PhyloSystemX is a computational tool designed to reconstruct ancestral biological interaction networks by integrating phylogenetic species trees with modern interaction data. While systems biologists commonly use networks to represent complex biological interactions, contemporary network analysis alone cannot reveal the evolutionary history that shaped these systems. PhyloSystemX addresses this gap by implementing parsimony-based algorithms (Dollo and Sankoff adjusted algorithm) to infer both ancestral vertices (homology groups) and their interactions across evolutionary time. Unlike tools limited to specific interaction types, PhyloSystemX can analyze any network whose vertices can be labeled with homology identifiers, including protein-protein interactions, gene co-expression networks, and metabolic regulation systems. By inferring ancestral networks, researchers can uncover evolutionarily conserved functional modules, identify lineage-specific innovations, and explore how biological systems adapt over time. The current implementation assumes vertical inheritance of traits, with limitations regarding introgression, hybridization, and horizontal gene transfer that future versions may address.

One of the prokaryotic partners at the origin of eukaryotes was an archaeon, but do archaea age, and if so, how? Uncovering aging in Archaea might provide more general clues about cellular senescence and rejuvenation and their origins. To fill this knowledge gap, we focused on Saccharolobus islandicus, a model archaeon for which the cell cycle can be synchronized and controlled. We generated longitudinal transcriptomes of synchronized S. islandicus populations that capture typical expression and co-expression profiles associated with chronological aging. These experiments also allowed us to infer average cellular death rates during the cell cycle. Our results are compatible with general patterns of biological aging observed in single cells. However, at the population level, we observe a peak of mortality shortly after cell division in S. islandicus, which we interpret as "negative" demographic aging or ontogenescence, i.e., pre-reproductive mortality decline. To reconcile these observations, we propose a model of S. islandicus aging and rejuvenation. Our research constitutes a first step into the study of aging in archaea on the basis of gene expression, gene co-expression patterns during the cell cycle and biodemography analyses, and proposes a hypothetical new model to explore how cellular senescence and rejuvenation in eukaryotes may have prokaryotic roots. Alternative interpretations of our transcriptomic results however are possible, encouraging future experimental validation of aging in Archaea.

Metagenomics has considerably broadened our knowledge of microbial diversity, unravelling fascinating adaptations and characterising multiple novel major taxonomic groups, e.g. CPR bacteria, DPANN and Asgard archaea, and novel viruses. Such findings profoundly reshaped the structure of the known Tree of Life and emphasised the central role of investigating uncultured organisms. However, despite significant progresses, a large portion of proteins predicted from metagenomes remain today unannotated, both taxonomically and functionally, across many biomes and in particular in oceanic waters. Here, we used an iterative, network-based approach for remote homology detection, to probe a dataset of 40 million ORFs predicted in marine environments. We assessed the environmental diversity of 53 core gene families broadly distributed across the Tree of Life, with essential functions including translational, replication and trafficking processes. For nearly half of them, we identified clusters of remote environmental homologues that showed divergence from the known genetic diversity comparable to the divergence between Archaea and Bacteria, with representatives distributed across all the oceans. In particular, we report the detection of environmental clades with new structural variants of essential SMC (Structural Maintenance of Chromosomes) genes, divergent polymerase subunits forming deep-branching clades in the polymerase tree, and variant DNA recombinases in Bacteria as well as viruses. These results indicate that significant environmental diversity may yet be unravelled even in strongly conserved gene families. Protein sequence similarity network approaches, in particular, appear well-suited to highlight potential sources of biological novelty and make better sense of microbial dark matter across taxonomical scales.

Similar to many eukaryotes, the thermoacidophilic archaeon Saccharolobus islandicus follows a defined cell cycle program, with two growth phases, G1 and G2, interspersed by a chromosome replication phase (S), and followed by genome segregation and cytokinesis (M-D) phases. To study whether and which other processes are cell cycle-coordinated, we synchronized cultures of S. islandicus and performed an in-depth transcriptomic analysis of samples enriched in cells undergoing the M-G1, S, and G2 phases, providing a holistic view of the S. islandicus cell cycle. We show that diverse metabolic pathways, protein synthesis, cell motility and even antiviral defense systems, are expressed in a cell cycle-dependent fashion. Moreover, application of a transcriptome deconvolution method defined sets of phase-specific signature genes, whose peaks of expression roughly matched those of yeast homologs. Collectively, our data elucidates the complexity of the S. islandicus cell cycle, suggesting that it more closely resembles the cell cycle of certain eukaryotes than previously appreciated.

