Does a Flea See??
An Invesitgation of Flea Opsin (Flopsin)
Nature uses visual cues to fulfill a multitude of roles in communicating with the environment. Common uses include mating, mimicry, individual communication, prey allocation, defense and survival.
Visual pigments are responsible for conferring visual perceptual abilities to animals. Various pigments are tuned to absorb light of different wavelengths and to provide contrast and movement perception. All pigments consist of an opsin protein and a chromophore molecule, usually a carotene derivative. Interactions between the chromophore and amino acid residues of opsin fine-tune visual pigments for the absorption of different wavelengths. Opsin variants can tune visual pigments to absorb wavelengths in the ultraviolet, blue, green, and red regions of the spectrum.
Insects represent a group of organisms in which visual perception and the associated structural components are highly developed. Siphonaptera (fleas), in contrast, show a transformation or a complete loss of the multifaceted eyes and ommatidia (simple eyes) of most insects. Some fleas, for instance, exhibit an eye “spot”, however, SEM studies show that no ommatidia are present.
The extent of visual perception exhibited by these parasites remains unclear. However, behavioral studies proved that certain circadian rhythms related to sexual behavior and feeding habits are not only influenced by the activity patterns of the prospective hosts but also the environmental photoperiodic cycle (Amin 1970 and Kheisin and Lavrenko 1956). Thus, light sensitivity appears to be correlated to host finding behavior. No formal study concerning the presence of opsin genes in Siphonaptera has been attempted.
Whiting (2002) recently demonstrated that Siphonaptera is closely related to Boreidae (snow fleas), and more distantly related to Panorpidae (scorpionflies). Scorpionflies have well developed eyes and are highly visually oriented. Snow fleas, on the other hand, have only limited visual perception. These insects represent a unique opportunity to study the correlation between opsin gene evolution, eye structure, and visual acuity.
We have successfully amplified and sequenced the long-wavelength opsin gene from six individuals of fleas representing five families, and six individuals of scorpionflies. With just a brief comparison of the DNA sequence, it was apparent that the flea sequence was significantly different from that of the scorpionflies. The fleas have several unique long inserts of DNA present in the gene. However, these could easily be explained as introns, segments of DNA removed prior to coding of a new protein. By removing these inserts and translating the DNA code into a protein sequence, the flea opsin showed an open reading frame (no mutations giving a premature stop signal) and a remarkable similarity to the scorpionfly protein sequence. Interestingly, however, when put in the context of a phylogenetic tree along with other major insect lineages, the flea opsin showed less similarity to the scorpionfly opsin than did other more distantly related insects, such as grasshoppers, bees, and moths.
These data suggest several interesting conclusions that deserve further exploration. First, the presence of an intact open reading frame in the opsin sequence suggests that the opsin has some retained functionality and confirms the hypothesis that fleas do posses some visual acuity or at least some photosensitivity. We assume therefore that it is most likely functioning in the vestigal eye. However, it is also possible that it may be used elsewhere in some unique photosystem developed by these parasites. This project opens new questions that can now be explored with more in depth investigation of the biochemistry and biology of the flea photosystems.