Dear Students!
Our next speaker on the 21th of February will be Rob Williams who will talk about: "Animal models of autism and schizophrenia"
You will need to read his paper called "Genetic architecture supports mosaic brain evolution and independent brain-body size regulation"!
Please post your comments not later than Tuesday afternoon!
In this paper, Hager et al. argue that 1) brain size and body size are dissociable phenomena in both development and evolution and that 2) the development of different brain parts are dissociable from the development of the brain as a whole. They present evidence from a sample of 10,000 mice to show that the development of different brain parts are regulated by different and distinct genes. These data have important evolutionary implications because they show that different brain areas can respond to selection pressure independently of other brain areas. Brain development and evolution is not a bundled package that moves as a single unit. Instead, the relative independence and functional specialization of diverse structures within the brain allows for a richer and much greater variety of ways that the organ can develop within and across species.
ReplyDeleteThe paper focused on the weight of individual brain structures that have relatively distinct anatomic boundaries. However, several of the readings in the course have highlighted the importance of the interconnectivity between different brain regions (e.g. brain networks activated in joint attention in Mundy’s work and Dehaene’s global work space theory). I am curious as to whether interconnectivity between specific brain regions can be genetically dissociated from interconnectivity as a global measure. Is the genetic mechanism responsible for the regulation of interconnectivity (or propensity for interconnectivity) a global mechanism that, when impaired, will result in lower levels of interconnectivity overall, or can certain connections/networks develop independently of other networks? For example, is the wide distributed network of neurons in Dehaene’s Global Workspace a natural consequence of greater global connectivity overall or is it highly specialized network that has been under selection pressure independent of other networks? If it is the natural consequence of greater connectivity across the board, it would imply that human consciousness from animal consciousness (as understood by Dehaene) is a matter of degree. The greater the propensity for denser interconnectivity of brain networks in a species, the more human-like consciousness the species would be likely to have. On the other hand, if those specific networks have been selected independently of other networks, the capacity for human-like consciousness would be concentrated in those species most closely related to us.
Article
ReplyDeleteThis paper describes the individual evolution of brain parts and their relation to each other in terms of growth. Authors emphasize on similar brain weights among mammals and their correlation with sizes of individual parts of brain. Two hypotheses were used as base cases to infer the underlying evolutionary process for our brain. First hypotheses place a proposition on the evolution of different brain parts versus the whole brain. Whereas the second hypotheses propose that there exists a developmental factor which affects the growth in different parts of the brain. Results for the brain and the body size regulation indicated another important aspect of brain evolution which is that the selection pressure on brain and body act independently which further points towards genes for brain and body size evolution. Since selection pressure plays an important role in the evolution of brain and its parts, therefore can cognitive abilities or better yet, intelligence be directly proportional to the brain mass?
Jessica Yoo
ReplyDeleteHagar et al. (2012) discusses a few theories on brain evolution- if different parts of brain develop independently (mosaic evolutionary hypothesis) or together as a whole (evolutionary paradigm). Both theories have been studied for evidences, but not much for mammals. The study explores the aforesaid independency of brain parts and also the relation of brain and body sizes in terms of evolution and development through scanning the genome of 10,000 mice from a long-term research data. The results indicate that the sizes of brain parts do not affect one another, as well as the whole brain size itself under selection pressure. The results also demonstrated that different genes control brain size and body size separately. Low phenotypic correlation in body and overall brain size was shown, which is in line with mosaic evolutionary hypothesis. However, it must me questioned if this results will also directly show in accordance to that of humans, because of a great variance even among classes of mammals. Also, is it possible that these "different underlying genes" may show different contributions to the growth and development of brain in people with mental disorders such as in Schizophrenia? How does this play with evolution?
