Inverted chromatin shines light on nocturnal vision

An extraordinary inversion of the chromatin structure in rod photoreceptor cells of nocturnal animals helps the cell’s nuclei to act as collecting lenses, German researchers have found.

The normal nuclear architecture of eukaryotic cells consists of heterochromatin clumping at the periphery of the nucleus while euchromatin resides in the interior.

In the rod photoreceptor cells of nocturnal animals, however, euchromatin is found at the edge of the nucleus while heterochromatin gathers in the centre.

The researchers, led by Boris Joffe of the Ludwig-Maximilians University in Munich, say that this inverted pattern is an adaptation to nocturnal vision specific to mammals.

In mice, a normal chromatin structure is observable at birth. At day six, chromatin started to change until the conventional arrangement was completely transformed by day 28.

In addition to mice, the researchers studied 16 other terrestrial mammals, finding that in all diurnal mammals the conventional structure was apparent. In all nocturnal animals, it was inverted.

Crepuscular species – those that are active at dusk and dawn – had a mixture, with deer species having an inverted structure and zebra and horses the conventional structure.

Only in the guinea pig, which the researchers say is undergoing an adaptation to a diurnal lifestyle, was there an anomaly – it still has an inverted pattern in a ‘diurnal’ retina.

The researchers say the inverted pattern is probably an adaptation to optimise light transmission through the outer nuclear layer.

Using phase contrast, they found a central dark area in the rod nucleus surrounded by a lighter rim, which corresponded remarkably to the distribution of hetero and euchromatin.

They suggest the structure reduces the scattering of light in the outer nuclear layer, allowing an advantage for vision at low-light conditions.

They conclude that the data show that the inverted pattern appeared early in the evolution of mammals, and that the conventional pattern was repeatedly reacquired in mammals that readopted a diurnal lifestyle.

The paper is published in Cell.