Eye evolution and the origin of Darwin's eye prototype

Hagfish to Illuminate the Developmental and Evolutionary Origins of the Vertebrate Retina

The vertebrate eye is a vital sensory organ that has long fascinated scientists, but the details of how this organ evolved are still unclear. The vertebrate eye is distinct from the simple photoreceptive organs of other non-vertebrate chordates and there are no clear transitional forms of the eye in the fossil record. To investigate the evolution of the eye we can examine the eyes of the most ancient extant vertebrates, the hagfish and lamprey. These jawless vertebrates are in an ideal phylogenetic position to study the origin of the vertebrate eye but data on eye/retina development in these organisms is limited. New genomic and gene expression data from hagfish and lamprey suggest they have many of the same genes for eye development and retinal neurogenesis as jawed vertebrates, but functional work to determine if these genes operate in retinogenesis similarly to other vertebrates is missing. In addition, hagfish express a marker of proliferative retinal cells (Pax6) near the margin of the retina, and adult retinal growth is apparent in some species. This finding of eye growth late into hagfish ontogeny is unexpected given the degenerate eye phenotype. Further studies dissecting retinal neurogenesis in jawless vertebrates would allow for comparison of the mechanisms of retinal development between cyclostome and gnathostome eyes and provide insight into the evolutionary origins of the vertebrate eye.

Slowly seeing the light: an integrative review on ecological light pollution as a potential threat for mollusks

Seasonal changes in the natural light condition play a pivotal role in the regulation of many biological processes in organisms. Disruption of this natural condition via the growing loss of darkness as a result of anthropogenic light pollution has been linked to species-wide shifts in behavioral and physiological traits. This review starts with a brief overview of the definition of light pollution and the most recent insights into the perception of light. We then go on to review the evidence for some adverse effects of ecological light pollution on different groups of animals and will focus on mollusks. Taken together, the available evidence suggests a critical role for light pollution as a recent, growing threat to the regulation of various biological processes in these animals, with the potential to disrupt ecosystem stability. The latter indicates that ecological light pollution is an environmental threat that needs to be taken seriously and requires further research attention.

Taxon-restricted genes at the origin of a novel trait allowing access to a new environment

Taxon-restricted genes make up a considerable proportion of genomes, yet their contribution to phenotypic evolution is poorly understood. We combined gene expression with functional and behavioral assays to study the origin and adaptive value of an evolutionary innovation exclusive to the water strider genus Rhagovelia: the propelling fan. We discovered that two taxon-restricted genes, which we named geisha and mother-of-geisha, specifically control fan development. geisha originated through a duplication event at the base of the Rhagovelia lineage, and both duplicates acquired a novel expression in a specific cell population prefiguring fan development. These gene duplicates played a central role in Rhagovelia's adaptation to a new physical environment, demonstrating that the evolution of taxon-restricted genes can contribute directly to evolutionary novelties that allow access to unexploited ecological niches.

Ancient genetic redundancy of eyeless and twin of eyeless in the arthropod ocular segment

Pax6 transcription factors are essential upstream regulators in the developing anterior brain and peripheral visual system of most bilaterian animals. While a single homolog is in charge of these functions in vertebrates, two Pax6 genes are in Drosophila: eyeless (ey) and twin of eyeless (toy). At first glance, their co-existence seems sufficiently explained by their differential involvement in the specification of two types of insect visual organs: the lateral compound eyes (ey) and the dorsal ocelli (toy). Less straightforward to understand, however, is their genetic redundancy in promoting defined early and late growth phases of the precursor tissue to these organs: the eye-antennal imaginal disc. Drawing on comparative sequence, expression, and gene function evidence, I here conclude that this gene regulatory network module dates back to the dawn of arthropod evolution, securing the embryonic development of the ocular head segment. Thus, ey and toy constitute a paradigm to explore the organization and functional significance of longterm conserved genetic redundancy of duplicated genes. Indeed, as first steps in this direction, recent studies uncovered the shared use of binding sites in shared enhancers of target genes that are under redundant (string) and, strikingly, even subfunctionalized control by ey and toy (atonal). Equally significant, the evolutionarily recent and paralog-specific function of ey to repress the transcription of the antenna fate regulator Distal-less offers a functionally and phylogenetically well-defined opportunity to study the reconciliation of shared, partitioned, and newly acquired functions in a duplicated developmental gene pair.

Immunolocalization of Arthropsin in the Onychophoran Euperipatoides rowelli (Peripatopsidae)

Opsins are light-sensitive proteins that play a key role in animal vision and are related to the ancient photoreceptive molecule rhodopsin found in unicellular organisms. In general, opsins involved in vision comprise two major groups: the rhabdomeric (r-opsins) and the ciliary opsins (c-opsins). The functionality of opsins, which is dependent on their protein structure, may have changed during evolution. In arthropods, typically r-opsins are responsible for vision, whereas in vertebrates c-opsins are components of visual photoreceptors. Recently, an enigmatic r-opsin-like protein called arthropsin has been identified in various bilaterian taxa, including arthropods, lophotrochozoans, and chordates, by performing transcriptomic and genomic analyses. Since the role of arthropsin and its distribution within the body are unknown, we immunolocalized this protein in a representative of Onychophora – Euperipatoides rowelli – an ecdysozoan taxon which is regarded as one of the closest relatives of Arthropoda. Our data show that arthropsin is expressed in the central nervous system of E. rowelli, including the brain and the ventral nerve cords, but not in the eyes. These findings are consistent with previous results based on reverse transcription PCR in a closely related onychophoran species and suggest that arthropsin is a non-visual protein. Based on its distribution in the central brain region and the mushroom bodies, we speculate that the onychophoran arthropsin might be either a photosensitive molecule playing a role in the circadian clock, or a non-photosensitive protein involved in olfactory pathways, or both.

