Polarizing optics in a spider eye

Many arthropods including insects and spiders exploit skylight polarization for navigation. One of the four eye pairs of the spider Drassodes cupreus is dedicated to detect skylight polarization. These eyes are equipped with a tapetum that strongly plane-polarizes reflected light. This effectively enhances the polarization-sensitivity of the photoreceptors, improving orientation performance. With a multidisciplinary approach, we demonstrate that D. cupreus exploits reflective elements also present in non-polarizing tapetal eyes of other species such as Agelena labyrinthica. By approximately orthogonal arrangement of two multilayer reflectors consisting of reflecting guanine platelets, the tapetum uses the mechanism of polarization by reflection for polarizing reflected ligh

Specialized ommatidia of the polarization-sensitive dorsal rim area in the eye of monarch butterflies have non-functional reflecting tapeta

Many insects exploit sky light polarization for navigation or cruising-course control. The detection of polarized sky light is mediated by the ommatidia of a small specialized part of the compound eye: the dorsal rim area (DRA). We describe the morphology and fine structure of the DRA in monarch butterflies (Danaus plexippus). The DRA consists of approximately 100 ommatidia forming a narrow ribbon along the dorsal eye margin. Each ommatidium contains two types of photoreceptor with mutually orthogonal microvilli orientations occurring in a 2:6 ratio. Within each rhabdomere, the microvilli are well aligned. Rhabdom structure and orientation remain constant at all retinal levels, but the rhabdom profiles, as seen in tangential sections through the DRA, change their orientations in a fan-like fashion from the frontal to the caudal end of the DRA. Whereas these properties (two microvillar orientations per rhabdom, microvillar alignment along rhabdomeres, ommatidial fan array) are typical for insect DRAs in general, we also report and discuss here a novel feature. The ommatidia of monarch butterflies are equipped with reflecting tapeta, which are directly connected to the proximal ends of the rhabdoms. Although tapeta are also present in the DRA, they are separated from the rhabdoms by a space of approximately 55 μm effectively inactivating them. This reduces self-screening effects, keeping polarization sensitivity of all photoreceptors of the DRA ommatidia both high and approximately equa

Anatomical Reconstruction and Functional Imaging Reveal an Ordered Array of Skylight Polarization Detectors in Drosophila

Many insects exploit skylight polarization as a compass cue for orientation and navigation. In the fruit fly, Drosophila melanogaster, photoreceptors R7 and R8 in the dorsal rim area (DRA) of the compound eye are specialized to detect the electric vector (e-vector) of linearly polarized light. These photoreceptors are arranged in stacked pairs with identical fields of view and spectral sensitivities, but mutually orthogonal microvillar orientations. As in larger flies, we found that the microvillar orientation of the distal photoreceptor R7 changes in a fan-like fashion along the DRA. This anatomical arrangement suggests that the DRA constitutes a detector for skylight polarization, in which different e-vectors maximally excite different positions in the array. To test our hypothesis, we measured responses to polarized light of varying e-vector angles in the terminals of R7/8 cells using genetically encoded calcium indicators. Our data confirm a progression of preferred e-vector angles from anterior to posterior in the DRA, and a strict orthogonality between the e-vector preferences of paired R7/8 cells. We observed decreased activity in photoreceptors in response to flashes of light polarized orthogonally to their preferred e-vector angle, suggesting reciprocal inhibition between photoreceptors in the same medullar column, which may serve to increase polarization contrast. Together, our results indicate that the polarization-vision system relies on a spatial map of preferred e-vector angles at the earliest stage of sensory processing

Circadian pacemaker coupling by multi-peptidergic neurons in the cockroach Leucophaea maderae

Lesion and transplantation studies in the cockroach, Leucophaea maderae, have located its bilaterally symmetric circadian pacemakers necessary for driving circadian locomotor activity rhythms to the accessory medulla of the optic lobes. The accessory medulla comprises a network of peptidergic neurons, including pigment-dispersing factor (PDF)-expressing presumptive circadian pacemaker cells. At least three of the PDF-expressing neurons directly connect the two accessory medullae, apparently as a circadian coupling pathway. Here, the PDF-expressing circadian coupling pathways were examined for peptide colocalization by tracer experiments and double-label immunohistochemistry with antisera against PDF, FMRFamide, and Asn13-orcokinin. A fourth group of contralaterally projecting medulla neurons was identified, additional to the three known groups. Group one of the contralaterally projecting medulla neurons contained up to four PDF-expressing cells. Of these, three medium-sized PDF-immunoreactive neurons coexpressed FMRFamide and Asn13-orcokinin immunoreactivity. However, the contralaterally projecting largest PDF neuron showed no further peptide colocalization, as was also the case for the other large PDF-expressing medulla cells, allowing the easy identification of this cell group. Although two-thirds of all PDF-expressing medulla neurons coexpressed FMRFamide and orcokinin immunoreactivity in their somata, colocalization of PDF and FMRFamide immunoreactivity was observed in only a few termination sites. Colocalization of PDF and orcokinin immunoreactivity was never observed in any of the terminals or optic commissures. We suggest that circadian pacemaker cells employ axonal peptide sorting to phase-control physiological processes at specific times of the day

