Researches have discovered an important factor in how the DNA of nocturnal mammals increases their capacity for seeing in the dark. Rather than being a specific expression or sequence of genes, it is believed that it is in fact the manner in which the chromatin is packed within specific cells in the eye which increases the perception of light.
The nuclear architecture of most non-dividing cells generally consists of DNA and proteins combing to form a region of more condensed “heterochromatin” surrounding a region of less condensed “euchromatin” in the interior. In the photoreceptive rod cells that were studied in mice however, this arrangement is inverted, with the heterochromatin concentrated in the middle of the nucleus. This formation was also found to be present in the rod cells of 40 other nocturnal mammals, showing a strong correlation between lifestyle and nucleic architecture.
Measurements of the interaction of these special rod cells with light show that the nuclei act as “collecting lenses” when the chromatin is packed in this way, whereas in the rod cells of diurnal mammals light is scattered. This means that when light enters a column of specially packed rod cells it is directed more efficiently towards the receptor segments, as it is passed from one nucleus to the next. As a result, nocturnal mammals are able to see in light levels one million times lower than than during the day, and their photoreceptors are believed to be sensitive down to the level of one or two photons.
