Biology subjects

Semon, M.

Publications and source records attributed to Semon, M..

12 recordsLinked to original sources

bioRxiv2026

Patterns of molecular conservation along tooth development are only partly shaped by evolutionary pressures on tooth

Although it is well established that certain stages of development are molecularly more conserved than others, the reasons for this phenomenon remain largely unknown. We study molecular conservation in the development of an organ, the molar, by comparing the temporal profiles of expression in mice and hamsters. We find that the cause of conservation of expression and of coding sequences changes over molar development. Gene expression levels display a classical increase of divergence as development progresses. In terms of genes expressed, the composition of early and late stages is better conserved and enriched in pleiotropic genes, yet each stage mobilizes different sets of pleiotropic genes, cell division for bud growth and secretion for tooth mineralization. Moreover similar patterns of higher divergence of gene sets and of coding sequences at mid development, are caused by different biological phenomena, in that case heterochronies and blood colonisation respectively. In conclusion, the patterns of molecular conservation in developing molars are shaped by a combination of processes intrinsic to the teeth, and by negative and positive selection on functions which are mostly extrinsic to the teeth. This is likely translatable to explain molecular conservation patterns in many other biological systems. AUTHOR SUMMARYFor species to evolve different adaptations to different life styles, their anatomy has to evolve correspondingly. This in turn implies evolution of the embryonic development of anatomical structures. Notably, tooth shape can evolve rapidly as an adaptation to different diets. Mice and hamsters are closely related rodents who yet differ in the shape of their molars, and thus in their development. In this study, we investigated why the genes active in molar development are more or less similar between the two species from early tooth bud to fully formed embryo molar. We found that early and late molar development were slow evolving, while mid-development was evolving faster. But surprisingly, this was in part due not to tooth evolution, but to the involvement of genes which are active in other processes in the body. For example an influx of immune cells also brings fast evolving immune genes. This helps us understand better the complexity of causes of apparently simple evolutionary patterns.

evolutionary biology↗

bioRxiv2025

Single-cell transcriptomics uncover conserved molecular mechanisms and functional diversification in multilayered epithelia

Multilayered epithelia, including skin, cervix, thymus, and prostate, arise from all three germ layers and perform diverse physiological functions, yet share a conserved layered architecture in which a supportive basal layer underlies differentiated suprabasal cells. To determine whether this common organization reflects a shared genetic program or tissue-specific regulation, we assembled a comprehensive single-cell atlas of 14 murine Multilayered epithelia. We identified a conserved p63-Notch axis that universally governs basal cell identity maintenance and suprabasal commitment. The p63-centered transcriptional program is shared, while Notch signaling recruits context-specific transcriptional modules to generate functional diversity. The layered architecture of epithelia spatially and temporally decouples conserved basal from context-specific suprabasal networks, compartmentalizing functional innovation across multilayered epithelia. Comparative genomics traces the p63-Notch axis to the root of vertebrates over 500 million years ago, showing that it was repeatedly co-opted through the incorporation of newly evolved genes in the suprabasal domain to generate novel epithelial functions. Our work establishes that evolution of multilayered epithelia operates through modular, compartmentalized diversification of a conserved scaffold, providing a unifying principle for how complex tissues achieve functional innovation while preserving structural integrity.

cell biology↗

bioRxiv2025

Pleiotropy accelerates tooth phenotypic and genomic evolution - An in silico study under the lens of development

