Creatively-applied chemicals can cause the brain of one fish to develop into the brain of another. Researchers reported today that they'd converted the brains of rock-dwelling fish called cichlids into those of a very different, but related, fish species.
The researchers from Georgia Tech report in Proceedings of the National Academy of Sciences that they dosed the embryos of rock-dwelling cichlids with lithium chloride. Within days, they had developed the brains of sand-dwelling cichlids. These sand-dwelling cousins of the fish have brains with very different characteristics, including extreme variations in the vision-processing regions of the brain.
According to a release from Georgia Tech, this discovery overturns some long-held theories about brain development:
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.
The sand-dwelling cichlids use their vision to detect plankton prey, so their brains are heavily devoted to integrating visual signals. However, the rock-dwelling species feed by scraping algae from rocks and possess larger cerebra, or telencephala, perhaps to aid in navigating their complex 3D environments.
"The genomes of these species are very similar," said Streelman, "almost as similar as those of any two humans, and yet their brains vary as much as some mammal groups, one from each other."
Does this mean you could treat a chimp embryo and make its brain human? Possibly - as long as you started very early in the process of development.
Full scientific paper available via PNAS