Proximate and ultimate insights in the evolution of color vision in tropical freshwater fish

Evolutionary biology aims to understand diversity and the different mechanisms shaping this organismal variation. Furthermore, several animals vary greatly in coloration patterns and the adaptive mechanisms they have to optimally perceive visual signals in their light environment. The visual system of fish, due to their extensive variation in spectral sensitivities and their numerous adaptations to the underwater light environment, offers a unique opportunity to disentangle this phenotypic diversity. Throughout this dissertation, I analyze the visual systems of two major groups of Neotropical teleosts: cichlids and characins. Through transcriptome, genome and physiological experiments, I characterized the extant opsin gene complements of their visual system, which is a product of highly dynamic opsin gene evolution, and their color vision, which is based on the expression of at least three spectrally different visual pigments. The diversity of visual pigments found in these fish is the product of several spectral tuning mechanisms, which they use to fine-tune their spectral sensitivities to specific wavelengths. Our results follow the sensitivity hypothesis because the visual sensitivities of cichlids and characins match the available light in Neotropical ecosystems. Furthermore, through behavioral assays complemented with visual modeling, I show that African cichlids possess true color vision, the capacity of discriminating color regardless of brightness. This is followed by behavioral experiments analyzing the limits of their chromatic discrimination and discussing the adaptive significance of color vision and its relevance in the visual ecology of Lake Malawi. This dissertation enhances our understanding of color vision in freshwater fish using molecular and behavioral methods. This work encompasses experiments analyzing the genetic complement of visual pigments, builds knowledge in the evolution of these molecules and their relationship with aquatic environments, and analyzes the color dimensionality of visual systems through behavioral trials. Overall, this dissertation demonstrates the evolution of fish color vision with several methodologies highlighting the importance of an integrative and comparative approach in vision research

The geometry of colour

This thesis explores the geometric description of animal colour vision. It examines the relationship of colour spaces to behavior and to physiology. I provide a derivation of, and explore the limits of, geometric spaces derived from the notion of risk and uncertainty aversion as well as the geometric objects that enumerate the variety of achievable colours. Using these principles I go on to explore evolutionary questions concerning colourfulness, such as aposematism, mimicry and the idea of aesthetic preference

Computing Chromatic Adaptation

Most of today’s chromatic adaptation transforms (CATs) are based on a modified form of the von Kries chromatic adaptation model, which states that chromatic adaptation is an independent gain regulation of the three photoreceptors in the human visual system. However, modern CATs apply the scaling not in cone space, but use “sharper” sensors, i.e. sensors that have a narrower shape than cones. The recommended transforms currently in use are derived by minimizing perceptual error over experimentally obtained corresponding color data sets. We show that these sensors are still not optimally sharp. Using different computational approaches, we obtain sensors that are even more narrowband. In a first experiment, we derive a CAT by using spectral sharpening on Lam’s corresponding color data set. The resulting Sharp CAT, which minimizes XYZ errors, performs as well as the current most popular CATs when tested on several corresponding color data sets and evaluating perceptual error. Designing a spherical sampling technique, we can indeed show that these CAT sensors are not unique, and that there exist a large number of sensors that perform just as well as CAT02, the chromatic adaptation transform used in CIECAM02 and the ICC color management framework. We speculate that in order to make a final decision on a single CAT, we should consider secondary factors, such as their applicability in a color imaging workflow. We show that sharp sensors are very appropriate for color encodings, as they provide excellent gamut coverage and hue constancy. Finally, we derive sensors for a CAT that provide stable color ratios over different illuminants, i.e. that only model physical responses, which still can predict experimentally obtained appearance data. The resulting sensors are sharp

Avian photoreceptors.

SIGLEAvailable from British Library Document Supply Centre-DSC:DXN023209 / BLDSC - British Library Document Supply CentreGBUnited Kingdo

Testing assumptions of coevolution in an egg-rejecting brood parasite host: Uncovering sensory, cognitive, and evolutionary drivers of responses to parasitism in American robins (Turdus migratorius)

