Most people are lucky to encounter one surprise during the course of their research, but when Adriana Briscoe began investigating opsin gene duplication in butterfly eyes, she hit the surprise-jackpot. All Briscoe knew when she began investigating the expression of photopigment genes (opsins) was that the eyes of Lycaena rubidus butterflies expressed four photopigments,rather than the three found in most other butterflies. Her long-time colleague, Gary Bernard, had also found that the distributions of these photopigments were different between the sexes. From this starting point Briscoe, Bernard and the rest of her team decided to clone the genes(p. 3079) to find out whether they were dealing with a gene duplication or an allele (slightly different copies of the same gene on different chromosomes).
Extracting mRNA from butterfly eyes, Marilou Sison-Mangus cloned all four butterfly eye opsin genes, and could clearly see that the extra opsin wasn't an allele; one of the other three regular opsin genes had been duplicated to give rise to the extra photopigment. But which one? Briscoe explains that insect eyes usually express one ultraviolet (UV)-sensitive pigment, one blue-sensitive pigment and a long wavelength sensitive pigment. In most cases,it's the long wavelength gene that has doubled up. But when Briscoe and the team aligned the butterfly's gene sequences, they realised this couldn't be the case. The extra gene had all the hallmarks of a blue opsin: surprise number one.
But which blue gene gave rise to which blue pigment? Knowing that the photopigments' distributions were different in the male and female's eyes,Briscoe decided to match the photopigments' locations with the gene expression patterns to identify the gene that was responsible for the extra blue photopigment. Bernard mapped the photopigment distributions and found that the only blue opsin that occurred in the dorsal regions of both male and female eyes was the opsin tuned to 437 nm. Next, Marilou Sison-Mangus painstakingly explored each gene's expression pattern with RNA probes and identified the blue genes responsible for the 437 nm and 500 nm photopigments.
The gene mapping also threw up the second and third surprises. Firstly, the opsin gene expression patterns in the dorsal region of the male's eye were unique and unlike the patterns in any other butterflies' eyes, and secondly,some visual receptors in the dorsal region of the female's eye expressed two opsins simultaneously in a single cell. No one had ever seen a receptor cell expressing both blue and long wavelength opsins before; usually they only express one.
Finally, Briscoe explains that in most butterfly eyes each ommatidium is composed of 9 photoreceptor cells; 2 of the 9 cells (R1 and R2) express either the UV or the blue opsin, while the remaining 6 or 7 express the long wavelength opsin. The fourth surprise in this roller coaster ride came when Briscoe realised that by expressing different combinations of the two blue opsins and UV opsin in the R1 and R2 cells of the ventral eye L. rubidus has increased the number of ommatidia from the three found in most butterfly eyes, to six. Briscoe suggests that this increased colour sensitivity, coupled with the early evolution of the second blue opsin gene,could have driven many lycaenid butterflies to evolve their startlingly blue wings.
