The eye as a replicating and diverging, modular developmental unit

Sparked by new discoveries in developmental genetics, few topics have generated as much debate as eye evolution. This is somewhat surprising because the central controversy is not unique to eyes, but is a general theme of developmental genetics: evolutionarily conserved genes are deployed during the development of highly divergent morphological features. In the case of eyes, this paradox has engendered opposing camps entrenched in what has been termed a ‘scientific war’. One camp highlights conserved genetic features, concluding that eyes stem from an ancestral prototype. The opposing camp emphasizes variation, arguing that some eyes must have recruited the same genes after separate morphological origins. Here, I blur the line between these camps and suggest that eyes have often evolved by replication, perhaps through the ectopic expression of a conserved, modular regulatory cascade to produce serially homologous structures that often diverged during evolution. Therefore, morphologically diverse eyes could stem from a single ancestral prototype, yet also result from multiple morphological origins.     

Evolution of eyes

Seeing is important for most species and it has been a key selective advantage throughout evolution. Consequently, there is a remarkable diversity among types of eyes. Animals have converged on eight optical solutions for collecting and focusing light; in contrast, all eyes share the same molecular strategy for absorbing photons. Recent studies have identified similarities in the genetic information that is used in the development of eyes, leading to the hypothesis that distinctly different eye types might have had a monophyletic origin. Across many species, there is a remarkable continuity of the developmental genes that participate in the construction of similar--but not necessarily homologous--eyes.     

Colonial origin for Emetazoa: major morphological transitions and the origin of bilaterian complexity

A new hypothesis for the evolution of Bilateria is presented. It is based on a reinterpretation of the morphological characters shared by protostomes and deuterostomes, which, when taken together with developmental processes shared by the two lineages, lead to the inescapable conclusion that the last common ancestor of Bilateria was complex. It possessed a head, a segmented trunk, and a tail. The segmented trunk was further divided into two sections. A dorsal brain innervated one or more sensory cells, which included photoreceptors. "Appendages" or outgrowths were present. The bilaterian ancestor also possessed serially repeated "segments" that were expressed ontogenetically as blocks of mesoderm or somites with adjoining fields of ectoderm or neuroectoderm. It displayed serially repeated gonads (gonocoels), each with a gonoduct and gonopore to the exterior, and serially repeated "coeloms" with connections to both the gut and the exterior (gill slits and pores). Podocytes, some of which were serially repeated in the trunk, formed sites of ultrafiltration. In addition, the bilaterian ancestor had unsegmented coeloms and a contractile blood vessel or "heart" formed by coelomic myoepithelial cells. These cells and their underlying basement membrane confine the hemocoelic fluid, or blood, in the connective tissue compartment. A possible scenario to account for this particular suite of characters is one in which a colony of organisms with a cnidarian grade of organization became individuated into a new entity with a bilaterian grade of organization. The transformation postulated encompassed three major transitions in the evolution of animals. These transitions included the origins of Metazoa, Eumetazoa, and Bilateria and involved the successive development of poriferan, cnidarian, and bilaterian grades of organization. Two models are presented for the sponge-to-cnidarian transition. In both models the loss of a flow-through pattern of water circulation in poriferans and the establishment of a single opening and epithelia sensu stricto in cnidarians are considered crucial events. In the model offered for the cnidarian-to-bilaterian transition, the last common ancestor of Eumetazoa is considered to have had a colonial, cnidarian-grade of organization. The ancestral cnidarian body plan would have been similar to that exhibited by pennatulacean anthozoans. It is postulated that a colonial organization could have provided a preadaptive framework for the evolution of the complex and modularized body plan of the triploblastic ancestor of Bilateria. Thus, one can explore the possibility that problematica such as ctenophores, the Ediacaran biota, archaeocyaths, and Yunnanozoon reflect the fact that complexity originated early and involved the evolution of a macroscopic compartmented ancestor. Bilaterian complexity can be understood in terms of Beklemishev "cycles" of duplication and colony individuation. Two such cycles appear to have transpired in the early evolution of Metazoa. The first gave rise to a multicellular organism with a sponge grade of organization and the second to the modularized ancestor of Bilateria. The latter episode may have been favored by the ecological conditions in the late Proterozoic. Whatever its cause, the individuation of a cnidarian-grade colony furnishes a possible explanation for the rapid diversification of bilaterians in the late Vendian and Cambrian. The creation of a complex yet versatile prototype, which could be rapidly modified by selection into a profusion of body plans, is postulated to have affected the timing, mode, and extent of the "Cambrian explosion." During the radiations, selective loss or simplification may have been as creative a force as innovation. Finally, colony individuation may have been a unique historical event that imprinted the development of bilaterians as the zootype and phylotypic stage. (ABSTRACT TRUNCATED)     

