#OM0LEX5S Introduction Complexus 2006;3:211–216 DOI: 10.1159/000095881 Received: March 3, 2006 Accepted after revision: June 28, 2006 Published online: September 21, 2006 Wings for Flying, Lymphocytes for Defense: Spandrels, Exaptation and Specific Immunity Gustavo C. Ramos a Nelson M. Vaz c Kay Saalfeld b a Departamento de Farmacologia and b Departamento de Ecologia e Zoologia, UFSC, Florianópolis, Brazil c Departamento de Bioquímica e Imunologia, UFMG, Belo Horizonte, Brazil Key Words Evolution Immunology Adaptation Exaptation Spandrels Abstract The evolution of the immune system of jawed vertebrates with its vast array of clonally expressed lymphocyte receptors is usually viewed as optimized for the defense of the organism. There is a clear association between neo-Darwinism, the dominant view in current biology, and the usual description of immunological activity as adaptive immunity. Herein, using the concepts of spandrels and exaptation created by Gould and coworkers, we review data from comparative immunology and claim that the immune system was not formed as a system optimized for the defense of the organism, but rather may be viewed as a spandrel, a consequence of processes not originally linked to interactions with foreign materials. Copyright © 2006 S. Karger AG, Basel Fax +41 61 306 12 34 E-Mail firstname.lastname@example.org www.karger.com © 2006 S. Karger AG, Basel 1424–8492/06/0034–0211 $23.50/0 Accessible online at: www.karger.com/cpu Gustavo Campos Ramos, Departamento de Farmacologia, Centro de Ciências Biológicas, Bloco ‘D’, Caixa Postal 476, Universidade Federal de Santa Catarina, Trindade, Florianópolis, SC 88049-900 (Brazil) Tel. +55 48 3331 9491, ext. 216, Fax +55 48 3337 5479 E-Mail email@example.com Over the last century, a way of seeing the world based on neo-Darwinism became so strong that it is now considered as a fact and an a priori condition for biological thinking. This way of seeing the world, based on adaptations and functions, which was called the adaptationist program by Gould and Lewontin , has guided the conduction of biological and medical research and teaching, and oriented whole branches of medical endeavor. Modern immunology is a good example of this influence . From the late 1950s, immunology has been based on and corroborated by neoDarwinian concepts, which it also helps to support. In neo-Darwinian (Burnetian) immunology , an ‘immune system’, the main function of which is the defense of the organism, is viewed as a pool of isolated lymphocyte clones unconnected from each other and from body components. This approach has been very important in understanding many features of immunological phenomena, but many others remain unsolved, such as autoimmunity. Herein, we propose that, due to the strong association between neo-Darwinism and Burnetian immunology, important alterations in immunological thinking require the uprooting of its neo-Darwinian fundaments. In attempting to do this, we apply to immunology concepts and metaphors proposed by Gould and coauthors [1, 4] that contradict the adaptationist program. The main implication of this initiative is to suggest the adoption of research programs based on the structure and organization of living systems rather than on their functions or their ability to compete, as is currently the norm. The Adaptationist Program The adaptationist program assumes that the organism, in its changes, follows chan ges in its medium and that Downloaded by: Univ.of Adelaide 220.127.116.11 - 10/26/2017 1:38:17 PM ORIGINAL RESEARCH PAPER 212 seen in neo-Darwinism. For example, El-Hani and Emmeche  say: ‘Change arises out of an external process of sorting (natural selection) and, according to this point of view, assumptions about causal agency within organisms are unnecessary. Selection becomes merely an external force. Mutation is the inner force that creates variation, but this force is random and, thus, without connections to the needs of the organism and its functions. The organism becomes a passive meeting point of forces alien to the very organism. The irony is that the organism becomes irrelevant to evolutionary biology, merely a medium where the outer selective force meets with the inner producer of variation. This may bring along a tendency to reify the living organism as a mere passive thing.’ For the majority of scientists, neoDarwinism (the synthetic theory of evolution) is consolidated as a fact and is the only way to contemplate living systems. However, the theory has serious problems. In brief: ism and its form are generated, rather than simply with the constitution of a particular kind of molecules. This consideration becomes clear in the words of Waddington [see 11]: ‘Changes in genotypes only have ostensible effects in evolution if they bring with them alterations in the epigenetic processes by which phenotypes come into being; the kinds of change possible in the adult form of an animal are limited to the possible alterations in the epigenetic system by which it is produced.’ (2) Biological Information Is Not Specified neither Solely in Nucleic Acid Structure nor in Any Other Component of Biological Systems Kimura  demonstrated in his neutralist theory that the relation between genotype and phenotype is wider than it was assumed by the neo-Darwinist tradition, by showing that molecular evolution and the evolution of form are separate phenomena. For example, mutation rates of different species are rather constant according to the molecule studied and the time range. Species that are considered as living fossils (e.g., sharks), and which, therefore, should carry fewer mutations (since they changed less over time), display mutation rates very similar to any other species, when certain loci are studied . Furthermore, this problem regarding genotype/phenotype relation has also been extensively discussed by developmental biologists [10–12]. An important aspect of developmental approaches is the preoccupation with how the organ- In neo-Darwinism, all biological information is assumed to be specified in the genes. Thus, studying evolution would amount to understanding the fluctuation of genotypic frequencies in populations throughout their lives. These ideas have been extensively discussed by many authors [see 13, for a lucid summary]. A well-known example which contradicts genetic determinism is the experience of Kollar and Fisher , which deals with hen’s teeth. Birds are toothless and it is generally agreed that they do not have genes for teeth. However, experiments in which grafts of chick epithelium combined with mouse molar mesenchyme produced a variety of dental structures. In other words, the formation of teeth is not determined only by the nuclear content, but rather depends on the developmental context [discussed in 15]. Oyama  also presents important reflections concerning this issue. This author points out that an organism’s nature – the characteristics that define it at a given time – is not a genetic program or plan causing development, but a product of development. Thus, Oyama  argues that nature is not transmitted, but constructed, and what passes from one generation to the Complexus 2006;3:211–216 Exaptive Immunity (1) The Genotype/Phenotype Relation Is neither Simple nor Direct Downloaded by: Univ.of Adelaide 18.104.22.168 - 10/26/2017 1:38:17 PM natural selection determines practically all organic forms, functions and behaviors. The organism becomes a set of individualized structures (traits), each one of which performs actions that were optimized by natural selection, with a specific function . This teleological, functionalist view focuses mainly on understanding function; the structure and organization of living systems becomes a secondary issue. Gould and Lewontin  and Gould and Vrba  challenged this dominant adaptationist (functionalist) program by introducing two new concepts: spandrels and exaptation, to indicate ‘characters which are constructed and are useful in virtue of their internal organization and structure, rather than by a process of natural selection’. A clear example of exaptation is the relation between feathers, wings and flying. Bird feathers and wings have always been seen as adapted for flying. Obviously, they make flying possible, but since feathers were already present in nonflying small theropod dinosaurs , possibly involved in thermal insulation, they can no longer be seen as adaptations for flying. The relation between wings and flying in insects also appears to be exapted from aquatic surface skimming . The importance of the concepts of exaptation and spandrels is in indicating that the adaptationist program may explain some evolutionary patterns but is neither sufficient, nor necessary to the understanding of evolutionary processes . By being generated at random and either activated and expanded by external antigens (immunity), or inhibited (suppressed, regulated) by body components (natural tolerance), the ‘immune system’ has no predictable form (organization) of its own and becomes a passive meeting point of external forces. This is exactly the way the organism is Ramos /Vaz /Saalfeld Complexus 2006;3:211–216 (3) In General, Evolution Is neither Slow nor Gradual Eldredge and Gould  in their theory of punctuated equilibrium stress that evolution of form is neither slow nor gradual. They argue that evolution includes long periods of stasis, with few morphological changes (in spite of constant mutation rates), which are interrupted by short periods of intense morphological change, which are ‘punctual’ on a geologic time scale. This is supported by the fossil record, in which there are virtually no intermediary forms between species [reviewed by Gould and Eldredge, 17]. If evolution were a slow and gradual process of transformations, the fossil record should be expected to contain innumerable intermediate species. Gould’s Approach: The Spandrels of San Marco and Exaptations According to Gould , the differentiation between an adaptive and an exaptive characteristic can be made based on the historic order of emergence of a certain characteristic and then one may observe whether its current utility arose together with the historic origin or a posteriori. Utility arising a posteriori signals an exaptation. Therefore, this inference can be made based on the fossil record in a phylogenic context. We suggest that comparative immunology offers other examples of exaptation. Comparative Immunology In comparative immunology, critical points in the development of the immune system are: the emergence of the MHC (major histocompatibility complex), and of molecules containing variable regions generated somatically: immunoglobulins (BCR or Ig) and T cell receptors (TCR) and also of a unique process of receptor diversification . Both BCRs and TCRs are formed in somatic processes of rearrangement of two or three gene segments, which transcribe variable (V), junctional (J) and diversity (D) regions, which are then joined to a constant (C) region. In this process, variations may also occur on the exact point of section of the genetic