Brain diversity evolves via differences in patterning

Abstract

Differences in brain region size among species are thought to arise late in development via adaptive control over neurogenesis, as cells of previously patterned compartments proliferate, die, and/or differentiate into neurons. Here we investigate comparative brain development in ecologically distinct cichlid fishes from Lake Malawi and demonstrate that brains vary among recently evolved lineages because of early patterning. Divergence among rock-dwellers and sand-dwellers in the relative size of the telencephalon versus the thalamus is correlated with gene expression variation in a regulatory circuit (composed of six3, fezf2, shh, irx1b, and wnt1) known from model organisms to specify anterior-posterior (AP) brain polarity and position the shh-positive signaling boundary zona limitans intrathalamica (ZLI) in the forebrain. To confirm that changes in this coexpression network are sufficient to produce the differences we observe, we manipulated WNT signaling in vivo by treating rockdwelling cichlid embryos with temporally precise doses of LiCl. Chemically treated rock-dwellers develop gene expression patterns, ZLIs, and forebrains distinct from controls and untreated conspecifics, but strongly resembling those of sand-dwellers. Notably, endemic Malawi rock-and sand-dwelling lineages are alternately fixed for an SNP in irx1b, a mediator of WNT signaling required for proper thalamus and ZLI. Together, these natural experiments in neuroanatomy, development, and genomics suggest that evolutionary changes in AP patterning establish ecologically relevant differences in the elaboration of cichlid forebrain compartments. In general, variation in developmental patterning might lay the foundations on which neurogenesis erects diverse brain architectures.A rguably the most-studied vertebrate organ, the brain has played an important role in the evolution of our own species. Modifications of brain structure are responsible for novel behaviors that galvanized evolutionary radiation of the major vertebrate groups (1). Following decades of research in model organisms, we now know a great deal about how the process of development makes a brain (2). We know much less about evolutionary mechanisms of brain diversification. The brain develops under the iterative influence of antagonistic anterior and posterior signaling molecules, inductive and repressive transcription factors that receive those signals, and lineage restriction boundaries that define compartments (2, 3). Just after gastrulation, the initial anterior-posterior (AP) polarity of the brain is established by a tug-of-war between posteriorizing signals (e.g., wnt1) secreted from the midbrain-hindbrain boundary (MHB) and WNT antagonists (e.g., six3, tlc) expressed from the anterior neural ridge (ANR). The MHB develops to demarcate the hindbrain from the fore-plus midbrain (Fig. S1). With the subsequent formation of the diencephalon-midbrain boundary and the zona limitans intrathalamica (ZLI), the forebrain and midbrain begin to follow separate paths of development.These ini...

Sox2-mediated differential activation of Six3.2 contributes to forebrain patterning

“…In the retina, however, a lower Six3.2 activity would not be sufficient to repress Rx3, the activation of which would consequently repress Foxg1-mediated telencephalic fate, as previously suggested (Kennedy et al, 2004;Stigloher et al, 2006). In support of this hypothesis, a recent study has shown that differences in the extent of the Six3 expression domain positively correlate with the size of the telencephalon in cichlid fishes (Sylvester et al, 2010 compatible with studies of other Six3 orthologues showing that activator and repressor roles depend on context-specific interactions. Indeed, Six3 binds a variety of proteins, including other TFs (Tessmar et al, 2002), transcriptional co-repressors of the Groucho family (Kobayashi et al, 2001;Lopez-Rios et al, 2003), coactivators such as EYA4 (Abe et al, 2009) and components of the chromatin remodelling complexes, including MTA1 and HDAC2 (Manavathi et al, 2007).…”

“…In the retina, however, a lower Six3.2 activity would not be sufficient to repress Rx3, the activation of which would consequently repress Foxg1-mediated telencephalic fate, as previously suggested (Kennedy et al, 2004;Stigloher et al, 2006). In support of this hypothesis, a recent study has shown that differences in the extent of the Six3 expression domain positively correlate with the size of the telencephalon in cichlid fishes (Sylvester et al, 2010 (Tessmar et al, 2002), transcriptional co-repressors of the Groucho family (Kobayashi et al, 2001;Lopez-Rios et al, 2003), coactivators such as EYA4 (Abe et al, 2009) and components of the chromatin remodelling complexes, including MTA1 and HDAC2 (Manavathi et al, 2007).Second, our results show that the medaka Six3 paralogues have undergone at least partial subfunctionalisation, with a major function for Six3.1 in the eye and significant involvement of Six3.2 in telencephalon formation. Six3.2 gain-of-function did not induce the formation of ectopic retinal tissue, in contrast to the effect of Six3.1 overexpression (Loosli et al, 1999;Lopez-Rios et al, 2003), but rather appeared to recruit proliferating progenitors into the telencephalic domain.…”

Development of the hypothalamus: conservation, modification and innovation

“…Thus, their repositioning would not only alter the location of individual neuronal populations, but would also allow new combinations of genes to be co-expressed and thus create novel neuronal subtypes and circuits. In fact, such a mechanism has been demonstrated in recently diverged species of cichlid fish, where changes in Wnt regulation and activity can mimic changes in forebrain structure between species (Sylvester et al, 2010).…”

Building brains that can evolve: Challenges and prospects for evo-devo neurobiology

“…Similarly, other scientists have shown that, among a group of teleost fishes, adult differences in the size of specific forebrain areas correlate with species differences in the expression of genes involved in forebrain patterning. Indeed, manipulating the expression of these genes can recreate (i.e., phenocopy) some of the adult species differences (Sylvester et al, 2010). What remains unknown in these studies is the molecular cause of the species differences in embryonic gene expression.…”