Photoreceptor cells and eyes in Annelida

The evolution of photoreceptor cells and eyes in Metazoa is far from being resolved, although recent developmental and morphological studies provided strong evidence for a common origin of photoreceptor cells and existence of sister cell types in early metazoans. Photoreceptor cells are of two types, rhabdomeric and ciliary, depending on which part of the cells is involved in photoreception proper. A crucial point in understanding eye evolution is the explanation of the enormous structural diversity of photoreceptor cells and visual systems, given the general tendency for molecular conservation. One example of such diversity occurs in Annelida. In this taxon three types of photoreceptor cells exist: rhabdomeric, ciliary and phaosomous sensory cells. Whether the latter evolved independently or have been derived from one of the former cell types is still unresolved, since cilia and microvilli are found in these cells. These different photoreceptor cells are present in cerebral ocelli and eyes, in various ectopic ocelli and eyes situated in different places as well as in various photoreceptor-like sense organs. Whereas rhabdomeric cells mostly occur in connection with pigmented supportive cells, the other types are usually found with unpigmented supportive cells. Thus for the latter cells clear evidence for photoreception is still lacking in most cases. However, initial molecular-developmental investigations have shown that in fact ciliary photoreceptor cells exist within Annelida. Certain visual systems are only present during the larval phase and either replaced by the adult eyes or completely reduced during postlarval and adult stages. In the present paper the diversity of cerebral and extracerebral photoreceptor cells and ocelli as well as corresponding organs devoid of shading pigment is reviewed in Annelida.     

进化细胞生物学的提出及其任务

作者提出应创建一门源于进化生物学与细胞生物学两者的交叉学科一进化细胞生物学(细胞的进化生物学)。其根本任务在于用进化的观点考察真核细胞的一切方面,从它们的起源和演化来认识它们的现在。文中列举了其具体的研究内容,并分析了其研究方法上的特点,指出在这里需要把进化生物学的综合性分析与细胞生物学的实验研究最紧密地结合起来。文中还论述了真核细胞的细胞器的“不进化”现象,指出其根本原因在于进化焦点的转移。     

The genetic control of eye development and its implications for the evolution of the various eye-types

Mutations in the Pax 6 homologs of mammals and insects prevent eye development and targeted expression of both mammal and insect Pax 6 homologs is capable of inducing functional ectopic eyes. Supported by RNA interference experiments in planarians and nemerteans, these findings indicate that Pax 6 is a universal master control gene for eye morphogenesis. Since all metazoan eyes use rhodopsin as a photoreceptor molecule and the same master control gene for eye development, we postulate a monophyletic origin of the various eye types. The finding of well developed eyes in jellyfish which essentially lack a brain, leads us to propose that the eye as a sensory organ evolved before the brain which is an information processing organ. The finding of highly developed eyes with a lens, vitreous body, stacked membranes like a retina and shielding pigment in unicellular dinoflagellates, raises the possibility that the prototypic eyes might have been acquired from symbionts.     

Ferredoxins in the evolution of photosynthetic systems from anaerobic bacteria to higher plants

Ferredoxins are present in a wide range of organisms, from the primitive anaerobic bacteria to higher plants and animals, where they function in diverse electron transfer processes. They are relatively small proteins with molecular weights of 6000 to 12000, contain 2–8 Fe atoms and an equivalent amount of inorganic sulphur per molecule, and they transfer electrons at low redox potentials.Anaerobic bacteria, like the clostridia, contain 8 Fe ferredoxins with a peptide chain of 55 amino acid residues which could be arranged in two similar halves suggesting the evolution of the molecule, from a prototype of 26 amino acid residues, by gene duplication. Since these ferredoxins contain a high predominance of certain amino acids detected in meteorites and lunar samples and synthesized under simulated prebiotic environment and since iron and sulphus could be incorporated easily into the apoprotein in anaerobic conditions, the ferredoxin molecule could have been formed in the early periods of the origin of life. From the available chemical compositions and amino acid sequences of various ferredoxins the following evolutionary scheme can be postulated: anaerobic bacteriagreen photosynthetic bacteriared photosynthetic bacteriasulphate reducing bacteriablue-green algaegreen algae and higher plants.Special Symposium on Photochemistry and the Origins of Life, Sixth International Congress on Photobiology, Bochum, Germany.     