Pleiotropy, which can occur when a gene affects multiple traits, is a central property of living organisms, influencing their response to mutations and their evolutionary trajectories. Despite many studies and discussions, it remains very difficult to reconcile molecular, developmental and quantitative evolutionary genetics viewpoints on pleiotropy and appreciate how much it puts constraints on genetic evolution, phenotypic evolution and adaptation. Here, we revisit this question by simulating evolution in silico. Our model captures multiple levels of integration observed in complex organisms: from genes to development to phenotype to fitness, additionally allowing to remove pleiotropy to directly test its effect. We focus on the pleiotropic interactions between two organs, specifically teeth. Fitness is determined from the functional interaction between these two teeth, which are produced by an in silico model of tooth morphogenesis. The developmental parameters are produced by a genome consisting of pleiotropic genes which may be influenced by one tooth-specific transcriptional regulator per tooth. Our simulations with and without pleiotropy confirm several acknowledged consequences of pleiotropy. It reduced genetic and phenotypic exploration, and facilitated the rapid accumulation of adaptive mutations in the modular cis-regulatory regions of pleiotropic genes, which are specific to each tooth. Unexpectedly, pleiotropy promoted fitness improvements, morphological complexity, and the accumulation of genetic divergence. Mutations in pleiotropic genes contributed significantly to adaptation, and removing pleiotropy did not increase the proportion of adaptive mutations. Thus pleiotropy does not act as a conservative force, but a channeling force promoting genetic divergence. Significance StatementThe notion of "Gene pleiotropy", just as the notion of "gene", crosses different fields of biology, but appreciations vary on how frequent it is and how much it favors conservation or divergence at the molecular, developmental or phenotypic scale. One reason is the discrepancies of viewpoints between these three scales, another the difficulty to manipulate pleiotropy experimentally. Here, we performed in silico evolution experiments with a computational toy model focusing on the pleiotropy between two organs while bridging the molecular, developmental and phenotypic scales. Moreover, we compared evolutionary scenarios where the two organs shared or not pleiotropic genes. Our results reconcile observations and show that sharing pleiotropic genes accelerates genetic divergence, even though non-pleiotropic genes diverge faster than pleiotropic genes.

evolutionary biology↗

bioRxiv2025

Inverse hourglass pattern of conservation in rodent molar development.

Although it is well established that certain stages of development are more conserved than others, the reasons for this phenomenon remain largely unknown. We study molecular conservation in the development of an organ, the molar, by comparing the temporal profiles of expression in mice and hamsters. We find that molar development is characterized by a rarely observed pattern of conservation of expression level and coding sequences forming an inverse hourglass, with more conservation at the beginning and end of morphogenesis than at intermediate stages. As the development of the rodent molar is well described, we were able to link this pattern to the properties of the expressed genes and the activity of different developmental processes. Early and late stages mobilize different sets of pleiotropic genes, cell division for bud growth and secretion for tooth mineralization. The particularities of dental morphogenesis and homeostasis, with the degradation of certain tissues at the end of development and the hosting of immune cells, as well as heterochronies linked to adaptations, also probably contribute to the pattern. Our study of the different actors explains the inverted hourglass of molars by a combination of processes intrinsic to the teeth, and by negative and positive selection which is mostly extrinsic to the teeth. This is likely translatable to explain molecular conservation patterns in many other biological systems.

evolutionary biology↗

bioRxiv2024

Convergent transcriptomic and genomic adaptation in xeric rodents

Repeated adaptations rely in part on convergent genetic changes. The extent of convergent changes at the genomic scale is debated and may depend on the interplay between different factors. Rodents have repeatedly adapted to life in arid conditions, notably with altered renal morphology and physiology. This occurred at different time periods, allowing us to test the importance of time in convergent genomic evolution. We analyzed kidney transcriptomes from 34 species to quantify and characterize convergent evolution at the level of gene expression, tissue composition, and coding sequences. We found that several genes showed convergent expression changes, some of which also carried convergent changes in their coding sequence. We then subdivided these data to test the influence of evolutionary history. First, within the subfamily Murinae, we found more convergent gene expression, reflecting convergent changes in cell proportions. Second, we compared data for recent (within genera) and ancient (between genera) adaptations, and observed more convergent changes in the latter. Our study shows that adaptation to xeric environments in rodents involves repeated changes in tissue composition, gene expression and coding sequences, and that the degree of convergent evolution increases with both the age of the adaptations and species relatedness.

evolutionary biology↗

bioRxiv2022

The genomic basis of hybrid male sterility in Ficedula flycatchers

Identifying genes involved in genetic incompatibilities causing hybrid sterility or inviability is a long-standing challenge in speciation research, especially in studies based on natural hybrid zones. Here we present the first high-probability candidate genes for hybrid male sterility in birds by using a combination of whole genome sequence data, histology sections of testis and single cell transcriptomics of testis samples from male pied-, collared-, and hybrid flycatchers. We reveal failure of meiosis in hybrid males and propose candidate genes involved in genetic incompatibilities causing this failure. Based on identification of genes with non-synonymous fixed differences between the two species and revealing miss-expression patterns of these genes across the various stages of hybrid male spermatogenesis we conclude aberrant chromosome segregation and/or faulty chromatin packing. A lower proportion of spermatids produced by hybrid males implies that a proportion of the aberrant spermatids undergo apoptosis. Finally, we report an overrepresentation of Z-linkage of the revealed candidate incompatibility genes. Our results challenge the assumption that speciation processes are driven by fast evolving genes by showing that a few changes in genes with highly conserved and central functions may quickly ensure reproductive isolation through post-zygotic isolation.

