If you see a bee flitting between flowers, it's a good bet that it's relying on UV light for its deft navigation. Given that we've known for almost a century that bees can see UV, we know surprisingly little about the nature and location of UV visual pigments in bees' brains. To explore the possible functions of such pigments, Adriana Briscoe and Johannes Spaethe set out to locate UV visual pigments in bumblebees' brains().
Briscoe explains that insects' visual pigments consist of opsin proteins. If you want to find out where an opsin protein is expressed, the obvious place to look is in an insect's retina. But Briscoe knew that some butterfly visual pigments are only expressed in the butterfly's brain, not in its retina. Could the same be true for bumblebees?
Before they could tackle this question, Briscoe and Spaethe needed to identify the gene for bumblebees' UV-sensitive opsin protein. They extracted RNA from bumblebee eyes, and from it sequenced a putative UV opsin gene. To find out if their sequence really encodes a UV opsin, they searched other insect genomes for similar sequences. They found that their putative bumblebee UV opsin only produced hits to other known insect UV opsins. But was their bumblebee opsin a functional UV-sensitive visual pigment? To show that the bumblebee sequence contains a critical amino acid residue that confers UV sensitivity, Briscoe and Spaethe aligned their bumblebee sequence with the known UV opsin sequence of Drosophila. Briscoe was relieved to find that `the bumblebee sequence contained the same mutation found in Drosophila that we know is functionally important to make a UV-sensitive pigment.'
Now, Briscoe and Spaethe could probe where this UV opsin protein is expressed in the bumblebee brain. They had an anti-opsin antibody for a butterfly UV visual pigment, which they discovered also revealed UV opsin expression in bumblebees' retinas and brains. In the bumblebee retina, they found that the ommatidia – the repeating structures that make up the compound eye – come in three types; they can contain two, one or no UV opsin-expressing cells. Briscoe didn't expect to find such heterogeneity because honeybees, bumblebees' closest relatives, are thought to have just one ommatidial type. Briscoe and Spaethe also found that the UV opsin is expressed in bees' ocelli, the three simple eyes that detect polarised light and allow bees to forage until dusk. `This is the first opsin to be localised in bee ocelli,' Briscoe says.
But when Briscoe and Spaethe examined bumblebees' brains, they were in for a few more surprises. They found UV opsin expression in various brain parts,but two in particular caught their eye – the optic and antennal lobes,the regions that process vision and olfaction. Briscoe and Spaethe knew that some insects express period, a circadian clock protein, in the outer layer of the optic lobe. Curious to find out if this is also true for bumblebees, they searched for period expression in bumblebee brains. Sure enough, bumblebees express period in the optic and antennal lobes. Since period and the UV opsin are both expressed in the optic and antennal lobes, Briscoe and Spaethe conclude that UV light might play a role in bumblebee circadian rhythm regulation, mediated through these two brain regions.