Reconstruction of possible paths of the origin and morphological evolution of bacteriophages

The problem of the origin and evolution of viruses and, in particular, the origin and evolution of bacteriophages is of considerable interest. However, so far, this problem has not been solved with quantitative methods of molecular systematics. In the present study, an attempt to reconstruct the possible paths of appearance and evolution of bacteriophages based on their structural features and morphogenesis, as well as general characteristics of their life cycles and genome organization, was carried out. A scheme describing phylogeny of the main bacteriophage groups and evolution of their life cycles is suggested. Existence of two independently evaluating types of morphogenesis ("budding outward" and "budding inward") is postulated.     

Ultrastructure of pigmented adult eyes in errant polychaetes (Annelida): implications for annelid evolution

The evolution of photoreceptor cells and eyes in Metazoa is far from being resolved, although recent developmental and structural studies have provided strong evidence for a common origin of photoreceptor cells and existence of sister cell types already in early metazoans. These sister cell types are ciliary and rhabdomeric photoreceptor cells, depending on which part of each cell is involved in photoreception proper. However, a crucial point in eye evolution is how the enormous structural diversity of photoreceptor cells and visual systems developed, given the general molecular conservation of the photoreceptor cells. One example of this diversity can be observed in Annelida. Within the polychaetes the errant forms, taxon Aciculata, constitute the only group possessing true multicellular eyes in the adult stage. Thus far these organs have been investigated only in taxa of Phyllodocida, a subgroup of Aciculata. Data on Eunicida and Amphinomida as well as certain phyllodocidan taxa had been lacking. The ultrastructure of these adult eyes was investigated in various species of errant polychaetes, belonging to Amphinomidae, Eunicidae and Hesionidae, to elucidate whether they provide any phylogenetic clues regarding either the evolution of visual systems in Annelida or lophotrochozoan phylogeny in general. These eyes are composed of numerous supportive pigment cells and rhabdomeric photoreceptor cells and sometimes additional cell types. As a rule the pigment and rhabdomeric cell types form a continuous epithelium in which the two types intermingle. Presence of granules with shading pigment in sensory cells is a common feature but is apparently restricted to a taxon comprising Phyllodocida and Eunicida s. str. Very likely a lens-like structure does not belong to the ground pattern of annelid eyes, despite its presence in Phyllodocida. These lens-like structures are formed by secretions or cellular processes of the pigment cells. In many species the eye cup communicates with the exterior via a small cuticularized canal. This canal is interpreted as a rudiment due to the mode of formation in the epidermis. With respect to current phylogenetic hypotheses, these multicellular eyes have either been developed in the stem species of a taxon Aciculata nested within the polychaetes or have been evolved in the stem lineage of Annelida. Similarities to gastropod eyes are interpreted as convergent and not as indication of common origin. Except for the photoreceptor cells proper, the structure of the adult eyes in polychaetes most likely does not help to resolve lophotrochozoan phylogeny.     

Genetics and hybridization in surface and cave Astyanax (Teleostei): a comparison of regressive and constructive traits

Change in ecological conditions, as seen in surface and cave populations of Astyanax (Teleostei), has caused the divergent evolution of a large number of traits like eyes, coloration, taste, lateral line, and different kinds of behaviour like schooling, sleep or feeding posture. Because of the interfertility of surface and cave forms these fish are an exceptional object to study the morphological and genetic basis of the evolution of such complex regressive and constructive traits. Classical crossing analyses and genomic studies are contributing to growing understanding. Both kinds of traits mostly rely on multiple genetic bases and the phenotypic manifestation in the various crosses is similar. The gene effect underlying the phenotypic manifestation may exhibit an exponential increase at differing amounts in the various traits and crosses. Missing or presence of such genetic interaction helps determine whether the variability of eyes or pigmentation exhibited by Astyanax cave fish populations like Micos, is due to a more recent origin or to secondary hybridization with the surface fish. Neither crossing analysis nor QTL mapping revealed that eye reduction is pleiotropically antagonistically related to the increase of taste buds or lateral line sense. Independent inheritance of traits suggests that Astyanax cave fish are subjected to mosaic evolution.     

