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This article was originally published as a chapter in the book “Design and Catastrophe: 51 Scientists Explore Evidence in Nature"
Regardless of the worldview adopted, much evidence in the fields of geology and paleontology points to the occurrence of catastrophic events during the history of our planet. Based on the disasters that occur today, we can conjecture that past global catastrophic events caused climatic changes and affected the availability of food. Therefore, in a post-catastrophic context, surviving organisms were challenged to adapt to hostile living conditions.
In addition to the survival of the fittest advocated by natural selection, organisms in these post-catastrophic situations are also able to respond adaptively. This adaptation involves genetic and epigenetic changes. To understand how this might have occurred, we will consider the worm Caenorhabditis elegans (C. elegans). This nematode has been widely used as a model in different research fields, resulting in some scientific studies awarded with Nobel Prizes. In genetic studies, it is possible to correlate findings in C. elegans with several other organisms including humans, since 40% of its sequences are homologous to those found in humans.
Studies analyzing the response of C. elegans to different types of stress have demonstrated that this nematode not only presents beneficial epigenetic modifications during stress situations but is also capable of passing these changes to subsequent generations, allowing them to better deal with stresses of a similar nature.[1] Among other effects, epigenetic alterations can modify the genomic structure through the contraction or expansion of repetitive DNA. In addition, the modulation of these repetitive regions may increase the rate of evolution, resulting in drastic phenotypic changes over a few generations. Therefore, during stressful conditions related to post-catastrophic environments, surviving organisms could present genomic and phenotypic changes over a short time.[2] However, this is only one of the mechanisms capable of generating genomic and morphological changes much faster than those for which Darwinian gradualism advocates. Another mechanism that will be addressed in this chapter is the dynamic of transposing genetic elements in response to stress situations.
Also known as “transposons,” these elements have the ability to move from one genomic site to the other through the action of transposase enzymes that act by breaking the phosphodiester bonds present between the nucleotides and DNA double strands. The activities of the transposons can result in deletions, inversions, and chromosomal fusions having the potential to cause some diseases as well as to generate new genetic information. About 12% of the genome of C. elegans is formed by transposons, and the vast majority of these elements have lost their transposition capacity.
The ability to adjust to external disturbances is an essential property for all living organisms. In this context, the dynamic nature of the transposition elements active in C. elegans is essential for the organization of the genome in response to different stress situations. Temperature variations and oxidation are some examples of stressful conditions capable of inducing the expression of transposon genes.[3] Recent analyses have shown that even moderate exposure to these stresses is already capable of increasing the excision of transposons in a concentration-dependent manner.[4] The transposons respond to different stresses through transcriptional activator sites located in these elements that act as a system of defense against genomic disorganization. Some of these sites, for example, are very similar to those found in heat shock proteins. Interestingly, some heat shock proteins can act by regulating the action of the transposing elements in C. elegans during stress.[5]
In order for the response of stress-transposable elements to be adequate, there are mechanisms that regulate transposition at the transcriptional and post-transcriptional levels by means of epigenetic modifications and silencing by RNA interference (RNAi), respectively. At the transcriptional level, DNA compaction dramatically compromises, or decreases, the expression of transposable elements. Therefore, depending on the transcriptional modifications, especially of the N-terminal amino tails of the histones, the expression of transposable elements can be induced or repressed. In addition to this, it should be considered that under stress conditions DNA tends to decompress. Therefore, it is clearly noticeable that these two mechanisms of adaptation to stress situations are closely related.
The genomic dynamics in response to stresses in various organisms, including C. elegans, depend on the harmony between external stimuli and mechanisms of internal gene regulation and allow the genome of that organism to adapt to stress situations. Thus, stressful conditions related to post-catastrophic scenarios could cause epigenetic modifications and mobilization of transposition elements, and these events are interconnected. In addition, inhospitable post-catastrophic scenarios provide the opportunity for surviving populations to quickly occupy different niches and adapt to them.
The fine-tuning between these mechanisms of stress response and the emergence of adaptive changes is a strong evidence that the genomes of organisms (e.g., C. elegans) were designed to adapt to adverse conditions ensuing a catastrophe. Moreover, the unpredictable nature of disasters makes it very difficult to defend the Darwinian concept that mechanisms of genomic adaptation to such catastrophic scenarios could evolve gradually. Considering the information presented in this chapter, we can conjecture that epigenetic mechanisms and genetic transposition could have facilitated microevolutionary processes allowing the origin, over a few generations, of a large biodiversity from ancestral populations that had occupied the empty niches left after global catastrophes, such as the one described in chapters 6 to 8 of the book of Genesis.
NOTES
[1] A Klosin, E Casas, C Hidalgo-Carcedo, T Vavouri, B Lehner. Transgenerational transmission of environmental information in C. elegans. Science 2017; 356(6335):320–323.
[2] DM Ruden, MD Garfinkel, L Xiao, X Lu. Epigenetic regulation of trinucleotide repeat expansions and contractions and the “biased embryos” hypothesis for rapid morphological evolution. Current Genomics 2005; 6(3):145–155.
[3] CE Paquin, VM Williamson. Temperature effects on the rate of Ty transposition. Science 1984; 226(4670): 53–54.
[4] CP Ryan, JC Brownlie, S Whyard. Hsp90 and physiological stress are linked to autonomous transposon mobility and heritable genetic change in nematodes. Genome Biology Evolution 2016; 8(12):3794–3805.
[5] Ibid.; YM Cowley, RJ Oakey. Transposable elements rewire and fine tune the transcriptome. PLoS Genetics 2013; 9:e1003234.
Tiago A. J. de Souza is professor of science and religion and genetics/evolution at the Centro Universitário Adventista de São Paulo, and he is an invited lecturer at the Department of Biology at Andrews University. He earned his PhD in Genetics at the University of São Paulo. He has expertise in cytogenetics, molecular genetics, nanotoxicology, and evolution; has authored articles and book chapters on these subjects; and is a peer-reviewer for leading academic journals.