The eye lens protein alphaA-crystallin of the blind mole rat Spalax ehrenbergi: effects of altered functional constraints

Evidence for repeated loss of selective constraint in rhodopsin of amblyopsid cavefishes (Teleostei: Amblyopsidae)

The genetic mechanisms underlying regressive evolution-the degeneration or loss of a derived trait--are largely unknown, particularly for complex structures such as eyes in cave organisms. In several eyeless animals, the visual photoreceptor rhodopsin appears to have retained functional amino acid sequences. Hypotheses to explain apparent maintenance of function include weak selection for retention of light-sensing abilities and its pleiotropic roles in circadian rhythms and thermotaxis. In contrast, we show that there has been repeated loss of functional constraint of rhodopsin in amblyopsid cavefishes, as at least three cave lineages have independently accumulated unique loss-of-function mutations over the last 10.3 Mya. Although several cave lineages still possess functional rhodopsin, they exhibit increased rates of nonsynonymous mutations that have greater effect on the structure and function of rhodopsin compared to those in surface lineages. These results indicate that functionality of rhodopsin has been repeatedly lost in amblyopsid cavefishes. The presence of a functional copy of rhodopsin in some cave lineages is likely explained by stochastic accumulation of mutations following recent subterranean colonization.

A quantitative NMR spectroscopic examination of the flexibility of the C-terminal extensions of the molecular chaperones, αA- and αB-crystallin

The principal lens proteins αA- and αB-crystallin are members of the small heat-shock protein (sHsp) family of molecular chaperone proteins. Via their chaperone action, αA- and αB-crystallin play an important role in maintaining lens transparency by preventing crystallin protein aggregation and precipitation. αB-crystallin is found extensively extralenticularly where it is stress inducible and acts as a chaperone to facilitate general protein stabilization. The structure of either αA- or αB-crystallin is not known nor is the mechanism of their chaperone action. Our earlier (1)H NMR spectroscopic studies determined that mammalian sHsps have a highly dynamic, polar and unstructured region at their extreme C-terminus (summarized in Carver (1999) Prog. Ret. Eye Res. 18, 431). This C-terminal extension acts as a solubilizing agent for the relatively hydrophobic protein and the complex it makes with its target proteins during chaperone action. In this study, αA- and αB-crystallin were (15)N-labelled and their (1)H-(15)N through-bond correlation, heteronuclear single-quantum coherence (HSQC) NMR spectra were assigned via standard methods. (1)H-(15)N spin-lattice (T(1)) and spin-spin (T(2)) relaxation times were measured for αA- and αB-crystallin in the absence and presence of a bound target protein, reduced α-lactalbumin. (1)H-(15)N Nuclear Overhauser Effect (NOE) values provide an accurate measure, on a residue-by-residue basis, of the backbone flexibility of polypeptides. From measurement of these NOE values, it was determined that the flexibility of the extension in αA- and αB-crystallin increased markedly at the extreme C-terminus. By contrast, upon chaperone interaction of αA-crystallin with reduced α-lactalbumin, flexibility was maintained in the extension but was distributed evenly across all residues in the extension. Two mutants of αB-crystallin in its C-terminal region: (i) I159A and I161A and (ii) K175L, have altered chaperone ability (Treweek et al. (2007) PLoS One 2, e1046). Comparison of (1)H-(15)N NOE values for these mutants with wild type αB-crystallin revealed alteration in flexibility of the extension, particularly at the extremity of K175L αB-crystallin, which may affect chaperone ability.

Regressive evolution of an eye pigment gene in independently evolved eyeless subterranean diving beetles

Regressive evolution, the reduction or total loss of non-functional characters, is a fairly common evolutionary phenomenon in subterranean taxa. However, the genetic basis of regressive evolution is not well understood. Here we investigate the molecular evolution of the eye pigment gene cinnabar in several independently evolved lineages of subterranean water beetles using maximum likelihood analyses. We found that in eyeless lineages cinnabar has an increased rate of sequence evolution, as well as mutations leading to frame shifts and stop codons, indicative of pseudogenes. These results are consistent with the hypothesis that regressive evolution of eyes proceeds by random mutations, in the absence of selection, that ultimately lead to the loss of gene function in protein-coding genes specific to the eye pathway.

Adaptive loss of ultraviolet-sensitive/violet-sensitive (UVS/VS) cone opsin in the blind mole rat (Spalax ehrenbergi)

In previous studies, fully functional rod and long-wavelength-sensitive (LWS) cone photopigments have been isolated from the eye of the subterranean blind mole rat (Spalax ehrenbergi superspecies). Spalax possesses subcutaneous atrophied eyes and lacks any ability to respond to visual images. By contrast this animal retains the ability to entrain circadian rhythms of locomotor behaviour to environmental light cues. As this is the only known function of the eye, the rod and LWS photopigments are thought to mediate this response. Most mammals are dichromats possessing, in addition to a single rod photopigment, two classes of cone photopigment, LWS and ultraviolet-sensitive/violet-sensitive (UVS/VS) with differing spectral sensitivities which mediate colour vision. In this paper we explore whether Spalax is a dichromat and has the potential to use colour discrimination for photoentrainment. Using immunocytochemistry and molecular approaches we demonstrate that Spalax is a LWS monochromat. Spalax lacks a functional UVS/VS cone photopigment due to the accumulation of several deleterious mutational changes that have rendered the gene nonfunctional. Using phylogenetic analysis we show that the loss of this class of photoreceptor is likely to have arisen from the visual ecology of this species, and is not an artefact of having an ancestor which lacked a functional UVS/VS cone photopigment. We conclude that colour discrimination is not a prerequisite for photoentrainment in this species.