Sterility and Gene Expression in Hybrid Males of Xenopus laevis and X. muelleri

BACKGROUND: Reproductive isolation is a defining characteristic of populations that represent unique biological species, yet we know very little about the gene expression basis for reproductive isolation. The advent of powerful molecular biology tools provides the ability to identify genes involved in reproductive isolation and focuses attention on the molecular mechanisms that separate biological species. Herein we quantify the sterility pattern of hybrid males in African Clawed Frogs (Xenopus) and apply microarray analysis of the expression pattern found in testes to identify genes that are misexpressed in hybrid males relative to their two parental species (Xenopus laevis and X. muelleri). METHODOLOGY/PRINCIPAL FINDINGS: Phenotypic characteristics of spermatogenesis in sterile male hybrids (X. laevis x X. muelleri) were examined using a novel sperm assay that allowed quantification of live, dead, and undifferentiated sperm cells, the number of motile vs. immotile sperm, and sperm morphology. Hybrids exhibited a dramatically lower abundance of mature sperm relative to the parental species. Hybrid spermatozoa were larger in size and accompanied by numerous undifferentiated sperm cells. Microarray analysis of gene expression in testes was combined with a correction for sequence divergence derived from genomic hybridizations to identify candidate genes involved in the sterility phenotype. Analysis of the transcriptome revealed a striking asymmetric pattern of misexpression. There were only about 140 genes misexpressed in hybrids compared to X. laevis but nearly 4,000 genes misexpressed in hybrids compared to X. muelleri. CONCLUSIONS/SIGNIFICANCE: Our results provide an important correlation between phenotypic characteristics of sperm and gene expression in sterile hybrid males. The broad pattern of gene misexpression suggests intriguing mechanisms creating the dominance pattern of the X. laevis genome in hybrids. These findings significantly contribute to growing evidence for allelic dominance in hybrids and have implications for the mechanism of species differentiation at the transcriptome level

Behavioral analysis of light intensity discrimination and spectral sensitivity in the honey bee, Apis mellifera

1. The spontaneous choice behavior of bees between two lights—a reference light of constant intensity (I_c) and a test light of variable intensity (I_v)—was tested by means of a binary choice apparatus. 2. The intensity discrimination threshold for white light was determined in an intensity range of five powers of ten. Defining 65% choice frequency as reaction threshold, an intensity difference of 14.5% is perceived at the highest intensity tested (I_c = 3.6 W/m² = relative intensity 1). This difference increases only slightly, to 18.7%, with a decrease in relative intensity to 10⁻³, but at a relative intensity of 10⁻⁴ it increases to 57.2% (Fig. 3). 3. According to these findings the least potential difference noted at receptor niveau is about 1 mV as can be determined by means of the intensity characteristic of the photoreceptors (Fig. 8). 4. The intensity discrimination sensitivity in ultraviolet, blue, and green monochromatic light was determined. No significant differences were found (Fig. 4). 5. An action spectrum, i.e., the spectral sensitivity function of the spontaneous phototactic choice behavior was determined by measuring the intensities of different monochromatic lights which are equally attractive as the constant reference light (I_c). It has maxima in the ultraviolet (350–360 nm) and in the green range (530–540 nm; Fig. 5). The relative amplitude of the maxima depends on the intensity of the reference light: The green sensitivity decreases when the intensity of the reference light is increased (Fig. 7, insert)

Behavioral analysis of light intensity discrimination and spectral sensitivity in the honey bee, Apis mellifera

1. The spontaneous choice behavior of bees between two lights—a reference light of constant intensity (I_c) and a test light of variable intensity (I_v)—was tested by means of a binary choice apparatus. 2. The intensity discrimination threshold for white light was determined in an intensity range of five powers of ten. Defining 65% choice frequency as reaction threshold, an intensity difference of 14.5% is perceived at the highest intensity tested (I_c = 3.6 W/m² = relative intensity 1). This difference increases only slightly, to 18.7%, with a decrease in relative intensity to 10⁻³, but at a relative intensity of 10⁻⁴ it increases to 57.2% (Fig. 3). 3. According to these findings the least potential difference noted at receptor niveau is about 1 mV as can be determined by means of the intensity characteristic of the photoreceptors (Fig. 8). 4. The intensity discrimination sensitivity in ultraviolet, blue, and green monochromatic light was determined. No significant differences were found (Fig. 4). 5. An action spectrum, i.e., the spectral sensitivity function of the spontaneous phototactic choice behavior was determined by measuring the intensities of different monochromatic lights which are equally attractive as the constant reference light (I_c). It has maxima in the ultraviolet (350–360 nm) and in the green range (530–540 nm; Fig. 5). The relative amplitude of the maxima depends on the intensity of the reference light: The green sensitivity decreases when the intensity of the reference light is increased (Fig. 7, insert)