collected samples of embryos of six fruit fly species, and measured gene expression across the genome at different times of embroygenesis. The analysis took another year.

What they found was that there was a very strong pattern.

"We knew we'd discovered something very early on", lead scientist Pavel Tomancak told Deutsche Welle.

Tomancak's team found that not only in morphology but also in the expression pattern of the genes, the similarities between embryos of different species in one animal phylum are greatest during the phylotypic stage. Before and after this phase, the differences between the species are greater.

"Darwin had a point"

But the question remained: which model could be applied to prove that they were right?

The pattern was so strong that it couldn't be random, Tomancak observed.

"We found that natural selection constrains genes most strongly during the phylotypic stage", he told Deutsche Welle.

The results show that the similarity between different animal species in the middle of their embryonic development is shaped by selection.

Their discoveries 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", said Alex T. Kalinka, a researcher from the Dresden group, in an interview with the journal, Nature.

In a different study, researchers at the Max-Planck-Institute for Evolutionary Biology in Ploen demonstrated with zebrafish 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 scientists also found something else: In adult zebrafish progressively older genes are also activated with the increasing age of the animals. They found the same to be true in fruit flies and threadworms.

As a follow-up the researchers in Dresden are thinking about manipulating genes at the phylotypic stage to see what happens. While fossils allow scientists to look at what species looked like when they were living on our planet.

"Our findings are like a time machine, we can look at what the genome was like 600 million years ago," Tomancak said.

Author: Nina Haase
Editor: Cyrus Farivar