elements, creating further diversity. Moreover, extra nucleotides are added in the junctions of genetic elements, generating the so-called N regions. These processes of gene rearrangement and the subsequent process of polypeptide chain pairing to form the complete receptors are potentially able to generate an immense diversity of clonally expressed receptors individually expressed on lymphocytes . In the processes of gene rearrangement, two recombinases known as recombination activating gene (RAG), RAG1 and RAG2, play a crucial role. These two enzymes are found only in jawed vertebrates and their emergence in phylogeny coincides with that of adaptive 213 Downloaded by: Univ.of Adelaide 22.214.171.124 - 10/26/2017 1:38:17 PM In what is now a classic paper on discussions about evolution, Gould and Lewontin  used an example from architecture to demonstrate the preponderance of structural features over functional interpretations. Inverted triangular structures may result from building arches to support the ceiling. In the Saint Mark’s Basilica, in Venice, these inverted triangles are covered with mosaic figures so elaborate and harmonious that one may be led to believe that these works of art are the reason for the surrounding architecture. However, there is an inversion in this reasoning. The primary construction plan of the building included, but did not aim at, the generation of large inverted triangles which were later decorated with colorful mosaics: the triangles inevitably arose from building the supporting arches. In architecture and art these structures that arise a posteriori are called spandrels, i.e., a spandrel is a structure that is formed as a consequence of previous architectonic structures, and not as the primary intention. Gould and Lewontin  suggested that spandrels may be used as metaphors for biological structures, that is things that ‘design the physical properties – form, position, constitution and number – which must arise as inevitable consequences of primary reasons or alterations in complex structures’. A few years later, Gould and Vrba  introduced another new concept into evolutionary biology, exaptation, which is related to the concept of spandrels and which provokes a reevaluation on the use of the term adaptation. These authors argue that the word adaptation has a clear teleologic connotation: ad + aptus – in the direction of an aptitude. Inside the neo-Darwinian perspective, this teleology is explicable by the selective pressures that guide evolution in the direction of the fittest. However, to see every useful trait as ‘adapted’ reinforces the adaptationist program and hinders the consideration of other possibilities. Therefore, Gould and Vrba  separate ‘adaptation’ – those characteristics built by selection for a defined function – from exaptation – those characteristics which are not built by selection, but are useful for their effects. The major implication of the term exaptation is to refute the functionalism essential in biological thinking. Gould and Vrba  end their paper on exaptation stating that the time and frequency of observed exaptations will confirm whether this term will become relevant. The identification and discussion of examples of exaptation in immunology will have the double meaning of subsidizing the importance of the concept of spandrels and to uproot the strong association between immunology and neo-Darwinism. next is an entire developmental system, not only genes. 214 transfer, a nondirected form of evolution, has proven to be a wider phenomenon, leading Brosius  to put forward the idea that ‘Genomes were forged by massive bombardments with retroelements and retrosequences.’ Among the several characteristics of mammalian organisms acquired by horizontal transfer, one of the most dramatic is that related to syncytin, an important mediator in mammalian placenta morphogenesis, which was identified as a retroviral envelope protein . Another interesting issue is the regulation of the expression of RAG genes. The gene responsible for the activation of these recombinases was found in Amphioxus (Cephalochordata), a member of a close group of organisms that received the microbial genes . Unless it is assumed that these genes were already ‘predicting and waiting’ for the insertion of a viral gene, it seems quite clear that this loop of gene activation also represents an exaptive phenomenon, devoid of intentionality (directionality). This exaptation of viral integrases into recombinases in vertebrate organisms seems to coincide with an important period in the evolution of adaptive immunity. The emergence of adaptive immunity is intriguing because instead of a slow and gradual evolution, it arose ‘as a big bang’ . In the words of Bartl et al. : ‘In conclusion, at this time adaptive immunity appears to have evolved ‘‘with a bang’’ or all at once in an ancestor of the jawed vertebrates with all components appearing out of nowhere.’ This sudden emergence, as well as many data from comparative biology of immune systems, seem to demonstrate two aspects: first, that this is an exaptive condition; second, that it is an outstanding example of Gould and Eldredge’s punctuated equilibrium. To argue in favor of this punctual event in the genesis of the immune system, we must examine the time scales on which these processes took place. The first vertebrates appeared approximately at 525 million years ago. It is estimated that those organisms had no lymphocytes, nor any functional component of adaptive immunity . At about 450 million years ago, there was a separation of taxa forming the jawless (Agnatha) and the jawed (Gnatostomatha) vertebrates. Living representatives of Agnatha are the lampreys, which have no lymphocytes, but have recently been shown to generate a diversified repertoire of receptors based on lysine repeats [27, 28]. On the other hand, all the living jawed vertebrates, including cartilaginous fish (sharks, rays and kin), bony fish and other groups that evolved from them, display an adaptive immune system. Thus, it is assumed that the lymphocytic system must have arisen at a point after the split between jawless and jawed fish. Also, as the system is present in cartilaginous and bony fish, it is accepted it was formed before this separation. And this is the curious thing: the period between the formation of jaws and the separation between cartilaginous and bony fish is extremely short (punctual, in geologic time units) and presents no intermediate forms. Furthermore, after its inception, approximately 450 million years ago, the system remained practically unaltered, corresponding to a period of stasis, as posited in the theory of punctuated equilibrium. It is not our purpose to describe in detail the evolution of the immune system, but rather to emphasize that this evolution was not driven by selective pressures, and the immune system was not formed as a system optimized for the defense of anti-infectious the organism. The suggestion of a nondirective exaptive emergence of the process of diversification of lymphocyte receptors implies that immunological defense is an exaptation that corroborates the theory of punctuated equilibrium. Rinkevich  also refutes the idea of a pathogen-directed Complexus 2006;3:211–216 Exaptive Immunity Downloaded by: Univ.of Adelaide 126.96.36.199 - 10/26/2017 1:38:17 PM immunity. In spite of many efforts, it has been impossible to find phylogenetic relations between these recombinases with other molecules present in ancestor animal lineages, as would be necessary to indicate a slow and gradual process culminating in the current processes of gene rearrangement. The genes coding for these recombinases seem to have been acquired by horizontal transmission – directly from the genome of a microorganism or a virus to the genome of a jawed ancestor vertebrate . The main facts corroborating this hypothesis are: (1) Despite many attempts, molecules showing homology to RAG1 and RAG2 have not been found in multicellular organisms out of the group of jawed vertebrates. (2) Microbial integrases and integration host factors present high homology to RAG1 and RAG2, respectively. (3) These RAG genes resemble those of microbes in lacking introns. (4) The spacer segment between RAG 1 and RAG 2 contains defective pieces of retroposons. (5) Constructs consisting of RAG1 and RAG2 may act as transposons . These data strongly suggest an exaptive character in the evolution of the process of lymphocyte clonal receptor diversification. In other words, these genes were not initially adapted to optimize host defense mechanisms: they originated in the context of a different structural dynamics, derived from a microorganism or a virus, participating in nonimmunological processes, such as the integration of viral DNA to the genome of other cells. In a fortuitous punctual moment of evolution, these genes were integrated into the genome of a jawed ancestor vertebrate and this became part of the evolution of its genome. This is quite different from imagining selective pressures in the generation of a diversified repertoire of receptors aimed at antiinfectious defense. Horizontal genetic Ramos /Vaz /Saalfeld Concluding Remarks There is a series of exaptive events in the evolution of the immunological activity which are not consistent with the idea of a system adapted for defensive functions of the organism. Therefore, it is misleading to present anti-infectious defense as the main reason guiding the evolution of immune systems. We argue that immunological defense is a possible result of what takes place, not a mechanism driving what takes place – a commentary that says nothing about the systems involved in its generation, or, in the words of Atlan and Cohen : ‘danger, like self-nonself, is a human’s teleological interpretation of what evolution ought to be doing’. Accepting that this is so opens the possibility for different questions and approaches in immunology, and argues for the rupture of the conceptual knot between immunology and neo-Darwinism (functionalism). Data from comparative immunology support the idea of an exaptive (non-pathogen-instructed) phylogenic drift of the lymphocytic system instead of an evolution adapted for defensive functions. Furthermore, seeing the immune network as a spandrel allows us to focus the attention on the organization and structure of the immune system and its insertion in the organism. Although an alternative theory is not proposed, the application of Gould’s terminology to immunology, as in evolutionary biology, demands abandoning the functionalist notions of neo-Darwinism. Acknowledgments We are grateful for having received suggestions from and having discussed central aspects of this manuscript with Dr. Jorge Mpodozis, Dr. Francisco Botelho and Dr. Evelyn Fox Keller. This study was supported by the CNPq-Brasil grant 30.5043/2003-0 to N. Vaz. Complexus 2006;3:211–216 References 1 Gould SJ, Lewontin R: The spandrels of San Marco and the panglossian paradigm: a critique of the adaptationist programme. 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