Jorge Herkovits
Instituto de Ciencias Ambientales y Salud, Fundacion PROSAMA, Paysandu 752 (1405) Buenos Aires, Argentina. herkovit@retina.ar
Key words: ontogenesis, developmental biology, environment, biomarkers, evolution, record, evo-devo, evoecotoxicology, paleoecotoxicology, gaia theory
Abstract. Although it is generally accepted that environmental features can select resistant individuals within species and resistant species within ecosystems, most of our understanding of the evolutionary process´ interactions between the environment and life forms is extremely limited and controversial. This article focuses on the hypothesis that living organisms at ontogenic stages could be considered biomarkers of the successive interactions with the environmental agents during the evolutionary process including their influence on the environment toward a living planet. A proposal for a universal brand name to link all fields related to ontogenesis and evolution is suggested.
- The established links between ontogenesis and evolution
The link of ontogenesis to evolution originates in the comparative embryology of the nineteenth century in the work of von Baer and Haeckel whose “laws” -embryonic divergence and recapitulation- were presented as being generally applicable to the way in which phylogenesis evolves. Although ontogenesis had a minor role both in the work of Lamarck, Darwin/Wallace and the establishment of the modern synthesis in evolution, the developmental patterns of otogenesis help to explain the branching evolutionary divergences in the metazoan and the relationships among phyla (Maier 1999). As developmental genetics advanced it became increasingly evident that every species is like a history book. Present day evo-devo erupted out of the discovery of the homeobox and the conservation of spatiotemporal expression pattern of those developmental genes. It is increasingly evident from evo-devo studies on organisms ranging from plants to mammals that the evolution of distinct morphologies relies on the reutilization of a relatively small set of master regulatory genes such as the Hox genes, which establishes a segmental pattern of activation (Meyerowitz 2002). This process of developmental reprogramming implies that if a particular ontogeny is represented by a trajectory through multidimensional phenotypic space, then many types and degrees of morphological differences can be caused by programming a different trajectory (Swalla, 2002; Davidson 2002). For instance, the great extent to which changes in plant forms have been engendered was exemplified by heterochrony (temporal shifts in developmental pathways) or heterotopy (spatial shifts in developmental pathways). Assessment of heterochrony or heterotopy is usually attributed to morphological features and seems to occur due to the expression of developmental genetic pathways in a new context. The ontogenetic trajectory is often influenced by environmental factors (the reaction norm). This sometimes occurs in a rapidly adapting manner, as in the case of aquatic plants that make different kinds of leaves above and below water (heterophylly), or the insects that produce winged and wingless forms at different population densities. Moreover, the whole ontogenetic trajectory could be deflected in a continuously variable way in response to environmental conditions. Eco-evo-devo, a study focused on the interactions between an organism’s environment, genes, and development, contributes to understand the evolutionary implications of these phenomena. Its major goals are to uncover the rules that underlie the interactions between an organism’s environment, genes, and development, and to incorporate these rules into evolutionary theory; for a recent review see Abouheif et al (2014).
2. The record of evolutionary environmental signatures during the ontogenesis of living organisms.
Environmental features essential for ecosystems, conservation of biodiversity and human health evolved from the time of the Earth´s accretion around 4.5 billion years ago. These features were crucial for the development of biota from its early forms, around 3.8 billion years ago. Present day biodiversity, estimated to comprise more than 100 million species, has developed on the basis of the ability of life forms to adapt to the evolving environmental scenarios at a planetary level and multiply at a rate that surpassed both background and mass extinction events. Aside from the abiotic inputs of chemicals in the environment, a living planet was established with environmental conditions increasingly of biological origin as life expanded worldwide and biodiversity increased. For example, the increased O2 in the water and atmosphere was due to the achievement of photosynthetic water-splitting capacity about 2.4 billion years ago (Anbar and Knoll 2002). However, our understanding of the evolutionary processes linking the various life forms and their environment is very limited, especially for the initial 3.3 billion years of evolution. This fact is due to the sampling intensity from different periods of time, as well as the preservation of fossils which bias our knowledge on the existent biodiversity and environmental conditions during different periods of the evolutionary process. The uncertainty for these ancient times is so profound that even in a concept article on the development of multicellular biodiversity, no time for this basic evolutionary step is suggested (Wolpert and Szathnary 2002). Thus, it could be highly valuable to explore alternative avenues to contribute to the understanding of the evolutionary processes on the Earth. In this article, I will revisit case studies to research how radical changes in metabolic features and the susceptibility to noxious agents during ontogenesis can be used as biomarkers of environmental features during the evolutionary process (Herkovits, 2006).
