The ascidian homolog of the vertebrate homeobox gene Rx is essential for ocellus development and function

The tadpole larvae prosencephalon of the ascidian Ciona intestinalis contains a single large ventricle, along the inner walls of which lie two sensory organs: the otolith (a gravity-sensing organ) and the ocellus (a photo-sensing organ composed of a single cup-shaped pigment cell, about 20 photoreceptor cells, and three lens cells). Comparison has been drawn between the morphology and physiology of photoreceptor cells in the ascidian ocellus and the vertebrate eye. The development of vertebrate and invertebrate eyes requires the activity of several conserved genes and it is regulated by precise expression patterns and cell fate decisions common to several species. We have isolated a Ciona homeobox gene (Ci-Rx) that belongs to the paired-like class of homeobox genes. Rx genes have been identified from a variety of organisms and have been demonstrated to have a role in vertebrate eye formation. Ci-Rx is expressed in the anterior neural plate in the middle tailbud stage and subsequently in the larval stage in the sensory vesicle around the ocellus. Loss of Ci-Rx function leads to an ocellus-less phenotype that shows a loss of photosensitive swimming behavior, suggesting the important role played by Ci-Rx in basal chordate photoreceptor cell differentiation and ocellus formation. Furthermore, studies on Ci-Rx regulatory elements electroporated into Ciona embryos using LacZ or GFP as reporter genes indicate the presence of Ci-Rx in pigment cells, photoreceptors, and neurons surrounding the sensory vesicle. In Ci-Rx knocked-down larvae, neither basal swimming activity nor shadow responses develop. Thus, Rx has a role not only in pigment cells and photoreceptor formation but also in the correct development of the neuronal circuit that controls larval photosensitivity and swimming behavior. The results suggest that a Ci-Rx "retinal" territory exists, which consists of pigment cells, photoreceptors, and neurons involved in transducing the photoreceptor signals.     

Caudal eyespots on fish predators influence the inspection behaviour of Trinidadian guppies, Poecilia reticulata

The effect of the false caudal eyespots (ocelli) of fish predators on guppy inspection behaviour was tested by use of predator ( Crenicichla ) models with and without a caudal ocellus. When a false eyespot was present, inspecting guppies spent significantly less time near the tail compared to a control model without a tail eyespot.     

Synaptic organization of the cabbage looper moth ocellus

Summary Chemical and electronic synapses are present in the ocellar synaptic region of the moth, Trichoplusia ni. The chemical synapses all appear to be of the conventional type. Four different chemical synaptic contacts were observed: Receptor cell axons presynaptic to receptor cell axons, receptor cell axons presynaptic to 1st order interneurons, 1st order interneurons presynaptic to receptor cell axons, and 1st order interneurons presynaptic to 1st order interneurons. Two different types of contact made by electronic synapes were observed: Contacts between receptor cell axons and 1st order interneurons, and contacts between 1st order interneurons. The significance of this synaptic arrangement for the generation of on and off responses in the 1st order interneurons is discussed.Supported by NSF Grant BMS 75-07645 and by the VPI & SU Research Division     

昆虫单眼的结构和功能

大多数昆虫的视觉器官除了复眼外还有一些简单的小眼,称为单眼。昆虫成虫和半变态类若虫的单眼称为背单眼,位于头顶两复眼之间。背单眼在数目和结构上都有较大变化,但基本结构包括角膜晶体、一层角膜生成细胞(覆盖在角膜晶体上)、视网膜(由大约1000个感光细胞构成,视类群而不同)。背单眼对弱光比较敏感,但在图像感知方面的作用并不显著;它是一种“激发器官”,可以增加复眼的感知能力。全变态昆虫的幼虫既没有复眼也没有背单眼,但在其头部两侧有些类似复眼小眼的侧单眼。侧单眼的结构也与小眼相似,包括角膜,晶体和由一些视网膜细胞组成的视杆。侧单眼是完全变态类昆虫幼虫仅有的感光器官,与复眼一样,它们可以感知颜色、形状、距离等等。     

PORGUENIA PERUVIANA GEN. ET SP. NOV., A MARINE CENTRIC DIATOM WITH AN UNUSUAL OCELLUS1

A new diatom genus and species, Porguenia peruviana Sullivan, is described from an Eocene marine deposit from the Paracas Peninsula, Peru. The valve outline is circular and the areolation is best described as pseudoloculate; spines of any type are lacking. A ring of elongated, densely packed rimoportulae is situated on the secondary marginal ridge. Externally, each rimoportula consists of a long, flattened, fluted upper portion supported by a shorter cylindrical stalk. The processes are winged and exhibit various degrees of curvature of the major axis. Typically, six reniform ocelli of unusual structure are situated centrifugally to the ring of rimoportulae and interrupt the true marginal ridge. Because the perforation plate differs from that of all other ocellus-bearing diatoms, a new term has been introduced for this structure, the diaphoron. The placement and structure of this newly discovered “perforation plate” do not allow Porguenia to be placed in any circumscribed family, although the Triceratiaceae would appear at present to be the most closely related family.     