evolutionary biology↗

bioRxiv2022

Single-Cell Transcriptomics reveals relaxed evolutionary constraint of spermatogenesis in two passerine birds as compared to mammals

Spermatogenesis is a complex process where spermatogonia develop into haploid, mobile sperm cells. The genes guiding this process are subject to an evolutionary trade-off between preserving basic functions of sperm while acquiring new traits ensuring advantages in competition over fertilization of female gametes. In species with XY sex chromosomes, the outcome of this trade-off is found to vary across the stages of spermatogenesis but remains unexplored for species with ZW sex chromosomes. Here we characterize avian spermatogenesis at single cell resolution from testis of collared and pied flycatchers. We find evidence for relaxed evolutionary constraint of genes expressed in spermatocyte cells going through meiosis. An overrepresentation of Z-linked differentially expressed genes between the two species at this stage suggests that this relaxed constraint is associated with the lack of sex-chromosome silencing during meiosis. We conclude that the high throughput of bird spermatogenesis, at least partly, is explained by relaxed developmental constraint.

evolutionary biology↗

bioRxiv2021

Distinct gene expression dynamics in developing and regenerating limbs

Regenerating animals have the ability to reproduce organs that were originally generated in the embryo and subsequently lost due to injury. Understanding whether the process of regeneration mirrors development is an open question in most regenerative species. Here we take a transcriptomics approach to examine to what extent leg regeneration shows the same temporal patterns of gene expression as leg development in the embryo, in the crustacean Parhyale hawaiensis. We find that leg development in the embryo shows stereotypic temporal patterns of gene expression. In contrast, global patterns of gene expression during leg regeneration show a high degree of variation, related to the physiology of individual animals. A major driver of this variation is the molting cycle. After dissecting the transcriptional signals of individual physiology from regeneration, we obtain temporal signals that mark distinct phases of leg regeneration. Comparing the transcriptional dynamics of development and regeneration we find that, although both processes use largely the same genes, the temporal patterns in which these gene sets are deployed are different and cannot be systematically aligned. HIGHLIGHTSO_LISingle-limb data on transcriptional dynamics of leg development and regeneration C_LIO_LIDeveloping embryonic legs show stereotypic transcriptional profiles C_LIO_LIRegenerating leg transcriptomes show a high degree on individual variation C_LIO_LIRegenerating leg transcriptomes are influenced by adult physiology, especially molting C_LIO_LIRegenerating leg transcriptomes reveal distinct phases of leg regeneration C_LIO_LILeg development and regeneration use overlapping sets of genes in different temporal patterns C_LI

developmental biology↗

bioRxiv2020

Comparison of developmental genome expression in rodent molars reveals extensive developmental system drift.

Serial appendages are similar organs found at different places in the body, such as fore/hindlimbs or different teeth. They are bound to develop with the same pleiotropic genes, apart from identity genes. These identity genes have logically been implicated in cases where a single appendage evolved a drastically new shape while the other retained an ancestral shape, by enabling developmental changes specifically in one organ. Here, we showed that independent evolution involved developmental changes happening in both organs, in two well characterized model systems. Mouse upper molars evolved a new dental plan with two more cusps on the lingual side, while the lower molar kept a much more ancestral morphology, as did the molars of hamster, our control species. We obtained quantitative timelines of cusp formation and corresponding transcriptomic timeseries in the 4 molars. We found that a molecular and morphogenetic identity of lower and upper molars predated the mouse and hamster divergence and likely facilitated the independent evolution of molars lingual side in the mouse lineage. We found 3 morphogenetic changes which could combine to cause the supplementary cusps in the upper molar and a candidate gene, Bmper. Unexpectedly given its milder morphological divergence, we observed extensive changes in mouse lower molar development. Its transcriptomic profiles diverged as much as, and co-evolved extensively with, those of the upper molar. Consistent with the transcriptomic quantifications, two out of the three morphogenetic changes also impacted lower molar development. Moving to limbs, we show the drastic evolution of the bat wing also involved gene expression co-evolution and a combination of specific and pleiotropic changes. Independent morphological innovation in one organ therefore involves concerted developmental evolution of the other organ. This is facilitated by evolutionary flexibility of its development, a phenomenon known as Developmental System Drift. AUTHOR SUMMARYSerial organs, such as the different wings of an insect or the different limbs or teeth of a vertebrate, can develop into drastically different shapes due to the position-specific expression of so-called "identity" genes. Often during evolution, one organ evolves a new shape while another retains a conserved shape. It was thought that identity genes were responsible for these cases of independent evolution, by enabling developmental changes specifically in one organ. Here, we showed that developmental changes evolved in both organs to enable the independent evolution of the upper molar in mice and the wing in bats. In the organ with the new shape, several developmental changes combine. In the organ with the conserved shape, part of these developmental changes are seen as well. This modifies the development but is not sufficient to drastically change the phenotype, a phenomenon known as "Developmental System Drift", DSD. Thus, the independent evolution of one organ relies on concerted molecular changes, which will contribute to adaptation in one organ and be no more than DSD in another organ. This concerted evolution could apply more generally to very different body parts and explain previous observations on gene expression evolution.