- i) The oxygen signature. The oxygen consumption of amphibian embryos exhibit marked changes as embryonic development advances: at the egg cell stage 5.7, tail bud 35, open mouth 118 and at complete operculum 180uL/hr for 100 embryos (Herkovits and Jatimliansky 1982). Conversely, survival times in anoxia shifted from more than 30 h at 2 days after fertilization to 20 h at 10 days of age, to only 2-4h at 14 days of age (Adolph 1983). A shift in the mitochondrial enzymes towards an aerobic metabolism was reported at the gastrula stage (Lovtrup-Rein and Nelson, 1982). Anaerobic metabolism in early embryonic stages was reported in a wide range of species and therefore could be generalized to embryonic development. For instance, the energy that mammals require in the preimplantation embryo are generated by anaerobic metabolism. (Adolph 1983, Robkin 1997; Burton 2003). Moreover, reducing oxygen concentration from atmospheric levels during in vitro culture generally improves early embryonic development across a range of species (Booth et al 2005). The same pattern of low oxygen uptake by early stage embryos occurs in invertebrates such as intertidal crabs, which increases its O2 consumption over 10-fold by the time of hatching (Taylor and Leelapiyanart 1997). Artemia is an extreme example, as 60% of early life stage embryos could survive 4 years of continuous anoxia at physiological temperatures (Clegg 1997). The anaerobic metabolism at early developmental stages in different species can be interpreted as an evolutionary trait that protects the embryo from oxidative stress damage and/or as an array of adaptations that enable them to survive a wide variety of environmental extremes. As Though the embryo should successfully pass through all of its developmental stages, this does not seem to be advantageous for present day environmental conditions (which have existed through the last 2 billion years), to achieve anaerobic metabolism as an adaptive feature just for the initial developmental stages (Herkovits, 2006).
Is this critical metabolic change during ontogenesis related to an environmental signature during the evolutionary process? From an O2 perspective, it is generally accepted that the biological and geochemical history of Earth can be separated into two supereons (Anbar and Knoll 2002). Based on independent geochemical evidence on oxygen availability, it can be deduced that surface water was already oxygenated between 2.4 and 2 billion years ago (Rue and Bruland 1995; Johnson et al 1997). O2 is usually considered the first biogenerated environmental pollutant to appear in large quantities on the planet, after which anaerobes died or restricted themselves to environments that O2 did not penetrate, while other organisms began the evolutionary process of evolving the use of O2 for metabolic transformation e.g. for efficient energy production (the mitochondrial oxidative phosphorylation system). In this context, the fact that living organisms exhibit anaerobic metabolism at early developmental stages is generally accepted to be the case for living forms in the ancient anoxic Earth. This provides support for the hypothesis that living organisms recapitulate their metabolic features at embryonic stages during their phylogenetic evolution and conversely, they could be considered as biomarkers of environmental features such as the transition from an anoxic to an oxic Earth. Moreover, taking into account that intercellular adhesiveness exists in protists and is achieved in amphibian embryos at very early blastula stage (Herkovits 1978), the earliest multicellular life forms seem to be a very ancient achievement in the anoxic Earth. In any case, it is noteworthy that the recapitulation at early embryonic stages of ancient life has features like anaerobic metabolism, which seem to be essential for the accomplishment of normal ontogenesis even for vertebrates. Anaerobic multicellular organisms exist in present day, but their evolution in complexity is very limited compared to those achieved by aerobic metazoans.
The transition towards an aerobic metabolism from the gastrula stage in living organisms corresponds within the EVO-DEVOenv perspective to an evolutionary period with major achievements in cell differentiation and morphogenesis in the rising free oxygen environment. In the case of amphibian phylogeny it implies the rearrangement of the cells toward a tridermic organism, and within this process the evolution led to early chordate life forms. Moreover, an increasing aerobic metabolism seems to be essential for the development of the cephalic nervous system (Herkovits, 2014). Based on a battery of studies including phylogenetics, structural biology, protein engineering, metabolism, competition and genomics, it was concluded that adaptation to environmental conditions was already in place 3.5 billion years ago (Zhu et al 2005). Therefore, the creatures from the Ediacaran, Tommotian, Oman, Chengjiang and Burgess Shale can be viewed as part of a broader picture with roots in multicellular life forms in the deep anoxic Earth history. The EVO-DEVOENV theory implies a new dating method, e.g. the shift from anaerobic to aerobic metabolisms during early embryonic stages versus the transition from an anoxic to an oxic Earth, allowing us to anticipate that multicellular organisms flourished over 2 Gy ago (Herkovits, 2006); they were discovered in 2010 (El Albani, et. al. 2010).
