Thursday, May 6, 2010

Something's Fishy

This week I'm apparently all about PNAS articles. This story also comes from that journal and is about changing the brains of fish.

Georgia Tech scientists have been applying chemicals to developing Cichlid fish embryos in order to manipulate genes. In these experiements they found differences in general brain patterns, differences evident in early development neurogenesis.

Let's step back for a second. There is a popular theory known as "late equals large" that explains the way brains evolve across species. The brain starts off as a blank slate, and as the embryo starts to develop, the anterior (front) and posterior (back) parts of the brain are specified. The next step is neurogenesis where precursor cells start to replicate and then mature into neurons. The later the this switch from precursor cell to neuron (the shutting off of replication and then maturation of cells) occurs the larger the brain. This is where the "late equals large" theory comes into play. It says that the brains of different species are similar in those early stages but differentiate in the later stages of development and neurogenesis.

So back to fish brains. The study looked at the brains of six species of Cichlid fish from Lake Malawi, three of those species from a rock-dwelling lineage and three from a sand-dwelling lineage (remember that Cichlids are known for their adaptive radiation). The authors of this paper found differences in the general brain patterning as early as 48 hours after fertilization. This is before neurogenesis begins.

When they repeated the experiments 2-4 days after fertilization they found that the sand-dwelling species had a larger expression of the gene wnt1, an important factor in posterior brain development, specifically the thalamus which is used in processing vision. Since the sand-dwellers are visual hunters this makes sense. The rock-dwelling species have larger cerebra (or telecephala) which is thought to help navigate 3D environments. However, the genomes of these species are very similar, but the variation you see in their brains is dramatic.

Ok, so now they knew the brains were similar, but different. At what point and how do these differences take place? According to the "late equals large" theory, most of the changes would occur in later stages of brain development.

To test this, developing fish brains were exposed to lithium chloride for 3-5 hours during an early stage of anterior-posterior patterning in order to alter gene expression. After that, samples were taken at various developmental stages. In each sample it was found that Wnt signaling was up-regulated, a change that reallocates brain precursors to the posterior thalamus. Ah-ha! The brains did not develop normally suggesting that a signal early in development, rather than late, causes differentiation in these species.

Overall, they have challenged the "late equals large" theory by showing that there are differences in the developmental process earlier than previously thought.

Here's the article:
Sylvester, Jonathan B., et al. (2010) Brain diversity evolves via differences in patterning. Proceedings of the National Academy of Sciences: published online. (DOI: 10.1073/pnas.1000395107)

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