How Our Evolution Is Written In Our Genome
Novel ideas and theories may approach a person of science and a believer of a parallel story of life other than Adam and Eve’s in a significantly different way. A curious being looking for a strong explanation behind explicit phenomena of life and its processes even pose a challenge towards the Nature in demand for science behind everything and every aspect. The question he asks to the Nature remains more or less the same when it first arises- “Man! How do you do it?”
Recently I came across a model organism in the field of genetics, developmental and population studies which was earlier not-so-much known to me. The Mexican blind cave fish or Astyanax mexicanus is a fabulous organism with lots of known and still unknown characteristics which may be helpful to the scientists to understand plethora of rules and theories and processes which have arisen questions like how evolution of life has resulted into such dramatic visible differences among us. Over two decades ago in 2001, a review by William R Jeffery (Jeffery WR, 2001) presented a very fruitful information that how two forms of Astyanyx — surface and cave dwelling, may help in understanding the phenotypic differences which have arisen during the course of evolution essential to accommodate in the dwelling habitat. The generation of novel phenotypic traits and modification in the present forms during the evolutionary process is an integral part of the Darwinian natural selection — the loss of eyes during the development, loss in sleep behavior, decrease in the level of social behavior and aggression, loss of skin pigmentation, enhancement in lateral line system and olfactory senses of a blind cave fish are the best examples that how animals for millions of years have continued to construct and deconstruct changes (Kowalko J., 2020). Within a population of surface dwelling Astyanyx, which accidentally reached and inhabited dark cave for the first time, through random mutations suddenly a ‘weirdo’ fish was born which had lost its eyes at the adult stage. Nature follows two criteria for the selection of any trait, first the trait should be inheritable and second, the trait should have competitive advantage over the others already prevalent within the population. As Dobzhansky once said “nothing in biology makes sense except in the light of evolution”, it remains true for the phenomenon of degeneration of eyes in the first ever blind cave fish, the visual system is one of the most energy-demanding system of the brain. This system comprises only 2% of the total body weight but it does consume 20% of the total oxygen and 25% of the total glucose used as a raw material for the energy supply (Wong-Riley, 2010). Nature selected the ‘blind’ one over the other ‘normal’ Astyanyx in the cave environment, and the basis of such selection was obvious — who needs a highly energy expediting visual system when there is literally none of its need in so much dark! This poses a perfect example that how the clever tactics of survival have their own way of qualifying to get written in the genomes of the descendants.
There are many molecular evolutionary routes at the level of genes through which evolution of novel or modified phenotypes may arise. The source of evolution of novel genes may be a function of gene duplications, exon shuffling, chimeric gene formation or gene formation from RNA intermediates (Long and Langley, 1992, Kaessmann et al., 2009, Kaessmann H., 2010). A well-studied classical example of young fusion gene has been observed in Drosophila sp. which was named jingwei (jgw)(Long and Langley, 1993). The gene jgw is a testis expressed gene in D. teissieri and D. yakuba which are sibling species. This fusion gene through the evolutionary course of time has been formed by the fusion of segmental duplicate copy of ynd gene and the retrocopy of Alcohol dehydrogenase gene (Adh). The part of ynd gene forms the regulatory elements of the novel gene. The novel gene thus formed (jgw) gained a new functional role in hormone and pheromone metabolism (Kaessmann et al., 2009).
The de novo genes or the genes which arise from previously non-coding genic regions are one of the recent and most interesting topics of studies in evolutionary molecular biology (Schlotterer C., 2015). A very recent article by van Oss and Carvunis (van Oss and Carvunis, 2019) explains the process of formation of de novo genes. Studies in Drosophila have revealed that de novo genes are both male-biased and X-linked and some parts of the X-chromosomes are more prone to giving birth to such de novo genes (Levine et al., 2006). Perhaps such biased births ensure the invasion of the de novo genes within the populations as male reproduction differs from the female reproduction in which recombination between sex chromosomes is restricted to specific regions only, this helps in bringing together beneficially interacting genes (Kauppi et al., 2012, Ponnikas et al., 2018).
The recent decades in the researches related to evolutionary biology have revealed many interesting and intricate details of being a male or a female. One of the most important aspects of such studies is ‘recombination suppression’. For thousands of years, recombination has played central role in bringing adaptations and changes in the genetics of populations in rescue towards severe environmental changes (Uecker and Hermisson, 2016). However recombination can be considered ‘bad’ also, as it disintegrates or breaks apart a favourable combination of alleles (Stapley et al., 2017). Recombination suppression is a phenomenon which is one of the two characteristic features of the evolution of the heterogametic sex chromosomes, between X and Y in humans and Z and W in birds (Yuan et al., 2018, Sun and Heitman, 2012 ). The phenomenon of recombination suppression explains the lower rate or comparative absence of recombination between sex chromosomes. This process is followed by the degeneration, loss of the regions of non-recombining sex-limited chromosomes (Y in humans) along with profound heterochromatization (Wright et al., 2016, Uecker and Hermisson, 2016). The exact reason why sex chromosomes recombination suppression has evolved throughout the history of life remains unclear, but its role in fixation of co-adapted gene complexes on the chromosomes is considered to be important. These co-adapted gene complexes, called the ‘supergenes’ are inherited as a single ‘unit’ to bring about phenotypic adaptations and speciation within the range of species (Wright et al., 2016). It has been speculated that during the course of evolution if dioecism has to be evolved from hermaphroditism or monoecism within the species, it is important that the recombination is suppressed between sex-determining genes and sex-antagonistic genes (Bergero and Charlesworth, 2009, Wright et al., 2016).
Y-chromosome evolution predicts that it would probably get disappeared in the coming ~5 million years. The gradual degradation of non-recombining regions has decided an inevitable fate for this sex chromosome, probably. The recombination suppression has resulted into higher degree of genetic drift and less efficient natural selection over the concentrating deleterious mutations, therefore a long-term impact is suggested to be a gradual genomic decay and loss of the Y-chromosome (Sun and Heitman, 2012).
‘Sexual antagonism’ or the ‘sexual conflict’ is another controversial topic of study these days which focuses upon the ‘arms race between two sexes’ in terms of evolution. The fitness optima of male and female differs, which may lead to conflict in the level of natural selection between the two sexes. We have known this for ages obviously, that how sex or reproduction between two individuals of opposite sexes helps in the transmission of genes of both the participants to the next generation, and the offspring acts as the vehicle or carrier of the their genes — this is what determines their fitness by natural selection. But this should not be confused that sex is an evolutionary cooperative enterprise between the two sexes, the selection also depends upon the choice of selection of the partner (Gangestad, 2003).
The evolutionary process seems to be a learning process for the genome of any organism, with natural selection acting like a teacher or a mentor whose job is to correct this process of learning. Provision of punishment is there to eliminate the misdeeds which may result into the decrement in the ‘performance’ of the pupil. This is a fascinating fact that how our genome can give answers to almost all the questions which we may ask about life and its origin, but it is the beauty of the Nature that most of the data would probably take another thousand years to get decoded!
References:
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Gangestad SW. Sexually Antagonistic Coevolution: Theory, Evidence, and Implications for Patterns of Human Mating and Fertility. In: National Research Council (US) Panel for the Workshop on the Biodemography of Fertility and Family Behavior; Wachter KW, Bulatao RA, editors. Washington (DC): National Academies Press (US); 2003. 8. Available from: https://www.ncbi.nlm.nih.gov/books/NBK97294/
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