The posterior median eyes of the dinopid spider Menneus

Evolution and development of complex eyes: a celebration of diversity

For centuries, the eye has fascinated scientists and philosophers alike, and as a result the visual system has always been at the forefront of integrating cutting-edge technology in research. We are again at a turning point at which technical advances have expanded the range of organisms we can study developmentally and deepened what we can learn. In this new era, we are finally able to understand eye development in animals across the phylogenetic tree. In this Review, we highlight six areas in comparative visual system development that address questions that are important for understanding the developmental basis of evolutionary change. We focus on the opportunities now available to biologists to study the developmental genetics, cell biology and morphogenesis that underlie the incredible variation of visual organs found across the Metazoa. Although decades of important work focused on gene expression has suggested homologies and potential evolutionary relationships between the eyes of diverse animals, it is time for developmental biologists to move away from this reductive approach. We now have the opportunity to celebrate the differences and diversity in visual organs found across animal development, and to learn what it can teach us about the fundamental principles of biological systems and how they are built.

Molecular Evolution of Spider Vision: New Opportunities, Familiar Players

Spiders are among the world's most species-rich animal lineages, and their visual systems are likewise highly diverse. These modular visual systems, composed of four pairs of image-forming "camera" eyes, have taken on a huge variety of forms, exhibiting variation in eye size, eye placement, image resolution, and field of view, as well as sensitivity to color, polarization, light levels, and motion cues. However, despite this conspicuous diversity, our understanding of the genetic underpinnings of these visual systems remains shallow. Here, we review the current literature, analyze publicly available transcriptomic data, and discuss hypotheses about the origins and development of spider eyes. Our efforts highlight that there are many new things to discover from spider eyes, and yet these opportunities are set against a backdrop of deep homology with other arthropod lineages. For example, many (but not all) of the genes that appear important for early eye development in spiders are familiar players known from the developmental networks of other model systems (e.g., Drosophila). Similarly, our analyses of opsins and related phototransduction genes suggest that spider photoreceptors employ many of the same genes and molecular mechanisms known from other arthropods, with a hypothesized ancestral spider set of four visual and four nonvisual opsins. This deep homology provides a number of useful footholds into new work on spider vision and the molecular basis of its extant variety. We therefore discuss what some of these first steps might be in the hopes of convincing others to join us in studying the vision of these fascinating creatures.

Post-embryonic development of the principal retina of a jumping spider. II. The acquisition and reorganization of rhabdomeres and growth of the glial matrix

An accompanying paper by Blest ( Phil. Trans. R. Soc. Lond. B 320, 489 (1988 b )) describes the conformational changes undergone by principal retinae of a salticid spider during post-embryonic development, and identifies the origins of the tiered receptor mosaics. We now discuss the ultrastructural morphogenesis of the individual receptive segments, with particular reference to the growth of microvilli, to the achievement of light-guide properties by rhabdomeres of the foveal mosaic of Layer I, and to putative mechanisms that ensure that inter-receptor spacings are optically apt. 1. Establishment of the tiered retina by conformational changes precedes the first appearance of plasmalemmal microvilli in all four Layers of receptive segments. Microvilli are first differentiated in the ventral retina. 2. Short, irregular microvilli, initially interdigitated, appear on the plasmalemmae of receptive segments around day 10 of post-embryonic development. They progressively lengthen, cease to be inter-digitated, and become more regular throughout subsequent development, during the moult to the second instar at day 12, and until day 15 when spiderlings become fully independent and principal retinae attain their final conformation. 3. Until late in development, after the moult to the second instar, foveal Layer I receptive segments are contiguous, and densely occupied by organelles. The six glial strands that flank each receptive segment are prominent. A transformation between days 14 and 15 variously reduces or eliminates the glial columns, and, by endocytosis, the plasmalemmal areas of the receptive segments. The Layer I cytoplasm is stripped of organelles other than microtubules. Consequently, receptive segments become separated from each other, and their rhabdomeres can perform as light-guides. 4. Over the same period, receptive segments of Layers II-IV slowly acquire the dense population of mitochondria that have been shown to equilibrate the refractive indices of rhabdomeres and their surrounds. 5. Final conformational changes to the retinae and the establishment of definitive spacings between receptive segments seem to be assured by the differential growth and local involution of the finely divided glial processes intercalated between them. Despite some caveats, it is argued that this protracted sequence of events is consistent with a limited degree of ontogenetic recapitulation. In particular, it is remarkable that the early states of the nascent microvilli so closely resemble those found in less advanced families of spiders.

Morphological correlates of visual pigment turnover in photoreceptors of the fly, Calliphora erythrocephala

The content of visual pigment in one eye of Calliphora 'chalky' was measured spectrophotometrically and related to the ultrastructure of the other eye and the density of membrane particles as revealed by the freeze-fracture technique. Electron microscopy of thin-sectioned material showed that the manifestations of synthesis and breakdown of photoreceptor membrane were most prominent in flies kept in blue light, in comparison to flies kept in green light or darkness, in which only a moderate breakdown of membrane was evident. By subjecting flies to different light regimes it was found that the density of membrane particles was related to the content of visual pigment, but not on a one-to-one basis. In particular, the particle density in flies with a low (less than 10%) rhodopsin content, produced by raising flies on a vitamin A-deficient diet or by exposure to green light, was still about 35% of that of normal flies. Taken together the results indicate that all rhodopsin molecules are particles but that some particles represent another protein that most probably serves to maintain the structural integrity of the photoreceptor membrane. Furthermore, membrane synthesis can take place in the absence of rhodopsin synthesis.

Ommatidial structure in relation to turnover of photoreceptor membrane in the locust

In the compound eye of the locust, Locusta, the cross-sectional area of the rhabdoms increases at "dusk" by 4.7-fold due to the rapid assembly of new microvillar membrane, and decreases at "dawn" by a corresponding amount as a result of pinocytotic shedding from the microvilli. The rhabdoms at night have more and longer photoreceptor microvilli than rhabdoms during the day. The orientations of the six rhabdomeres that comprise the distal rhabdom also change. The density of intramembrane particles on the P-face of the microvillar membrane, putatively representing mostly rhodopsin molecules, or aggregates thereof, does not change. An alteration in the size of the ommatidial field-stop, produced by the primary pigment cells, is concomitant with the change in rhabdom size. At night the increase in size of the field-stop must widen the angular acceptance of a rhabdom, increasing the capture of photons from an extended field. Conversely, during the day, when photons are more abundant, its decrease must narrow the acceptance angle, increasing angular resolution. Because of the presence of this field-stop, the optics of the ommatidium would not be greatly affected if the rhabdom were to remain always at its night size. It is argued, therefore, that the variable-size rhabdom must have resulted from some demand other than that of light/dark adaptation. Changes in size and organisation of the rhabdoms in response to various light regimes indicate that: (1) Rapid shedding of photoreceptor membrane is induced by the onset of light, but shedding also occurs slowly in darkness during the day. (2) Microvillar assembly is initiated by the onset of darkness, but also occurs at the normal time of dusk without a change in ambient lighting, provided there has been some light during the day. Therefore, both shedding and assembly of microvillar membrane are affected by the state of illumination, but also appear to be under some endogenous control.