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Pictured is the first comparative transcriptome analysis of the brain between amniote species (colored networks) addressing the thorniest problem in comparative neurobiology: understanding the evolution of the mammalian neocortex from its progenitor in our common ancestor with sauropsids (birds and reptiles). The branches represent the radiation of different amniotic lineages. In the background is sequence from Rorb, one member of a gene expression signature detected in functionally analogous circuits (see the article by T. G. Belgard, J. F. Montiel et al. in PNAS 2013 Jul 22). Image created by J. F. Montiel.
Pictured is the first comparative transcriptome analysis of the brain between amniote species (colored networks) addressing the thorniest problem in comparative neurobiology: understanding the evolution of the mammalian neocortex from its progenitor in our common ancestor with sauropsids (birds and reptiles). The branches represent the radiation of different amniotic lineages. In the background is sequence from Rorb, one member of a gene expression signature detected in functionally analogous circuits (see the article by T. G. Belgard, J. F. Montiel et al. in PNAS 2013 Jul 22). Image created by J. F. Montiel.

Adult pallium transcriptomes surprise in not reflecting predicted homologies across diverse chicken and mouse pallial sectors.

Belgard TG, Montiel JF, Wang WZ, García-Moreno F, Margulies EH, Ponting CP, Molnár Z. Proc Natl Acad Sci U S A. 2013 Jul 22.

The cerebral cortex is crucial for many advanced cognitive abilities of mammals. It is therefore remarkable that, although they lack a recognizable cortex, some birds possess impressive social and reasoning skills. The key brain structures required for these abilities develop from an embryonic region known as the pallium in both birds and mammals. Most subcortical structures have obvious similarities and agreed homologies. Yet, for the avian pallium and the mammalian pallium, which includes the neocortex, there are major discrepancies among models founded on cell lineage, anatomy, marker gene expression, connectivity, physiology, or behavioural properties. Studies using anatomy, development, physiology, and connectivity to infer homologies between the pallium in mammals and birds have drawn contradictory conclusions. These inconsistencies make the comparison of avian and mammalian brains the thorniest problem in evolutionary neurobiology.

By sequencing genes active in these regions, Gelgard, Montiel et al., (2013) recently published in PNAS demonstrated that these debated structures in adults of both species unexpectedly use very different genes. To uncover the evolutionary origin of the mammalian neocortex researchers have long attempted to identify the layer's counterparts in birds and reptiles.

Two groups from DPAG, lead by Zoltán Molnár and Chris Ponting performed the first comparative gene expression analysis to obtain expression profiles of 5,130 highly transcribed genes in a set of structures in the adult mouse and chicken brain that develop from one of four sectors of the pallium, the dorsal portion of a brain region known as the telencephalon. Their study report that structures that share a developmental origin but show functional divergence between birds and mammals did not exhibit greater similarity in transcription profiles than developmentally unrelated but functionally equivalent structures. However, regions such as the striatum and hippocampus, which share homology and function between birds and mammals, displayed conservation among groups of co-expressed genes. By sequencing genes active in several key regions of the avian and mammalian brain, the study provides an online portal to browse and search their data.

The authors suggest that the findings enable a molecular understanding of the development of specialized mammalian and avian brain regions. According to the authors, future studies of the evolution of the neocortex must include both gene expression data and homology studies based on cell lineages to uncover how molecular mechanisms were repurposed for neocortical evolution in mammals.