Blute Blog

Blute's blog about evolutionary theory: biological, sociocultural and gene-culture.

The evo-devo hour glass goes molecular but what explains it?

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In the nineteenth century the great German embryologist Karl Ernst von Baer proposed laws of embryological development – that general characteristics and structural relations develop before special ones; the form of an embryo does not converge on that of others but diverges from them; and that the embryo of a animal never resembles the adult of another animal but only its embryo. In modern language, embryological development is a process of differentiation in the senses that parts of an embryo become differentiated from each other and that members of different but related groups become more different from each other. Although Von Baer was not an evolutionist, Darwin thought that the early similarity of members of different groups was the best evidence for his theory of common descent. As he wrote in the Origin, “Community of embryonic structure reveals community of descent.” (Von Baer and the reception of his laws are discussed by Scott Gilbert here.)

While evolutionary changes in development themselves are presumably attributable to selection and/or drift, this pattern of change in development – less change earlier, more later – is most readily explained by constraints. A genetic mutation or recombination which affected the earliest stage of development would have more side effects down the road than would one which first acted later, and many of these would likely be maladaptive. Analogous phenomena are found in other selection processes. For example, a rat learning a maze with a series of choice points reinforced at the end eliminates mistakes in a backwards direction i.e. change takes place more readily in the later than in the earlier stages of the entire sequence. The philosopher of biology, William C. Wimsatt, dubbed the constraint principle in the evolution of development “generative entrenchment”.

In the 1990’s however, it turned out that the empirical generalization ‘more evolutionary change has taken place later than earlier in development’ does not quite hold – rather the difference between members of different but related groups resembles an hourglass (discussed for example by Raff in The Shape of Life). Groups differ more in the very earliest phase of development, converge to become more similar (the hour glass narrows to what is called the phylotypic stage in animals), and then diverge again quite a bit for the bulk of the rest of development. Kalinka et. al. writing in Nature (gated) have recently confirmed this hourglass pattern for gene transcription involved in key developmental processes by comparing six species of Drosophila.

I argued (in Darwinian Sociocultural Evolution pp. 146-8) that the constraints logic implies that the in-between phylotypic stage of minimal change/differences between groups must represent the actual historical origin of the taxa involved. This interpretation has now been borne out by Domazet-Lošo et. al. writing in the same issue of Nature. Using “phylostratigraphic” methods they had previously pioneered, they found from the transcriptome (all RNA molecules present), that the genes expressed in Zebra fish in the equivalent of the animal phylotypic stage are indeed older than those from other, including the earliest phase. They also confirmed this for some other groups using data from the literature.

But another question remains, again if the constraints logic is valid, how has so much evolutionary change in the earliest phase of development been possible? I argued that it must be because what appears to be the earliest phase of development is in actuality a later phase. Such would be the case if it were largely a maternal effect i.e. a later phase of the mother’s development. Intriguingly Domazet-Lošo et. al. did find significant differences between the age of genes expressed in the late juvenile and adult phases of males and females in Zebra fish with more newer in females than in males. The authors suggest this may be related to recent sexual selection (although it should be noted that Zebra fish are not notably sexually dimorphic and there is little evidence that female choice is more significant than male-male competition among them – if anything, the reverse may be the case). However, the sex difference is not really a test of the maternal effect hypothesis anyway because on my understanding (and I am not a molecular biologist) while maternally inherited RNAs would show up in the transcriptome, what would not show up would be if they were maternally inherited, or transcribed from maternally imprinted genes, or expressed as a consequence of maternally inherited protein transcription factors for example.

In short, I eagerly await whether the maternal effects hypothesis of ‘too much’ change early in development can and will be tested. If confirmed, Von Baer’s law would in a sense be restored.

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Written by Marion Blute

December 22, 2010 at 2:09 am

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