Hosts of brood parasitic birds face fitness costs associated with rearing unrelated offspring. In response, the recognition and rejection of parasitic eggs is a common host defense. Brown-headed cowbirds (Molothrus ater) challenge coevolutionary theory, because although they exploit over 200 host species, they lay non-mimetic eggs, and most hosts do not combat cowbird parasitism with egg rejection. American robins (Turdus migratorius) are one of a handful of cowbird hosts known to recognize and remove cowbird eggs from the nest. I addressed the mechanistic and evolutionary drivers of egg rejection in this host species, by disentangling the roles of spectral tuning and visual physiology on the behavioral outcome of egg rejection, by estimating the costs of parasitism which may drive egg rejection behavior, and by addressing the reciprocal effects parasitism on host egg color variation and its role in mediating rejection decisions. I also test assumptions underlying the evolvability of host egg rejection responses in this system. In Chapter 1, I lay out an overview of brood parasitism as a reproductive strategy and brood parasite-host ecology, and highlight evolutionary mechanisms and consequences of coevolution in these systems. In Chapter 2, I test the hypothesis that foreign egg rejection is driven proximately by perceivable differences in ground color between host and parasitic eggs across the entire avian spectral sensitivity range. I show that the rejection of artificially dyed eggs is mediated by input from all four avian single-cone photoreceptors, and that more divergent model `parasitic\u27 eggs are indeed rejected at higher rates. However, the cowbird egg does not conform to this prediction, because both model and real cowbird eggs are rejected in 100% of experimental trials despite their lower overall discriminability from robin eggs. This may indicate a cowbird-egg specific rejection response in robins. In Chapter 3, I test a critical assumption underlying the evolution of cowbird-specific egg rejection responses in robins, by assessing the hypothesis that cowbird parasitism imposes recoverable costs on robin hosts. My results indicate that cowbird chicks fare poorly when reared alongside robin chicks, but parasitism per se still reduces nesting success for robins; thus, rejection of cowbird eggs serves a function to eliminate the cost of parasitism. In Chapter 4, I examine a critical assumption underlying all of host-parasite coevolutionary theory, namely that host defenses can evolve genetically in response to parasitism. I address the hypothesis that egg rejection is repeatable in our study population, as repeatability is prerequisite to the evolution and spread of a behavioral trait, including a predictor of the trait\u27s genetic heritability. As predicted, egg rejection behavior in American robins was found to be highly repeatable for intermediately-rejected model egg colors within the same nesting attempt, irrespective of potentially confounding ecological and temporal factors. Finally, in Chapter 5, I test predictions stemming from alternate hypotheses that egg rejection evolved in response to cowbird (non-mimetic) versus conspecific (mimetic) parasitism, by investigating the degree of color variation within robins\u27 own clutches, and the effect of experimentally manipulating intraclutch color variation. I used both observational and experimental data, and found that egg color varies more between clutches than among egg within a single clutch, yet experimental manipulated intraclutch color variation did not affect rejection rates. These results support the scenario of historical parasitism by non-mimetic parasites. Variation among the findings of similar studies pertaining to hosts of mimetic parasites may be explained by hosts\u27 use of different cognitive mechanisms in the decision to reject foreign eggs, However, for hosts of non-mimetic parasites, investigating egg color variation and its effect on egg rejection is not informative about different cognitive decision-making rules, as predictions under each mechanism are similar - that there will be no effect of a history of parasitism on intraclutch color variation (observational patterns) or rejection rate (experimental data). This body of research presents compelling evidence in support of egg rejection by robins as a specific response to historical cowbird parasitism, and has highlighted important components of the sensory, cognitive, functional and evolutionary processes underlying egg rejection in this paradoxical brood parasite-host system

Characterisation of a multispectral digital camera System for quantitatively comparing complex animal Patterns in natural environments.

Animal coloration can be described by complex colour patterns including elements of varying size, shape and spectral profile which commonly reflect energy outside the spectral range visible for humans. Whilst spectrometry is currently employed for the quantitative study of animal coloration, it is limited on its ability to describe the spatial characteristics of spectral differences in patterns. Digital photography has recently been used as a tool for measuring spatial and spectral properties of patterns based on quantitative analysis of linear camera responses recovered after characterising the device. However current applications of digital imaging for studying animal coloration are limited to image recording within a laboratory environment considering controlled lighting conditions. Here a refined methodology for camera characterisation is developed permitting the recording of images under different illumination conditions typical of natural environments. The characterised camera system thus allows recording images from reflected ultraviolet and visible radiation resulting in a multispectral digital camera system. Furthermore a standardised imaging processing workflow was developed based on specific characteristics of the camera thus making possible an objective comparison from images. An application of the characterised camera system is exemplified in the study of animal colour patterns adapted for camouflage using as a model two Australian, endemic lizard species. The interaction between the spectral and spatial properties of the respective lizards produces complex patterns than cannot be interpreted by spectrophotometry alone. Data obtained from analysis of images recorded with the characterised camera system in the visible and near-ultraviolet region of the spectrum reveal significative differences between sex and species and a possible interaction between sex and species, suggesting microhabitat specialisation to different backgrounds