The colonization of terrestrial habitats by amphibians around 380 million years ago (Carol 1987) implied not only the shift from uptake of oxygen from water to the higher oxygen concentrations in the air, but also the possibility of mobilization requiring less energy demand from the media from which oxygen is obtained. Among biomarkers of the access to the higher oxygen environment and the concomitant need for enhanced protection against the increasing oxidative stress, a novel and more efficient glutathione S-transferases isoenzyme is highly expressed in post-metamorphic amphibian liver (Amicarelli et al 2004). It is well accepted that this last evolutionary step related to oxygen consumption in amphibian phylogeny corresponds to a process working its way towards occupying a new niche, the terrestrial habitat. The access to higher free oxygen levels at least 200 million years ago seems to have rendered living organisms with homoeothermic metabolism (Carol 1987). It is noteworthy that both the mammalian as well as the bird embryo have poikilothermic metabolism till birth (the homoeothermic condition is provided by the parent organism). The usual explanation of this fact is based on the energy needs of the embryo, which are considerably less than if it were a comparably sized homoeothermic organism. Although in present environmental conditions this explanation could be acceptable, from the EVO-DEVOENV perspective poikilotherm metabolism of the avian and mammalian embryo reflects the evolutionary process starting from anaerobic living conditions during the anoxic period of the Earth onwards to the last metabolic achievement.
3. The susceptibility to noxious agents at ontogenic stages versus environmental signature during evolution
Could the well documented stage dependent susceptibility to physicochemical agents during developmental stages (Herkovits et al 1997; Rutledge, 1997; Degitz 2000; Kast-Hutcheson et al. 2001; Fort et al., 2004) be related to environmental features during the evolutionary process? The high resistance at the blastula stage to physicochemical stress (Perez-Coll et al 1990; Herkovits et al 1997), enhanced in free living embryos by protective barriers like the vitelline membrane and jelly coats could reflect very aggressive environmental conditions during the evolution of early multicellular organisms. For instance, the high toxicity associated with UV-B irradiation in the anoxic Earth could be associated with the very high resistance of amphibian embryos at the blastula stage to UV-B (Castañaga et al, 2007), as well as other agents exerting oxidative stress (Pérez-Coll and Herkovits 1990; Herkovits et al. 1997; Vismara et al 2001). Again, the reciprocal elucidation of ontogenic features and environmental signatures during the evolutionary process provides support that metazoan organisms existed in the ancient anoxic Earth. Moreover, by focusing on these early developmental stages, there is a remarkable stage-dependent susceptibility within the blastula (Bustuoabad et al. 1977) reflecting environmental changes during the early phases of multicellular life forms in the anoxic Earth. This is not surprising when evaluated from an EVO-DEVOenv perspective, as blastula-like organisms existed during hundreds, if not over a billion years of evolution in the anoxic Earth.
The high resistance to environmental agents during the initial developmental stages contrasts with the high susceptibility of the organism as cell differentiation and morphogenetic processes achieve increasing complexity (Pérez-Coll and Herkovits 1990; Herkovits et al. 1997; Vismara et al 2001; Bogi, 2003; Christensen et al 2005). This juxtaposition is directly related to a gradual increase in aerobic metabolism and the associated oxidative stress. The fact that the organogenic stages are very susceptible to noxious agents in spite of the high capacity of the embryo at those developmental stages to recover from adverse effects (Herkovits and Faber 1978) contributes to the idea that the increasing complexity of cell differentiation and morphogenesis could be achieved during the evolutionary and ontogenetic processes under benign, low environmental stress conditions. Metamorphosis, also a complex cell differentiation and morphogenetic process in both invertebrate and vertebrate organisms, is also a period of very high susceptibility to a variety of environmental agents (Howe et al. 2004; Wilson 2004). Thus, the stage dependent resistance profile to noxious agents during the ontogenesis of any species could reflect the environmental stresses supported by their ancestors during the evolutionary process.
4. Ontogenesis and the construction of a living planet.
Global environmental conditions can be altered by both abiotic and biotic inputs (Herkovits, 2006). The biota has a significant effect on the Earth’s environment, such as the rise of free oxygen during the evolutionary process (Anbar and Knoll 2002) and oxygen production and consumption from that time onwards. According to the Gaia hypothesis the whole ecosystem can be seen as a giant organism in which life tends to optimizes both the physical and chemical environments to best fit their needs (Lovelock, 1986; Lenton 1998). The rise of O2 in the water and atmosphere initiated by photosynthetic cyanobacteria about 2.4 billion years ago represents an example of the magnitude of the impact of living organisms on the Earth´s environment and the effects of environmental changes on living forms. We discovered that amphibian embryos neutralize the acidic condition produced by different noxious agents like glyphosate, aluminum or even citrate buffer (Piazuelo et al., 2011; Herkovits et al., 2015). As acidic conditions were documented in ancient environmental scenarios (Knoll et al., 1996), the capacity of amphibian embryos to neutralize acidic pH could be considered a biomarker of ancestral organisms actively adjusting environmental conditions to their needs. Thus EVO-DEVOENV also provides the possibility of studying the effects of living organisms on ancestral environmental conditions. This could contribute to a better understanding of the coeveolution of living organisms and their environment, providing the possibility to obtain experimental data on the participation of individual species or a set of species in the buildup of environmental condition that benefit life. As a whole, based on the EVO-DEVO ENV synthesis, our study provides some evidence that a pH of 4 probably was the lower pH condition in the habitats of South American amphibian ancestors, it is the limit for embryo survival, their capacity to modify environmental pH and thus their lower limit within the resilience phenomenon. During the last few years several model systems have emerged for addressing the interconnectedness between an organism’s environment, its development, and its ecological interactions in natural populations (Van Valen’s 1973; Ledón-Rettig and Pfennig 2011). Our results point out that besides internal mechanisms of defense against toxicity, living organisms could contribute to modifying environmental pH toward their benefit.