Vertebrate cranial evolution: Contributions and conflict from the fossil record

In modern vertebrates, the craniofacial skeleton is complex, comprising cartilage and bone of the neurocranium, dermatocranium and splanchnocranium (and their derivatives), housing a range of sensory structures such as eyes, nasal and vestibulo-acoustic capsules, with the splanchnocranium including branchial arches, used in respiration and feeding. It is well understood that the skeleton derives from neural crest and mesoderm, while the sensory elements derive from ectodermal thickenings known as placodes. Recent research demonstrates that neural crest and placodes have an evolutionary history outside of vertebrates, while the vertebrate fossil record allows the sequence of the evolution of these various features to be understood. Stem-group vertebrates such as Metaspriggina walcotti (Burgess Shale, Middle Cambrian) possess eyes, paired nasal capsules and well-developed branchial arches, the latter derived from cranial neural crest in extant vertebrates, indicating that placodes and neural crest evolved over 500 million years ago. Since that time the vertebrate craniofacial skeleton has evolved, including different types of bone, of potential neural crest or mesodermal origin. One problematic part of the craniofacial skeleton concerns the evolution of the nasal organs, with evidence for both paired and unpaired nasal sacs being the primitive state for vertebrates.     

深海微生物高压适应与生物地球化学循环

深海是典型的高压环境,嗜压微生物是深海生态系统中的重要类群.随着深海采样技术的发展及高压微生物特殊培养设备的开发,已从深海环境中分离到一系列嗜压微生物,包括一些常压环境不能生长的严格嗜压菌.对这些嗜压菌的研究,不仅对微生物适应极端高压环境的机制有一定了解,而且发现了一些特殊的代谢产物.研究微生物高压嗜压机理,还有助于探索地球生命的温度压力极限及生命起源和演化等科学问题.从深海嗜压微生物多样性、深海微生物高压环境适应机理及深海微生物在生物地球化学循环中的作用等方面对嗜压微生物的研究进展进行综述.     

Evolution of incompatibility systems in plants: Complementarity and the mating locus in flowering plants and fungi

Summary The restriction of sexual pairing by a specificity gene is considered to be an ancient development in the plant kingdom. The diversity and general parallelism of incompatibility systems seen amongst the phyla at the present time can be rationalized in terms of the association of various derived forms of the ancestral specificity unit with differing spectra of accessory factors controlling sexual physiology in the different phyla. Sexual morphogenesis has become divided into distinct phases under the control of complementary genes. These phases are initiated by a regulatory system of Co-ordinator genes which control the order in which groups of morphogenetic genes are expressed during development. The entire sexual cycle will be completed only if all the complementary groups are activated in the appropriate sequence. The present article discusses essential features of the evolution of the breeding locus in different phyla. These features are consistent in themselves with the present data and are not dependent on the proposed ancient origin of the specificity gene.The above hypothesis throws light on the (1) evolution of the complex mating loci in flowering plants and fungi; (2) evolution of complementary incompatibility and heteromorphic incompatibility in flowering plants; (3) anomalous cross-compatibility behaviour of mutants in the fungus Schizophyllum commune; (4) nature of homothallism in higher fungi; (5) mode of origin of new functional self-incompatibility alleles; and (6) homogenic and heterogenic incompatibility.     

Feathers with Ocular Architecture: Implications for Functional and Evolutionary Similarities of Visual Signals and Receptors

Studies of visual receptors typically assume that only functionally similar structures are relevant to the evolution of complex eyes. This approach ignores growing evidence that different functional classes of organs often share structural and developmental patterns that pertain to biological sameness (deep homology). However, the potential relevance of non-receptor structures to eye evolution remains largely unexplored. An “ocular” feather color mechanism is described whose structural and optical features resemble those of chambered, image-forming eyes to a remarkable degree. These similarities include a laterally expanded, domed light receiving surface similar to that of an eye, an encapsulated spongy tissue mass whose coherent light scattering properties in the human-visible (destructive) and ultraviolet (constructive) wavelength ranges resemble those of cornea and lens, intervening spaces such as those with humors, and a laminar pigmented shelf whose structure and optics resemble a mirrored tapetum lucidum found behind many retinas. Fourier analysis and optical principles indicate that ocular structures adhere to the same light-handling properties regardless of higher function (receptor or signal). The extent to which chambered eyes and ocular feathers have evolved independently is surprisingly equivocal. On the one hand, broad differences in the location, composition, and development of chambered eyes and ocular feather signals suggest convergent evolution on an ocular organization. However, some level of evolutionary parallelism (generative homology) between chambered eyes and ocular feathers is implicated by similarities in constructional materials, tissue development, and signal transduction cascades. Structural, optical, and developmental similarities also occur between more primitive eyes and the colored dermal papillae responsible for avian skin ornamentation. Functional constraints on light-handling requirements, coupled with developmental constraints in high-stress environments on the body surface, may enhance the similar evolutionary outcomes in the different functional setting. Regardless of the mechanistic details, repeated evolution of eye-like structures in different functional settings reveals a biological potential to produce such organs that is much greater than would be inferred from a survey of receptor structures alone.     