The anatomy of the median ocellus of Limulus

Summary The median ocellus of Limulus consists of irregular groups of large photoreceptor cells which form a cup-shaped retina around the ocellar lens. Each group is surrounded and penetrated by guanophores and glia. The photoreceptor cells have extensive rhabdomeric regions, both along infoldings of cell membranes and between cells. Five-layered junctions occur between rhabdomeric microvilli. An occasional arhabdomeric (AR) cell is associated with a group of photoreceptors. Fine dendritic branches of the AR cell penetrate the rhabdomeric regions and form five-layered junctions with photoreceptor rhabdomeres. Axons of photoreceptor cells, and of at least some AR cells, gather at the proximal side of the cup to form an optic nerve.Supported in part by NIH EY00312 and EY00377.We would like to thank Dr. W. K. Stell, Dr. A. C. Bell, and Dr. W. H. Fahrenbach for their helpful discussions.     

Body size determines eyespot size and presence in coral reef fishes

Numerous organisms display conspicuous eyespots. These eye‐like patterns have been shown to effectively reduce predation by either deflecting strikes away from nonvital organs or by intimidating potential predators. While investigated extensively in terrestrial systems, determining what factors shape eyespot form in colorful coral reef fishes remains less well known. Using a broadscale approach we ask: How does the size of the eyespot relate to the actual eye, and at what size during ontogeny are eyespots acquired or lost? We utilized publicly available images to generate a dataset of 167 eyespot‐bearing reef fish species. We measured multiple features relating to the size of the fish, its eye, and the size of its eyespot. In reef fishes, the area of the eyespot closely matches that of the real eye; however, the eyespots “pupil” is nearly four times larger than the real pupil. Eyespots appear at about 20 mm standard length. However, there is a marked decrease in the presence of eyespots in fishes above 48 mm standard length; a size which is tightly correlated with significant decreases in documented mortality rates. Above 75–85 mm, the cost of eyespots appears to outweigh their benefit. Our results identify a “size window” for eyespots in coral reef fishes, which suggests that eyespot use is strictly body size‐dependent within this group.     

Comparative study of eye defective worm ‘menashi’ and regenerating wild‐type in planarian,Dugesia ryukyuensis

Inbreeding of the sexualized planarian, Dugesia ryukyuensis, produces eye‐defective worms, menashi, in the F₁ population. To study the effects of this mutation on the eye, we observed the eye‐region of menashi using electron microscopy and compared it with the regenerating eye in wild‐type worms. The intact eye of wild‐type planarians consisted of a few pigment cells and a number of visual cells. Pigment cells containing spherically‐shaped electron‐dense melanosomes contacted each other and enclosed rhabdomes of visual cells. Rhabdomes had numerous tubular microvilli extending radially and touching the pigment cells. However, in menashi, various lengths of tubular microvilli were irregularly distributed near the pigment cells, which contained numerous electron‐lucent premelanosomes, and no adhesive structures were found between the pigment cells. The premelanosomes of menashi were equal in size to those seen after 2 days of regeneration in wild‐type planarians and were similar in maturation to those found after 3 days of regeneration in wild‐type planarian. These results suggest that menashi is defective in the mechanism(s) of developing pigment granules and constructing visual cells. These findings also suggest that pigment cells in menashi are defective in the mechanism(s) involved with cell adhesion.     

The organization of synaptic vesicles at tonically transmitting connections of locust visual interneurons

Large, second‐order neurons of locust ocelli, or L‐neurons, make some output connections that transmit small changes in membrane potential and can sustain transmission tonically. The synaptic connections are made from the axons of L‐neurons in the lateral ocellar tracts, and are characterized by bar‐shaped presynaptic densities and densely packed clouds of vesicles near to the cell membrane. A cloud of vesicles can extend much of the length of this synaptic zone, and there is no border between the vesicles that are associated with neighboring presynaptic densities. In some axons, presynaptic densities are associated with discrete small clusters of vesicles. Up to 6% of the volume of a length of axon in a synaptic zone can be occupied with a vesicle cloud, packed with 4.5 to 5.5 thousand vesicles per μm³. Presynaptic densities vary in length, from less than 70 nm to 1.5 μm, with shorter presynaptic densities being most frequent. The distribution of vesicles around short presynaptic densities was indistinguishable from that around long presynaptic densities, and vesicles were distributed in a similar way right along the length of a presynaptic density. Within the cytoplasm, vesicles are homogeneously distributed within a cloud. We found no differences in the distribution of vesicles in clouds between locusts that had been dark‐adapted and locusts that had been light‐adapted before fixation. © 2002 Wiley Periodicals, Inc. J Neurobiol 50: 93–105, 2002; DOI 10.1002/neu.10018