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L. Peichl, I. Solovei, C. Lanctôt, S. Kösem, B. Joffe, T. Cremer; Rod Nuclear Architecture and Vision in the Evolution of Mammals. Invest. Ophthalmol. Vis. Sci. 2008;49(13):1263.
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© ARVO (1962-2015); The Authors (2016-present)
We have analyzed the rod chromatin arrangement in a range of mammals to elucidate the role of rod nuclear architecture for vision. In many mammals, the nuclei of rods look different from those of other retinal neurons by conventional light microscopy. Because the arrangement of chromatin is pivotal for nuclear function and highly conserved in evolution, modifications of the nuclear structure in rods are highly interesting.
The distribution of euchromatin (gene-rich, transcriptionally active) and heterochromatin (gene-poor, transcriptionally inactive) was assessed using 3D-FISH with probes for marker DNA sequences and antibodies against marker histone modifications. We studied nocturnal and diurnal rodents, artiodactyls, and primates.
In all retinal cells except rods, heterochromatin adjoined the nuclear envelope and the nucleolus, while euchromatin localized in the inner regions of the nucleus (conventional pattern). In contrast, the small rod nuclei of all studied nocturnal species had an inverted pattern with heterochromatin in the center and euchromatin as an outer shell. The rod nuclei of all diurnal species had the conventional pattern and were larger, irrespective of taxonomic position.
We suggest that the conventional pattern was altered in mammalian rods to facilitate nocturnal vision. Nocturnal retinae have high rod densities and hence thick outer nuclear layers. At the same time they critically depend on efficient light transmission through the retina. Smaller rod nuclei reduce retinal thickness, possibly improving transmission. Biophysically, the inverted nuclei with their dense heterochromatin core also may serve as series of lenses wave-guiding the light to the outer segments. Based on the general view that early mammals were nocturnal, we propose that the inverted pattern arose early in mammalian evolution, and that the conventional pattern was re-acquired by mammals that secondarily re-adopted a diurnal life style.
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