Chordate betagamma-crystallins and the evolutionary developmental biology of the vertebrate lens

Several animal lineages, including the vertebrates, have evolved sophisticated eyes with lenses that refract light to generate an image. The nearest invertebrate relatives of the vertebrates, such as the ascidians (sea squirts) and amphioxus, have only basic light detecting organs, leading to the widely-held view that the vertebrate lens is an innovation that evolved in early vertebrates. From an embryological perspective the lens is different from the rest of the eye, in that the eye is primarily of neural origin while the lens derives from a non-neural ectodermal placode which invaginates into the developing eye. How such an organ could have evolved has attracted much speculation. Recently, however, molecular developmental studies of sea squirts have started to suggest a possible evolutionary origin for the lens. First, studies of the Pax, Six, Eya and other gene families have indicated that sea squirts have areas of non-neural ectoderm homologous to placodes, suggesting an origin for the embryological characteristics of the lens. Second, the evolution and regulation of the betagamma-crystallins has been studied. These form one of the key crystallin gene families responsible for the transparency of the lens, and regulatory conservation between the betagamma-crystallin gene in the sea squirt Ciona intestinalis and the vertebrate visual system has been experimentally demonstrated. These data, together with knowledge of the morphological, physiological and gene expression similarities between the C. intestinalis ocellus and vertebrate retina, have led us to propose a hypothesis for the evolution of the vertebrate lens and integrated vertebrate eye via the co-option and combination of ancient gene regulatory networks; one controlling morphogenetic aspects of lens development and one controlling the expression of a gene family responsible for the biophysical properties of the lens, with the components of the retina having evolved from an ancestral photoreceptive organ derived from the anterior central nervous system.     

Evolutionary oscillation in prebiology: Igneous activity and the origins of life

The processes of chemical evolution are responsible for the origin of life. Three such processes have special importance: oscillation, creation, and competition.An oscillation from one kind of environment to another provides a mechanism for instituting processes that can only take place under conditions far removed from equilibrium. Oscillating evolutionary processes are likely to have played an important part in the origin of life. It is a mistake to assume that life originated in any one environment. It did not arrive in a moment of time. It was the result of a long period of chemical evolution during which it passed through a variety of environments. Biopoesis took place in an environment in which a variety of different kinds of protolife were assembled and concentrated.One essential form of protolife involved in these processes is the protocell. The experiments of Fox suggest that the creation of protocells involves violent oscillations of temperature and hydration. Igneous activity is especially characterised by oscillating conditions. Volcanic eruptions consist of violent changes from one extreme condition to another. Temperature, pressure, phase, concentration and hydration all oscillate violently, and are subject to shock pulses of many kinds. Protolife may well have passed through extremes of environment far wider than those that life itself can sustain.The most probable environment for the assembly of the various forms of protolife would be on mudbanks forming either at the mouth of streams draining regions of active vulcanicity, or round the edge of hot volcanic pools. In this situation one could find concentrated not only the various strands of protolife necessary for the final act of biopoesis, but also prebiologically formed nutrients necessary as food for the first eobionts. As soon as the first protocells start to grow, they start to compete with each other, and so initiate a new and additional evolutionary process, that of natural selection. Only after such competition has been initiated is life itself likely to be established.Given at the International Seminar Origin of Life, 2–7 August 1974, Moscow, U.S.S.R.     

Setting the stage: the history, chemistry, and geobiology behind RNA

No community-accepted scientific methods are available today to guide studies on what role RNA played in the origin and early evolution of life on Earth. Further, a definition-theory for life is needed to develop hypotheses relating to the "RNA First" model for the origin of life. Four approaches are currently at various stages of development of such a definition-theory to guide these studies. These are (a) paleogenetics, in which inferences about the structure of past life are drawn from the structure of present life; (b) prebiotic chemistry, in which hypotheses with experimental support are sought that get RNA from organic and inorganic species possibly present on early Earth; (c) exploration, hoping to encounter life independent of terran life, which might contain RNA; and (d) synthetic biology, in which laboratories attempt to reproduce biological behavior with unnatural chemical systems.