Nocturnal animals face an obvious challenge: collecting enough light to see clearly in the dark. We know about many of their tricks. They have bigger eyes and wider pupils. They have a reflective layer behind their retina called the tapetum, which reflects any light that passes through back onto it. Their retinas are loaded with rod cells, which are more light-sensitive than the cone cells that allow for colour vision.
But they also have another, far less obvious adaptation – their rod cells pack their DNA in a special way that turns the nucleus of each cell into a light-collecting lens. Their unconventional distribution is shared by the rods of nocturnal mammals from mice to cats. But it’s completely opposite to the usual genome packaging in the rods of day-living animals like primates, pigs and squirrels, and indeed, in almost all other eukaryotic cells.
In our cells, massive lengths of DNA are packaged into small spaces by wrapping them around proteins. These DNA-protein unions are known as chromatin, and they come in two different forms. Euchromatin is lightly packed and resembles a string of beads. Wrapping DNA in this way puts it within easy reach of other proteins and allows its genes to be actively transcribed. But imagine scrunching up that string of beads and you get heterochromatin – a tight, condensed ball of repressed genes that proteins cannot reach.
The two forms of chromatin are found in different areas, with euchromatin spread throughout the nucleus and heterochromatin concentrated at its edges. That pattern is nigh-universal and it applies from amoebae to plants to animals. There are only a few exceptions to this rule, including a minority of single-celled species and surprisingly, the rod cells in the eyes of nocturnal mammals. Now, Irina Solovei from the Ludwig-Maximilians University in Munich had found that this inverted distribution helps these species to see in the dark.