Although it is well-established that the mammalian brain employs specific areas for highly specialized functions, the underlying genetic components pertaining to the evolution and development of these functional systems have yet to be determined. Therefore, hypotheses regarding mammalian brain evolution and development remain mostly inconclusive. Some suggest that the brain develops as a whole due to developmental constraints rather than the independent evolution of functional areas, yet other studies suggest the opposite is true. To provide further clarification on this debated topic, Hager, Lu, Rosen, and Williams evaluate the underlying genetic components related to size variation in specific functional brain regions as well as brain to body size ratio. Using more than 10,000 mice bred to be reflective of genetic variation in large human families, they identify distinct loci that modulate the size of brain regions as well as brain and body size. Overall, this provides evidence to support that size variation in functional areas of the brain can be regulated independently of the whole and brain size varies independently of body size further supporting a mosaic model of brain evolution.
ReplyDeleteThis study used quantitative genetics to evaluate two hypotheses regarding development and evolution of brain and body size. Specifically, the mosaic evolutionary hypothesis predicts that the size of different systems evolve independently (e.g. brain region vs. body sizes), while the concerted evolutionary paradigm states that different regions evolve predominantly as a whole due to developmental constraints. To the credit of this study, a large-scale, 15-year research effort involving approximately 10,000 mice revealed compelling evidence for the mosaic hypothesis. Regions of the brain were regulated by genes that appear to be inherited independent of overall brain size; similarly, independent loci were found for variations in body and brain size. Thus, the authors suggest that brain part size can respond to selection pressures for specific tasks on a microevolutionary level; the apparent constraints observed at the macroevolutionary level (i.e., overall brain and body size) do not limit the development of brain regions in response to selection.
ReplyDeleteAre there developmental constraints on brain size? Hager et al. (2012) used a longitudinal study of mice to investigate the relationship between overall brain size and brain to body ratios. Moreover, the researchers attempt to determine whether brain parts are affected by selection pressure individually. There are two central hypotheses regarding brain evolution. First the mosaic evolutionary hypothesis. Due to varying selection pressures on the different tasks of brain parts, the size of these systems develops independently of overall brain size and other brain parts. Under this hypothesis, there are no constraints on the evolution of different regions. Instead, each region evolves as necessary and independent of other parts. However, the concerted evolutionary paradigm would disagree. The premise of the concerted evolutionary hypothesis is that the brain evolves as a whole. In this case, there are constraints on different regions. Because evidence supported both of these hypotheses, the researchers attempt to draw a distinction between the two and look at the inner workings of genetics in brain evolution. The overall goal of this research was to provide clarification on the issue of brain evolution and how it occurs through genetics. The findings of this study suggest that brain parts may not be constrained by developmental factors.
ReplyDeleteThere are two predominant hypotheses concerning the variations in brain size among species. The mosaic evolutionary hypothesis states that the size of brain regions evolve based on selective pressures associated with the function of that system. The concerted evolution hypothesis states that regions evolve together. This study sought to determine whether regions of the brain can respond independently to selective pressures or must evolve together, and how brain to body ratios evolve. Using a high-density linkage analysis, loci for brain size and size of the seven major brain regions were mapped in ~10,000 mice. Results indicated specific loci determined size of the brain, brain regions, and body size. Independent loci were found to be associated with the brain and body ratio, indicating that different loci determine brain and body sizes. These results support the mosaic evolutionary hypothesis, suggesting that selection pressures can cause variations in brain region sizes, and can be affected independently.
ReplyDeleteGenetic architecture supports mosaic brain evolution and independent brain–body size regulation
ReplyDeleteThe authors tested an hypothesis from Finlay and Darlington argued that different brain parts of the mammals are dependent on each other in selection and evolution. The study over 10,000 mice on seven brain parts including collecting brain sizes showed low or no phenotypic correlation among these parts, and variation in brain size is independently regulated from body size. As a results from regulation of brain size part, there's no evidence from gene-by-gene interaction between loci affecting overall brain size and those affecting the size of different brain parts. These results provides a strong support for brain mosaic evolution.