Retinal photoreception in southern hemisphere lampreys

[Truncated] The visual systems of animals appear to be specialised to the photic environment they inhabit and the behavioural tasks they perform. Presumably, these specialisations have been evolutionarily selected for to improve traits such as acuity, sensitivity, motion detection, object recognition and spectral sensitivity, allowing the animal to perceive the world around them. These adaptations include, but are not limited to, morphological, biochemical and physiological modifications within the eye.

The study reported in this thesis utilises a multidisciplinary approach to characterise the eyes of the southern hemisphere lampreys and relate specialisations in ocular and retinal structure and function, to behaviour and to the photic environments inhabited during the adult phase. This study focuses on the visual pigments of retinal photoreceptors in three species, Mordacia praecox, Mordacia mordax and Geotria australis. M. praecox and M. mordax exhibit differences in their feeding strategies and habitat following metamorphosis. These species are also unusual as they maintain the larval burrowing habit into adult life and are nocturnal. In contrast, G. australis is the only species known to date that occupies the brightly lit surface waters of the ocean during its parasitic phase.

Studying aspects of the visual system of lampreys provides a unique opportunity to assess the evolution of vertebrate vision. Lampreys are only one of two extant members (along with hagfishes) of a group which diverged from the lineage leading to jawed vertebrates, around the time when image forming eyes emerged in vertebrates.

The visual spectral sensitivity of M. praecox (after reaching maturity) and M. mordax (in the downstream migrant phase) was investigated using electroretinography (ERG) (Chapter 2). The application of strong monochromatic adaptation light produced clear shifts in spectral sensitivity for M. praecox but not M. mordax, indicating the presence of two spectrally distinct classes of photoreceptor in M. praecox and one in M. mordax. The spectral sensitivity of both species was affected by the presence of a yellow-green tapetum, which reflects long-wavelengths thereby enhancing long-wavelength sensitivity. The generation of an eye transcriptome by Next-Generation Sequencing revealed that both species express two visual opsins, lws (high tag count) and rh1 (very low tag count). Comparing the data obtained from ERGs and Next-Generation Sequencing suggests that the dominant spectral class in both species contains the lws pigment (based on vitamin A2 in M. praecox and a vitamin A1 in M. mordax), which is considerably short-wavelength shifted in comparison to the ancestral lws pigment. The minor spectral class in M. praecox was harder to define, as its contribution was low and strongly masked by the effects of the tapetum. Two explanations are therefore presented, the first is that two pigments are expressed, lws and rh1 (based on vitamin A2), and the second a paired pigment system is present with the minor spectral class expressing lws based on vitamin A1, although the rh1 pigment may not be expressed at a sufficiently biologically significant level to contribute to the spectral sensitivity measured by ERGs. The results suggest that M. praecox has the potential for dichromatic colour vision, whereas M. mordax is likely to be monochromatic (as spectral sensitivity was unaffected by strong adaptation light) at the developmental stage assessed.