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Dive into the research topics where Casper J. van der Kooi is active.

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Featured researches published by Casper J. van der Kooi.


New Phytologist | 2014

Iridescent flowers? Contribution of surface structures to optical signaling.

Casper J. van der Kooi; Bodo D. Wilts; Hein L. Leertouwer; Marten Staal; J. Theo M. Elzenga; Doekele G. Stavenga

The color of natural objects depends on how they are structured and pigmented. In flowers, both the surface structure of the petals and the pigments they contain determine coloration. The aim of the present study was to assess the contribution of structural coloration, including iridescence, to overall floral coloration. We studied the reflection characteristics of flower petals of various plant species with an imaging scatterometer, which allows direct visualization of the angle dependence of the reflected light in the hemisphere above the petal. To separate the light reflected by the flower surface from the light backscattered by the components inside (e.g. the vacuoles), we also investigated surface casts. A survey among angiosperms revealed three different types of floral surface structure, each with distinct reflections. Petals with a smooth and very flat surface had mirror-like reflections and petal surfaces with cones yielded diffuse reflections. Petals with striations yielded diffraction patterns when single cells were illuminated. The iridescent signal, however, vanished when illumination similar to that found in natural conditions was applied. Pigmentary rather than structural coloration determines the optical appearance of flowers. Therefore, the hypothesized signaling by flowers with striated surfaces to attract potential pollinators presently seems untenable.


Biological Reviews | 2014

On the fate of sexual traits under asexuality

Casper J. van der Kooi; Tanja Schwander

Environmental shifts and life‐history changes may result in formerly adaptive traits becoming non‐functional or maladaptive. In the absence of pleiotropy and other constraints, such traits may decay as a consequence of neutral mutation accumulation or selective processes, highlighting the importance of natural selection for adaptations. A suite of traits are expected to lose their adaptive function in asexual organisms derived from sexual ancestors, and the many independent transitions to asexuality allow for comparative studies of parallel trait maintenance versus decay. In addition, because certain traits, notably male‐specific traits, are usually not exposed to selection under asexuality, their decay would have to occur as a consequence of drift. Selective processes could drive the decay of traits associated with costs, which may be the case for the majority of sexual traits expressed in females. We review the fate of male and female sexual traits in 93 animal lineages characterized by asexual reproduction, covering a broad taxon range including molluscs, arachnids, diplopods, crustaceans and eleven different hexapod orders. Many asexual lineages are still able occasionally to produce males. These asexually produced males are often largely or even fully functional, revealing that major developmental pathways can remain quiescent and functional over extended time periods. By contrast, for asexual females, there is a parallel and rapid decay of sexual traits, especially of traits related to mate attraction and location, as expected given the considerable costs often associated with the expression of these traits. The level of decay of female sexual traits, in addition to asexual females being unable to fertilize their eggs, would severely impede reversals to sexual reproduction, even in recently derived asexual lineages. More generally, the parallel maintenance versus decay of different trait types across diverse asexual lineages suggests that neutral traits display little or no decay even after extended periods under relaxed selection, while extensive decay for selected traits occurs extremely quickly. These patterns also highlight that adaptations can fix rapidly in natural populations of asexual organisms, in spite of their mode of reproduction.


Evolution | 2014

Evolution of asexuality via different mechanisms in grass thrips (thysanoptera: Aptinothrips).

Casper J. van der Kooi; Tanja Schwander

Asexual lineages can derive from sexual ancestors via different mechanisms and at variable rates, which affects the diversity of the asexual population and thereby its ecological success. We investigated the variation and evolution of reproductive systems in Aptinothrips, a genus of grass thrips comprising four species. Extensive population surveys and breeding experiments indicated sexual reproduction in A. elegans, asexuality in A. stylifer and A. karnyi, and both sexual and asexual lineages in A. rufus. Asexuality in A. stylifer and A. rufus coincides with a worldwide distribution, with sexual A. rufus lineages confined to a limited area. Inference of molecular phylogenies and antibiotic treatment revealed different causes of asexuality in different species. Asexuality in A. stylifer and A. karnyi has most likely genetic causes, while it is induced by endosymbionts in A. rufus. Endosymbiont‐community characterization revealed presence of Wolbachia, and lack of other bacteria known to manipulate host reproduction. However, only 69% asexual A. rufus females are Wolbachia‐infected, indicating that either an undescribed endosymbiont causes asexuality in this species or that Wolbachia was lost in several lineages that remained asexual. These results open new perspectives for studies on the maintenance of mixed sexual and asexual reproduction in natural populations.


Proceedings of the Royal Society of London. Series B, Biological Sciences | 2016

How to colour a flower: on the optical principles of flower coloration.

Casper J. van der Kooi; J. Theo M. Elzenga; Marten Staal; Doekele G. Stavenga

The coloration of flowers is due to the wavelength-selective absorption by pigments of light backscattered by structures inside the petals. We investigated the optical properties of flowers using (micro)spectrophotometry and anatomical methods. To assess the contribution of different structures to the overall visual signal of flowers, we used an optical model, where a petal is considered as a stack of differently pigmented and structured layers and we interpreted the visual signals of the model petals with insect vision models. We show that the reflectance depends, in addition to the pigmentation, on the petals thickness and the inhomogeneity of its interior. We find large between-species differences in floral pigments, pigment concentration and localization, as well as floral interior structure. The fractions of reflected and transmitted light are remarkably similar between the studied species, suggesting common selective pressures of pollinator visual systems. Our optical model highlights that pigment localization crucially determines the efficiency of pigmentary filtering and thereby the chromatic contrast and saturation of the visual signal. The strongest visual signal occurs with deposition of pigments only on the side of viewing. Our systematic approach and optical modelling open new perspectives on the virtues of flower colour.