- Developing a unique brand name to link ontogenesis and evolution.
During the last 20 years, there has been a notable increase in contributions that are oriented to link ontogenesis and evolution from different perspectives. There are journals and interest groups devoted to this field like the European Society for Evolutionary Developmental Biology, the Society for the Advancement of Metadarwinism, the Pan American Society for Evolutionary Developmental Biology, etc. Generally, most of the traditional scientific societies in the biological field could be interested in contributions linking ontogenesis and evolution. As contributions have emerged from different disciplines, several brand names, acronyms and definitions have been presented in the literature e.g. eco-evo-devo in the case of the ecological impact on development, Evoecotoxicology in the case of ecotoxicological criteria to report the record of environmental signatures during the evolutionary process during the ontogenesis of living organisms, etc. EVO-DEVO is by far the most popular term and acronym. Thus, my proposal is to use EVO-DEVO as the brand acronym for all the studies oriented to reporting links between ontogenesis and evolution and by means of a superscript identifying the main discipline of each contribution. For instance GEN if the study focuses on genetic aspects (EVO-DEVOGEN), ENV in the case of environmental issues (EVO-DEVOENV), ECO in the case of ecological subjects (EVO-DEVOECO), PHYSIO in the case of physiology (EVO-DEVOPHYSIO), PATH in the case of pathology (EVO-DEVOPATH), etc. By using this method, we will construct a robust multidisciplinary and interdisciplinary brand name for all the contributions focused on establishing the links between ontogenesis and evolution orienting from the brand name the nature of each contribution.
- Conclusion
The possibility of considering living organisms at ontogenetic stages as biomarkers of the evolutionary process of both environmental features and living forms allows the reconstruction of the evolutionary process on Earth. Some biomarkers of environmental conditions, like those related to the rise of oxygen starting about 2.4 billion years ago and the subsequent changes towards aerobic metabolism, have global significance and therefore it could be anticipated that they appear in all aerobic organisms at specific developmental stage(s) according to their phylogenetic trajectory. Conversely, biomarkers related to very local features like the case of serpentinite-hosted hydrothermal field beneath the mid-ocean ridge (Kelley et al. 2005) will occur only in organisms living in those environmental conditions. From a methodological point of view, simple ecotoxicological studies can provide the unique opportunity to study in just one experiment the natural selection process affecting individuals (the survival of the most resistant individuals) and the capacity of a population to adjust to environmental conditions (e.g. pH in the case of glyphosate, aluminum, etc) to their benefit (e.g. Herkovits et. al., 2015). The reciprocal elucidation approach of ontogenic features and environmental signatures during the evolutionary process could integrate information from toxico-ecotoxicology, geochemistry, paleontology, cell differentiation, morphogenesis, physiology, metabolism, genetics, epigenetics, pathology, evolution, phylogenetics, etc. as a multidisciplinary and interdisciplinary approach to understanding the evolutionary process by means of a rational and holistic explantation. It includes the susceptibility/resistance features to noxious agents (Herkovits, 2006). As an overall picture, multicellular, blastula-like organisms existed at least 2.4 Gy ago, but may even date back to 3 Gy ago in the deep anoxic world, while aerobic tridermic organisms emerged around 2.4 billion years ago. Initially, living forms had to survive in very adverse environmental conditions, which is the reason for their high resistance to noxious agents, including UV-B irradiation, a fact reflected in the present day by the high resistance to noxious agents at early developmental stages. Conversely, organogenesis and metamorphosis, both exhibiting complex cell differentiation and morphogenetic processes, were achieved on an evolutionary scale during low level environmental stress conditions expressed in present day as high susceptibility to a wide range of different noxious agents. This reflects complex processes requiring very low stress levels in order to achieve success. In summary, EVO-DEVOENV provides the possibility for considering each species as a history book of both the environment and life forms contributing to a better understanding of the evolutionary process on Earth.
Jorge Herkovits is a scientist of the National Council of Science and Technology (CONICET), Argentina. This study received support from Fundacion PROSAMA and I thank the skilful revision of English by Lilly Backer
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