cientists at the Georgia Institute of Technology have found that by applying chemicals to manipulate genes in a developing embryo, they've been able to change the brain of one type of cichlid fish to resemble that of another. The researchers also discovered differences in the general patterning of the brain very early in development before functional neurons form in a process known as neurogenesis. This finding is at odds with a well-held theory known as "late equals large." The research appears in the Proceedings of the National Academy of Sciences Online Early Edition beginning May 3, 2010.



In the mid 1990s, the hypothesis called "late equals large" was put forth to explain the way brains evolve across species. The brain begins as a blank slate. In early development, the anterior, or front, part of the brain is specified from the posterior, or back, part. After that, neurogenesis occurs as precursor cells mature to become neurons. These precursors can replicate endlessly, but once they become functional neurons, replication ends. The later the switch from precursors to mature neurons, the larger the brain, or brain region, becomes. The "late equals large" model holds that the brains of different species, for example humans vs. mice, are similar early in development and differ because of the later process of neurogenesis.

"We found differences in the general patterning of the brain as early as 48 hours after fertilization, before neurogenesis begins," said J. Todd Streelman, associate professor in Georgia Tech's School of Biology.

Streelman, Ph.D. student Jonathan Sylvester, and their colleagues studied brain development in six species of cichlid from Lake Malawi stock, three species from the rock-dwelling lineage and three species of their sand-dwelling cousins.
"We repeated our tests from two to four days after fertilization and found that sand-dwelling cichlids exhibited a larger expression domain of the gene wnt1, known to be an important factor in the development of the posterior brain. This correlates with a larger thalamus, a posterior forebrain structure used in the processing of vision," said Sylvester."

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