Evolution of clitellate phaosomes from rhabdomeric photoreceptor cells of polychaetes - a study in the leech Helobdella robusta (Annelida, Sedentaria, Clitellata)

INTRODUCTION

In Annelida two types of photoreceptor cells (PRCs) are regarded as generally present, rhabdomeric and ciliary PRCs. In certain taxa, however, an additional type of PRC may occur, the so called phaosomal PRC. Whereas the former two types of PRCs are always organized as an epithelium with their sensory processes projecting into an extracellular cavity formed by the PRCs and (pigmented) supportive cells, phaosomes are seemingly intracellular vacuoles housing the sensory processes. Phaosomal PRCs are the only type of PRC found in one major annelid group, Clitellata. Several hypotheses have been put forward explaining the evolutionary origin of the clitellate phaosomes. To elucidate the evolution of clitellate PRC and eyes the leech Helobdella robusta, for which a sequenced genome is available, was chosen.

RESULTS

TEM observations showed that extraocular and ocular PRCs are structurally identical. Bioinformatic analyses revealed predictions for four opsin genes, three of which could be amplified. All belong to the rhabdomeric opsin family and phylogenetic analyses showed them in a derived position within annelid opsins. Gene expression studies showed two of them expressed in the eye and in the extraocular PRCs. Polychaete eye-typic key enzymes for ommochromme and pterin shading pigments synthesis are not expressed in leech eyes.

CONCLUSIONS

By comparative gene-expression studies we herein provide strong evidence that the phaosomal PRCs typical of Clitellata are derived from the rhabdomeric PRCs characteristic for polychaete adult eyes. Thus, they represent a highly derived type of PRC that evolved in the stem lineage of Clitellata rather than another, primitive type of PRC in Metazoa. Evolution of these PRCs in Clitellata is related to a loss of the primary eyes and most of their photoreceptive elements except for the rhabdomeric PRCs. Most likely this happened while changing to an endobenthic mode of life. This hypothesis of PRC evolution is in accordance with a recently published phylogeny of Annelida based on phylogenomic data. The data provide a nice example how morphologically highly divergent light sensitive structures emerged from a standard type of photoreceptor cell.

Cryptochrome in sponges: a key molecule linking photoreception with phototransduction

Sponges (phylum: Porifera) react to external light or mechanical signals with contractile or metabolic reactions and are devoid of any nervous or muscular system. Furthermore, elements of a photoreception/phototransduction system exist in those animals. Recently, a cryptochrome-based photoreceptor system has been discovered in the demosponge. The assumption that in sponges the siliceous skeleton acts as a substitution for the lack of a nervous system and allows light signals to be transmitted through its glass fiber network is supported by the findings that the first spicules are efficient light waveguides and the second sponges have the enzymatic machinery for the generation of light. Now, we have identified/cloned in Suberites domuncula two additional potential molecules of the sponge cryptochrome photoreception system, the guanine nucleotide-binding protein β subunit, related to β-transducin, and the nitric oxide synthase (NOS)-interacting protein. Cryptochrome and NOSIP are light-inducible genes. The studies show that the NOS inhibitor L-NMMA impairs both morphogenesis and motility of the cells. Finally, we report that the function of primmorphs to produce reactive nitrogen species can be abolished by a NOS inhibitor. We propose that the sponge cryptochrome-based photoreception system, through which photon signals are converted into radicals, is coupled to the NOS apparatus.

Conserved role for the Dachshund protein with Drosophila Pax6 homolog Eyeless in insulin expression

Members of the insulin family peptides have conserved roles in the regulation of growth and metabolism in a wide variety of metazoans. The Drosophila genome encodes seven insulin-like peptide genes, dilp1-7, and the most prominent dilps (dilp2, dilp3, and dilp5) are expressed in brain neurosecretory cells known as "insulin-producing cells" (IPCs). Although these dilps are expressed in the same cells, the expression of each dilp is regulated independently. However, the molecular mechanisms that regulate the expression of individual dilps in the IPCs remain largely unknown. Here, we show that Dachshund (Dac), which is a highly conserved nuclear protein, is a critical transcription factor that specifically regulates dilp5 expression. Dac was strongly expressed in IPCs throughout development. dac loss-of-function analyses revealed a severely reduced dilp5 expression level in young larvae. Dac interacted physically with the Drosophila Pax6 homolog Eyeless (Ey), and these proteins synergistically promoted dilp5 expression. In addition, the mammalian homolog of Dac, Dach1/2, facilitated the promoting action of Pax6 on the expression of islet hormone genes in cultured mammalian cells. These observations indicate the conserved role of Dac/Dach in controlling insulin expression in conjunction with Ey/Pax6.

Gene regulatory networks reused to build novel traits: co-option of an eye-related gene regulatory network in eye-like organs and red wing patches on insect wings is suggested by optix expression

Co-option of the eye developmental gene regulatory network may have led to the appearance of novel functional traits on the wings of flies and butterflies. The first trait is a recently described wing organ in a species of extinct midge resembling the outer layers of the midge's own compound eye. The second trait is red pigment patches on Heliconius butterfly wings connected to the expression of an eye selector gene, optix. These examples, as well as others, are discussed regarding the type of empirical evidence and burden of proof that have been used to infer gene network co-option underlying the origin of novel traits. A conceptual framework describing increasing confidence in inference of network co-option is proposed. Novel research directions to facilitate inference of network co-option are also highlighted, especially in cases where the pre-existent and novel traits do not resemble each other.