BackgroundTranscription factors that determine retinal development seem to be conserved in different phyla throughout the animal kingdom. In most representatives, however, only a few of the involved transcription factors have been sampled and many animal groups remain understudied. In order to fill in the gaps for the chelicerate group of arthropods, we tested the expression pattern of the candidate genes involved in the eye development in the embryo of the wandering spider Cupiennius salei. One main objective was to profile the molecular development of the eyes and to search for possible variation among eye subtype differentiation. A second aim was to form a basis for comparative studies in order to elucidate evolutionary pathways in eye development.ResultsWe screened the spider embryonic transcriptome for retina determination gene candidates and discovered that all except one of the retinal determination genes have been duplicated. Gene expression analysis shows that the two orthologs of all the genes have different expression patterns. The genes are mainly expressed in the developing optic neuropiles of the eyes (lateral furrow, mushroom body, arcuate body) in earlier stages of development (160 to 220 h after egg laying). Later in development (180 to 280 h after egg laying), there is differential expression of the genes in disparate eye vesicles; for example, Cs-otxa is expressed only in posterior-lateral eye vesicles, Cs-otxb, Cs-six1a, and Cs-six3b in all three secondary eye vesicles, Cs-pax6a only in principal eye vesicles, Cs-six1b in posterior-median, and posterior-lateral eye vesicles, and Cs-six3a in lateral and principal eye vesicles.ConclusionsPrinciple eye development shows pax6a (ey) expression, suggesting pax6 dependence, although secondary eyes develop independently of pax6 genes and show differential expression of several retinal determination genes. Comparing this with the other arthropods suggests that pax6-dependent median eye development is a ground pattern of eye development in this group and that the ocelli of insects, the median eyes of chelicerates, and nauplius eyes can be homologised. The expression pattern of the investigated genes makes it possible to distinguish between secondary eyes and principal eyes. Differences of gene expression among the different lateral eyes indicate disparate function combined with genetic drift.Electronic supplementary materialThe online version of this article (doi:10.1186/s13227-015-0010-x) contains supplementary material, which is available to authorized users.

BackgroundOpsins have been found in the majority of animals and their most apparent functions are related to vision and light-guided behaviour. As an increasing number of sequences have become available it has become clear that many opsin-like transcripts are expressed in tissues other than the eyes. Opsins can be divided into three main groups: rhabdomeric opsins (r-opsins), ciliary opsins (c-opsins) and group 4 opsins. In arthropods, the main focus has been on the r-opsins involved in vision. However, with increased sequencing it is becoming clear that arthropods also possess opsins of the c-type, group 4 opsins and the newly discovered arthropsins but the functions of these opsins are unknown in arthropods and data on their localisation is limited or absent.ResultsWe identified opsins from the spider Cupiennius salei and the onychophoran Euperipatoides kanangrensis and characterised the phylogeny and localisation of these transcripts. We recovered all known visual opsins in C. salei, and in addition found a peropsin, a c-opsin and an opsin resembling Daphnia pulex arthropsin. The peropsin was expressed in all eye types except the anterior median eyes. The arthropsin and the c-opsin were expressed in the central nervous system but not the eyes. In E. kanangrensis we found: a c-opsin; an opsin resembling D. pulex arthropsins; and an r-opsin with high sequence similarity to previously published onychophoran onychopsins. The E. kanangrensis c-opsin and onychopsin were expressed in both the eyes and the brain but the arthropsin only in the brain.ConclusionOur novel finding that opsins of both the ciliary and rhabdomeric type are present in the onychophoran and a spider suggests that these two types of opsins were present in the last common ancestor of the Onychophora and Euarthropoda. The expression of the c-opsin in the eye of an onychophoran indicates that c-opsins may originally have been involved in vision in the arthropod clade. The lack of c-opsin expression in the spider retina suggests that the role for c-opsin in vision was lost in the euarthropods. Our discovery of arthropsin in onychophorans and spiders dates the emergence of arthropsin to the common ancestor of Onychophora and Euarthropoda and their expression in the brain suggests a non-visual function.

The genes otd/otx, six3, pax6 and engrailed are involved in eye patterning in many animals. Here we describe the expression pattern of the homologs to otd/otx, six3, pax6 and engrailed in the developing Euperipatoides kanangrensis embryos. Special reference is given to the expression in the protocerebral/ocular region. E.kanangrensis otd is expressed in the posterior part of the protocerebral/ocular segment before, during, and after eye invagination. E.kanangrensis otd is also expressed segmentally in the developing ventral nerve cord. The E.kanangrensis six3 is located at the extreme anterior part of the protocerebral/ocular segment and not at the location of the developing eyes. Pax6 is expressed in a broad zone at the posterior part of the protocerebral/ocular segment but only week expression can be seen at early onset of eye invagination. In late stages of development, the expression in the eye is upregulated. Pax6 is also expressed in the invaginating hypocerebral organs, thus supporting earlier suggestions that the hypocerebral organs in onychophorans are glands. Pax6 transcripts are also present in the developing ventral nerve cord. The segment polarity gene engrailed is expressed at the dorsal side of the developing eye including only a subset of the cells of the invaginating eye vesicle. We show that engrailed is not expressed in the neuroectoderm of the protocerebral/ocular segment as in the other segments. In addition, we discuss other aspect of otd, six3 and pax6 expression that are relevant to our understanding of evolutionary changes in morphology and function in arthropods.

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Four manatees were trained to discriminate between a colored stimulus and a shade of gray in a two-fold simultaneous choice situation. The colors blue, green, red and blue-green were tested against shades of gray varying from low to high relative brightness. The animals distinguished both blue and green from a series of grays but failed to discriminate red and blue-green from certain steps of grays. The manatees could not discriminate between a UV-reflecting white target and an UV-absorbing white target. The results indicate that manatees possess color vision which is most likely dichromatic.

SUMMARYLike most other spiders Cupiennius salei (Keyserling 1877) has two different eye types, one pair of principal eyes and three pairs of secondary eyes. The principal eyes have two eye muscles each, which allow movement of the retina and are mainly used for the discrimination of stationary objects. The secondary eyes without such eye muscles are supposed to detect moving objects. Masking experiments were used to analyse the role of these two eye types in motion detection. In a white arena the animals were stimulated with short sequences of moving black bars. The principal eyes move involuntarily when objects are moving within the visual field of an ipsilateral secondary eye. The eye muscle activity of the principal eyes was recorded using single channel telemetry, and activity changes were taken as an indicator for the perception of motion. Masking the principal eyes with black paint and presenting a moving visual stimulus did not modify the induced muscle activity, whereas masking the secondary eyes eliminated the increase in eye muscle activity. This suggests that the secondary eyes are responsible for movement detection. We conclude that the animals are able to detect moving targets visually only with the secondary eyes. The principal eyes, by contrast, do not seem to be involved in the detection of moving targets.

Equine brightness discrimination ability and color discrimination were measured using a two-choice discrimination task. Two Haflinger horses (Equus caballus L., 1758) were trained to discriminate 30 different shades of grey varying from low to high relative brightness. Their ability to distinguish shades of grey was poor, with calculated Weber fractions of 0.42 and 0.45. In addition, a "neutral point" test to determine the dimensionality of color vision was carried out. Three hues of blue–green were tested versus a range of grey targets with brightnesses similar to those of the blue–green targets. A neutral point was found at about 480 nm. Thus, we can conclude that horses possess dichromatic color vision.