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Green HourglassThe Hourglass Model

It’s easy to see how plants and animals differ in many, many ways at the whole-organism level. (see previous posts here and here, for example.)

But at the cellular level, plants and animals are quite similar (mainly because they share a common eukaryotic cell ancestor).

New scientific evidence suggests that these similarities may also extend to the earliest stages of plant and animal life, namely, the formation of the embryo (embryogenesis).

Embryogenesis is the first developmental step in growing a multicellular life form, like a flowering plant or a cat.

In the 19th century, scientists studying vertebrate animal development noticed similar patterns in the embryogenesis of different taxa.

These vertebrate embryos appeared different in the earliest stages of development. But as embryogenesis progressed, the forms of the different embryos began to converge so that, at mid-stage embryogenesis, they all appeared similar. Finally, at the latter stages of embryogenesis, the forms of the embryos from different vertebrate animals again diverged.

Plotting these results along a vertical axis yielded a shape somewhat reminiscent of an hourglass. Hence, this early sequence of events in the formation of vertebrate animals was named the embryonic “hourglass model”.

Fast-forward to the 21st century — scientists doing genetic studies have recently shown that during the mid-stage of animal embryogenesis (the narrowing the middle of the “hourglass” when the embryos appear the same), the embryos all express similar “ancient genes”. These so-called “ancient genes”, dating back to the origin of eukaryotic cells, are shared by all animals tested so far.

However, there was no similar evidence for this “hourglass model” of embryogenesis in plants — until now.

The Green Hourglass

A paper published in the 4 October 2012 issue of Nature (see Ref. 1 below), Dr. Marcel Quint and colleagues have provided two lines of genetic evidence that support the idea that the hourglass model also is present in plant development.

Although embryogenesis evolved independently in both animals and plants, “… these findings indicate convergent evolution of the molecular hourglass and a conserved logic of embryogenesis across kingdoms.” (from Ref 1 below)

Convergent evolution of a molecular hourglass in animal and plant embryogenesis.


(Figure from Ref. 1 below.)

Perhaps the best summary of these interesting results is provided by the editor of Nature:

“It seems that both animals and plants have independently converged on a similar way of managing gene expression as they transform from a single celled zygote to multicellular organism, even though their morphological development is very different.”

References

1. Quint, M., et al. (2012) “A transcriptomic hourglass implant embryogenesis.” Nature, Vol. 490, pp. 98-101. (Abstract)

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