Visual characteristics of walleye pollock and Chinook salmon: Modeling theoretical visual space and target contrast of trawling materials in the Bering Sea

Walleye pollock (Gadus chalcogrammus) and Chinook salmon (Oncorhynchus tshawytscha) are economic and cultural resources in Alaska. Chinook salmon bycatch is a large concern within the pollock fishery. Current strategies to reduce salmon bycatch include modifying trawl gear by implementing artificial light near or on escapement panels to increase salmon escapement. The visual characteristics of pollock and Chinook salmon were investigated to understand the perception of trawl gear. The visual pigments of each species were measured using microspectrophotometry (MSP). Pollock were dichromats with spectral sensitivity ranging from 449nm–518 nm and Chinook salmon were trichromats with sensitivity ranging from 436 nm–545 nm. The green opsins within Chinook salmon will activate when stimulated by wavelengths that are outside of the spectral sensitivity of pollock. Microspectrophotometry data defining pollock and Chinook salmon visual pigments, spectral irradiance data from the Bering Sea, and spectral reflectance of commonly used trawl components were input into two visual models, VPModel® and the R package pavo. Visual models predict how the organism’s visual system responds to visual stimuli. Modeling the spectral distribution and physiological visual characteristics predicts how fish interact and adapt to the changing light environment. Spectral irradiance availability decreased with depth and increasing chlorophyll a concentration. Target contrast against the background space light was dependent on the light environment characteristics including depth. Using the physiological data and theoretical model output provides spectral range and intensity limitations to behavior experiments aiming to increase the escapement potential of Chinook salmon in the pollock fisher

Modelling Colour Appearance: Applications in Skin Image Perception

Humans are trichromatic, and yet their perception of colours is rich and complex. The research presented in this thesis explores the process of colour appearance of uniform patches and natural polychromatic stimuli. This is done through the measurement and analysis of the achromatic locus (Chapter 2), modelling of chromatic adaptation in a large dataset of unique hues settings (Chapter 3), and measurement of thresholds for uniform and polychromatic stimuli derived from simulated skin images (Chapter 4). Chapter 2 proposes a novel navigation scheme based on unique hues for traversing colour space. The results show that when colour adjustments are made using this novel scheme, the variability of achromatic settings made by observers is reduced compared to the classical method of making colour adjustments along the cardinal axes of the CIELUV colour space. This result holds across the tested luminance levels (5,20,50 cd/m^2) in each of the three tested ambient illumination conditions – dark, simulated daylight and cool white fluorescent lighting. The analysis also shows that the direction of maximum variance of the achromatic settings lies along the daylight locus. Chapter 3 evaluates models of chromatic adaptation by using unique hues settings measured under different ambient illumination conditions. It is shown that a simple diagonal model in cone excitation space is the most efficient in terms of the trade-off between accuracy and degrees of freedom. It is also found that diagonal and linear models show similar performances, reiterating their theoretical equivalence. Performances of these diagonalisable models are found to be worse for UR and UG unique hue planes compared to UY and UB planes. Chapter 4 presents a set of three experiments reporting estimations of perceptual thresholds for polychromatic and uniform stimuli in a 3-D chromaticity-luminance colour space. The first experiment reports thresholds for simulated skin images and uniform stimuli of the corresponding mean CIELAB colour. The second and third experiments investigate the effect of ambient illumination and the location of the stimuli in colour space. The thresholds for the polychromatic stimuli are found to be consistently higher than those for the uniform patches, for both the chromatic, and the luminance projections. The area of the chromaticity ellipses shows a gradual increase with distance from the illuminant chromaticity. The orientations of these ellipses for simulated skin are found to align with the vector joining the mean patch chromaticity and the illuminant chromaticity

Contemporary achievements in astronautics: Salyut-7, the Vega Project and Spacelab

The latest achievements in Soviet aeronautics are described; the new stage in the space program to study Venus using Soviet automated space probes, and the next space mission by cosmonauts to the Salyut-7 station. Information is also presented on the flight of the Spacelab orbiting laboratory created by Western European specialists