New Phytologist | 2015

Is floral iridescence a biologically relevant cue in plant–pollinator signaling?

Casper J. van der Kooi; Adrian G. Dyer; Doekele G. Stavenga

Flowers allure potential pollinators by displaying attractive colour patterns, which are generally created by their petal arrangement and pigmentation. In addition to the pigmentary coloration, in certain cases reflecting surface structures may contribute to the flowers’ appearance. For instance, a flower petal surface with periodic striations, acting as a grating reflector, can create an angle-dependent coloured reflection, i.e. iridescence. A striking demonstration of this phenomenon is reported by Vignolini et al. (2014). They show that with specific illumination of a Hibiscus trionum L. flower, the proximal part of the petals, where the surface is regularly striated, displays an iridescent, bluish shine in addition to the deep-red pigmentary colour. Inspired by a previous study by (largely) the same team on floral iridescence across various plant species, entitled ‘Floral iridescence, produced by diffractive optics, acts as a cue for animal pollinators’ (Whitney et al. 2009), we investigated the surface reflections of numerous plant species and confirmed the widespread presence of striations that have diffractive properties in controlled conditions. However, the iridescent signal vanished under natural illumination, and therefore we concluded that in no case did surface reflections add a noticeable iridescent signal to the pigmentary coloration that could be utilized by an insect pollinator in a biologically relevant scenario (van der Kooi et al. 2014). Here, we substantiate our previous statement that floral iridescence acting as signaling cue to pollinators is presently untenable by presenting photographs of a number of different flowers with striated epidermal cells and a new set of angle-dependent reflectance measurements


Journal of the Royal Society Interface | 2017

Functional optics of glossy buttercup flowers

Casper J. van der Kooi; J. Theo M. Elzenga; Jan Dijksterhuis; Doekele G. Stavenga

Buttercup (Ranunculus spp.) flowers are exceptional because they feature a distinct gloss (mirror-like reflection) in addition to their matte-yellow coloration. We investigated the optical properties of yellow petals of several Ranunculus and related species using (micro)spectrophotometry and anatomical methods. The contribution of different petal structures to the overall visual signal was quantified using a recently developed optical model. We show that the coloration of glossy buttercup flowers is due to a rare combination of structural and pigmentary coloration. A very flat, pigment-filled upper epidermis acts as a thin-film reflector yielding the gloss, and additionally serves as a filter for light backscattered by the strongly scattering starch and mesophyll layers, which yields the matte-yellow colour. We discuss the evolution of the gloss and its two likely functions: it provides a strong visual signal to insect pollinators and increases the reflection of sunlight to the centre of the flower in order to heat the reproductive organs.


Planta | 2016

Coloration of the Chilean Bellflower, Nolana paradoxa, interpreted with a scattering and absorbing layer stack model.

Doekele G. Stavenga; Casper J. van der Kooi

AbstractMain conclusionAn absorbing-layer-stack model allows quantitative analysis of the light flux in flowers and the resulting reflectance spectra. It provides insight in how plants can optimize their flower coloration for attracting pollinators. The coloration of flowers is due to the combined effect of pigments and light-scattering structures. To interpret flower coloration, we applied an optical model that considers a flower as a stack of layers, where each layer can be treated with the Kubelka–Munk theory for diffusely scattering and absorbing media. We applied our model to the flowers of the Chilean Bellflower, Nolana paradoxa, which have distinctly different-colored adaxial and abaxial sides. We found that the flowers have a pigmented, strongly scattering upper layer, in combination with an unpigmented, moderately reflecting lower layer. The model allowed quantitative interpretation of the reflectance and transmittance spectra measured with an integrating sphere. The absorbance spectrum of the pigment measured with a microspectrophotometer confirmed the spectrum derived by modeling. We discuss how different pigment localizations yield different reflectance spectra. The absorbing layer stack model aids in understanding the various constraints and options for plants to tune their coloration.


Current Biology | 2015

Parthenogenesis: Birth of a New Lineage or Reproductive Accident?

Casper J. van der Kooi; Tanja Schwander

Parthenogenesis - the ability to produce offspring from unfertilized eggs - is widespread among invertebrates and now increasingly found in normally sexual vertebrates. Are these cases reproductive errors or could they be a first step in the emergence of new parthenogenetic lineages?


Current Biology | 2016

Plant Biology: Flower Orientation, Temperature Regulation and Pollinator Attraction

Casper J. van der Kooi

The reproductive performance of plants depends on the temperature of the flower. A recent study reports the mechanistic basis of flower head orientation in sunflowers and provides intriguing hints as to its functional significance.The reproductive performance of plants depends on the temperature of the flower. A recent study reports the mechanistic basis of flower head orientation in sunflowers and provides intriguing hints as to its functional significance.


Annals of Botany | 2018

Functional significance of the optical properties of flowers for visual signalling

Casper J. van der Kooi; Adrian G. Dyer; Peter G. Kevan; Klaus Lunau

Background Flower coloration is a key enabler for pollinator attraction. Floral visual signals comprise several components that are generated by specific anatomical structures and pigmentation, and often have different functions in pollinator attraction. Anatomical studies have advanced our understanding of the optical properties of flowers, and evidence from behavioural experiments has elucidated the biological relevance of different components of floral visual signals, but these two lines of research are often considered independently. Scope Here, we review current knowledge about different aspects of the floral visual signals, their anatomical and optical properties, and their functional significance in plant-pollinator visual signalling. We discuss common aspects, such as chromatic and achromatic contrast, hue, saturation and brightness, as well as less common types of visual signals, including gloss, fluorescence, polarization and iridescence in the context of salience of floral colour signals and their evolution, and highlight promising avenues for future research.

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Marten Staal

University of Groningen

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Klaus Lunau

University of Düsseldorf

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