Evidence of convergent evolution of the central nervous system (CNS)

The nervous systems of animals come in many shapes and sizes, ranging from a handful of neurons to large, complex brains. A basic evolutionary question has been whether the centralized nervous systems –– found in many bilaterally symmetrical animals (bilaterians, i.e. having a left and right ‘side’), which include vertebrates (animals having a spine) and insects –– share a common evolutionary origin, or have evolved more than once. In terms of “evolution of the eye”, (i.e. any living thing’s perception of light vs darkness vs motion, which obviously has a survival advantage for finding food and avoiding prey), there is evidence that such a structure has spontaneously independently evolved at least 64 times..!!

At a superficial level, both flies and vertebrates boast a brain connected to a single nerve cord that extends into the trunk. In addition, molecular biology data indicate that key regulatory genes are deployed simi­larly during nervous-system development in vertebrates, flies, and another bilaterian, a seg­mented worm (an annelid). These similarities have been interpreted as evidence for evolu­tionary conservation of an ancient bilaterian developmental program for centralized nerv­ous systems. However, in the [attached] paper, authors provide evi­dence for the independent evolution of such nervous systems. In the mid-1980s, the ability to study this process received a boost, thanks to the discovery of a large family of genes that encode transcription factors con­taining a DNA-binding homeobox domain. Members of this homeobox-gene family, including the Hox complex, are expressed in the same order along the head-to-tail (anterior–posterior) axis during devel­opment in many distantly-related bilaterians, including flies and vertebrates. It was also shown that another signaling pathway –– genes that encode bone morphogenetic pro­teins (BMPs) is needed to establish the dor­sal–ventral (back-to-belly) body axis in amazingly diverse bilaterians.

It was therefore not surprising to find that a suite of homeobox genes is also expressed in strikingly similar patterns along the dorsal–ventral axis of the developing nervous systems of vertebrates and fruit flies. Along this axis, staggered homeobox-gene expression correlates with development of specific neuron types in different regions. The discovery that these genes are also expressed along the dorsal–ventral nervous-system axis in Platynereis dumerilii (an annelid distantly related to flies and vertebrates) was seen as evidence that bilaterian nerve cords are evolutionarily conserved.

Authors [attached article] studied representatives of Xenacoelomorpha, Rotifera, Nemertea, Brachiopoda, and Annelida (each one of these six represents a different Phylum in the Animal Kindgom) to assess the evolutionary conservation of the dorsoventral nerve cord patterning. None of the studied species shows a conserved dorsoventral molecular regionalization of their nerve cords –– not even the annelid Owenia fusiformis, whose trunk neuroanatomy parallels that of vertebrates and flies. These (pretty cool) data limit the use of molecular patterns to explain nervous system evolution, and suggest that similarities in dorsoventral patterning and trunk neuroanatomies evolved independently in these Bilateria.

Nature 4 Jan 2o18; 553: 45-50 & pp 34-36 [News-N-Views]

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