evolutionary biology↗

bioRxiv2019

Detecting convergent adaptive amino acid evolution

In evolutionary genomics, researchers have taken an interest in identifying in the genomes substitutions that subtend convergent phenotypic adaptations. This is a difficult question to address, because genomes contain billions of sites, many of which have substituted in the lineages where the adaptations took place, and yet are not linked to them. Those extra substitutions may be linked to other adaptations, may be neutral, or may be linked to mutational biases. Furthermore, one can think of various ways of defining substitutions of interest, and various methods that match those definitions have been used, resulting in different sets of candidate substitutions. In this manuscript we first clarify how adaptation to convergent phenotypic evolution can manifest itself in coding sequences. Second, we review methods that have been proposed to detect convergent adaptive evolution in coding sequences and expose the assumptions that underlie them. Finally, we examine their power on simulations of convergent changes, including in the presence of a confounding factor.

bioinformatics↗

bioRxiv2018

Accurate detection of convergent substitutions

In the history of life, some phenotypes have been acquired several times independently, through convergent evolution. Recently, lots of genome-scale studies have been devoted to identify nucleotides or amino acids that changed in a convergent manner when the convergent phenotypes evolved. These efforts have had mixed results, probably because of differences in the detection methods, and because of conceptual differences about the definition of a convergent substitution. Some methods contend that substitutions are convergent only if they occur on all branches where the phenotype changed towards the exact same state at a given nucleotide or amino acid position. Others are much looser in their requirements and define a convergent substitution as one that leads the site at which they occur to prefer a phylogeny in which species with the convergent phenotype group together. Here we suggest to look for convergent shifts in amino acid preferences instead of convergent substitutions to the exact same amino acid. We define as convergent shifts substitutions that occur on all branches where the phenotype changed and such that they correspond to a change in the type of amino acid preferred at this position. We implement the corresponding model into a method named PCOC. We show on simulations that PCOC better recovers convergent shifts than existing methods in terms of sensitivity and specificity. We test it on a plant protein alignment where convergent evolution has been studied in detail and find that our method recovers several previously identified convergent substitutions and proposes credible new candidates.

evolutionary biology↗

bioRxiv2016

Biased gene conversion drives codon usage in human and precludes selection on translation efficiency

In humans, as in other mammals, synonymous codon usage (SCU) varies widely among genes. In particular, genes involved in cell differentiation or in proliferation display a distinct codon usage, suggesting that SCU is adaptively constrained to optimize translation efficiency in distinct cellular states. However, in mammals, SCU is known to correlate with large-scale fluctuations of GC-content along chromosomes, caused by meiotic recombination, via the non-adaptive process of GC-biased gene conversion (gBGC). To disentangle and to quantify the different factors driving SCU in humans, we analyzed the relationships between functional categories, base composition, recombination, and gene expression. We first demonstrate that SCU is predominantly driven by large-scale variation in GC-content and is not linked to constraints on tRNA abundance, which excludes an effect of translational selection. In agreement with the gBGC model, we show that differences in SCU among functional categories are explained by variation in intragenic recombination rate, which, in turn, is strongly negatively correlated to gene expression levels during meiosis. Our results indicate that variation in SCU among functional categories (including variation associated to differentiation or proliferation)- result from differences in levels of meiotic transcription, which interferes with the formation of crossovers and thereby affects gBGC intensity within genes. Overall, the gBGC model explains 81.3% of the variance in SCU among genes. We argue that the strong heterogeneity of SCU induced by gBGC in mammalian genomes precludes any optimization of the tRNA pool to the demand in codon usage.

evolutionary biology↗