New insight into primate eye evolution
Published: Monday, May 18, 2009 - 16:51 in Biology & Nature
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Researchers comparing
the fetal development of the eye of the owl monkey with that of the
capuchin monkey have found that only a minor difference in the timing
of cell proliferation can explain the multiple anatomical differences
in the two kinds of eyes. The findings help scientists understand how a
structure as complex as the eye could change gradually through
evolution, yet remain functional. The findings also offer a lesson in
how seemingly simple genetic changes in the brain and nervous system
could produce the multiple evolutionary changes seen in more advanced
brains, without compromising function.
Analysis for this study was performed at St. Jude Children's
Research Hospital. The primates were housed at the Centro Nacional de
Primates in Brazil. Contributing researchers at Cornell University and
Universidade Federal do Pará, Brazil, approved all procedures. The
researchers published their findings in the early online issue of Proceedings
of the National Academy of Sciences.
"The molecular, cellular and genetic pathways that coordinate
proliferation during development have been fine-tuned since the first
multicellular organisms emerged millions of years ago," said Michael
Dyer, Ph.D., member of St. Jude Developmental Neurobiology and the
paper's first author. "When these pathways are deregulated during human
development, one of the consequences is childhood cancer. Therefore, by
studying how changes in the regulation of proliferation during
development can lead to dramatic changes in form and function during
evolution, we can gain a deeper understanding of these ancient pathways
that lie at the heart of many pediatric cancers."
The owl monkey's eye has numerous adaptations to make it effective
for nocturnal (active during the night) function. For example, it has a
greater number of rod photoreceptor cells than the capuchin monkey,
which is diurnal (active during the day). Rod cells are the most
light-sensitive cells in the retina making them effective for nighttime
vision. The owl monkey's nocturnal retina is also larger and lacks a
fovea, the central region of high-density cone photoreceptors that
gives the diurnal eye high acuity and daytime color vision.
For both owl and capuchin monkeys, the specialized cell types in the
eye all develop in the growing embryo from a single type of immature
cell, called a retinal progenitor cell.
"These two species evolved about 15 million years ago from a common
ancestor that had a diurnal eye," Dyer said. "So, we believe that
comparing how their eyes develop during embryonic growth could help us
understand what evolutionary changes would be required to evolve from a
diurnal to a nocturnal eye."
The researchers hypothesized that only speeding up or slowing down
the proliferation of the progenitor cells in the developing embryo
might actually change the types of cells that they became. Thus, the
evolutionary adaptation from diurnal to nocturnal eye might require no
more than a modest genetic change that affected that timing.
Such a concept—that timing of cell proliferation might profoundly
affect anatomy—has broader implications for understanding how the
complex human brain evolved from simpler mammalian brains, Dyer said.
In earlier comparative studies of the brains of more than 100 mammalian
species, the study's first author, Barbara Finlay, Ph.D., of Cornell
University, Ithaca, New York, had found that those parts of the brain
that are disproportionately larger in more complex brains develop last
during embryonic growth.
"This finding suggested that changes in the growth of the brain
during embryonic development could be a mechanism for evolutionary
change," Dyer said. "In other words, maybe the parts of the human brain
that are bigger than in other mammals are bigger simply because the
period of their growth is extended during fetal development."
In their analysis, Dyer and his colleagues compared the timing of
retinal progenitor cell proliferation into the different types of
mature retinal cells in owl and capuchin monkey embryos. They found
evidence that the extended period of progenitor cell proliferation in
the owl monkey eye did, indeed, give rise to the different population
of retinal cells that made the eye specially adapted for nocturnal
vision.
They also found evidence that this extended period of proliferation
also caused the size of the eye to be larger, which is necessary for
the eye to accommodate the larger light-gathering and light-sensing
structures necessary for nocturnal vision.
"The beauty of the evolutionary mechanism we have identified is that
it enables the eye to almost toggle back and forth between a nocturnal
and a diurnal structure," Dyer said. "It is an elegant system that
gives the eye a lot of flexibility in terms of specialization."
More broadly, Dyer said, the finding offers support for the idea
that important changes in brain structure can evolve via simple genetic
changes that affect the timing of development of brain regions.
Source: St. Jude Children's Research Hospital
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