Bacteria permeate every niche of the human body with major consequences on our health and senescence that have not been fully described. Here, we predict which bacteria and which bacterial proteins could interfere with proteins associated with human aging using bipartite networks showing interspecific protein interactions coupled with investigations of published experimental evidence and transcriptomic data. We introduce the term of "gerogenic" bacteria, literally bacteria that could induce some aging in their host and discuss the mechanisms by which such bacteria could serve as age-distorters of humans. Salmonella, Escherichia and Shigella appear as major candidate age-distorters, characterized by a higher experimentally demonstrated potential than other bacteria to interact with human proteins associated with human aging and human cellular senescence. Our analysis also highlights an evolutionary convergence among bacterial and viral candidate age-distorting proteins, since 14 human proteins associated with aging can be commonly targeted by bacteria and viruses in case of microbial infection. Since infections are common and Salmonella, Escherichia and Shigella are frequently found as pathogens in our microbiomes, characterizing bacterial influence on our aging and our cellular senescence through molecular hijacking could enhance the understanding of the causes of aging and suggest new anti-aging therapies.

The cell cycle is a series of events that occur from the moment of cell birth to cell division. In eukaryotes, cell growth, genome replication, genome segregation, and cytokinesis are strictly coordinated, defining discrete cell cycle phases. In contrast, these key processes may occur concurrently in bacteria. Thermoacidophilic archaea in the genus Saccharolobus follow a defined cell cycle program, with the first pre-replicative growth (G1) phase, followed by the chromosome replication (S) phase, the second growth (G2) phase, and rapid genome segregation (M) and cytokinesis (D) phases. However, whether other processes, such as metabolism, catabolism, protein translation, and antiviral defense also occur at specific cell cycle phases, as in eukaryotes, or are active throughout the cell cycle, as in bacteria, remains unclear. To address this question, we synchronized cultures of S. islandicus and performed an in-depth transcriptomic analysis of samples enriched in cells undergoing the M-G1, S, and G2 phases. Differential gene expression and consensus gene co-expression network analyses provided a holistic view of the S. islandicus cell cycle. In addition to the core transcriptome network, which is expressed throughout the cell cycle, we show that diverse metabolic pathways, protein synthesis, cell motility and even antiviral defense systems, are expressed in a cell cycle dependent fashion. Our data also refines understanding of the processes previously known to be linked to the cell cycle, such as DNA replication. We show that most DNA replication genes are expressed prior to the S phase, during the M-G1, whereas expression of the major chromatin genes, and accordingly, chromatinization are concomitant with replication. A statistical model was used to define sets of signature genes characteristic of each of the analyzed cell cycle phases, emphasizing transcriptional stratification of the phases. Signature genes are more conserved across Thermoproteota than non-signature genes and their peak expression, especially for the M-G1 and G2 specific genes, matches that of homologs in yeast. Collectively, our data elucidate the complexity of the S. islandicus cell cycle and suggest that it more closely resembles the cell cycle of eukaryotes than previously appreciated. ### Competing Interest Statement The authors have declared no competing interest.

Few studies have systematically analyzed how old aging is. Gaining a more accurate knowledge about the natural history of aging could however have several payoffs. This knowledge could unveil lineages with dated genetic hardware, possibly maladapted to current environmental challenges, and also uncover “phylogenetic modules of aging,” i.e., naturally evolved pathways associated with aging or longevity from a single ancestry, with translational interest for anti-aging therapies. Here, we approximated the natural history of the genetic hardware of aging for five model fungal and animal species. We propose a lower-bound estimate of the phylogenetic age of origination for their protein-encoding gene families and protein–protein interactions. Most aging-associated gene families are hundreds of million years old, older than the other gene families from these genomes. Moreover, we observed a form of punctuated evolution of the aging hardware in all species, as aging-associated families born at specific phylogenetic times accumulate preferentially in genomes. Most protein–protein interactions between aging genes are also old, and old aging-associated proteins showed a reduced potential to contribute to novel interactions associated with aging, suggesting that aging networks are at risk of losing in evolvability over long evolutionary periods. Finally, due to reshuffling events, aging networks presented a very limited phylogenetic structure that challenges the detection of “maladaptive” or “adaptative” phylogenetic modules of aging in present-day genomes.

Brown seaweeds are keystone species of coastal ecosystems, often forming extensive underwater forests, and are under considerable threat from climate change. In this study, analysis of multiple genomes has provided insights across the entire evolutionary history of this lineage, from initial emergence, through later diversification of the brown algal orders, down to microevolutionary events at the genus level. Emergence of the brown algal lineage was associated with a marked gain of new orthologous gene families, enhanced protein domain rearrangement, increased horizontal gene transfer events, and the acquisition of novel signaling molecules and key metabolic pathways, the latter notably related to biosynthesis of the alginate-based extracellular matrix, and halogen and phlorotannin biosynthesis. We show that brown algal genome diversification is tightly linked to phenotypic divergence, including changes in life cycle strategy and zoid flagellar structure. The study also showed that integration of large viral genomes has had a significant impact on brown algal genome content throughout the emergence of the lineage.

How, when, and why organisms age are fascinating issues that can only be fully addressed by adopting an evolutionary perspective. Consistently, the main evolutionary theories of ageing, namely the Mutation Accumulation theory, the Antagonistic Pleiotropy theory, and the Disposable Soma theory, have formulated stimulating hypotheses that structure current debates on both the proximal and ultimate causes of organismal ageing. However, all these theories leave a common area of biology relatively under-explored. The Mutation Accumulation theory and the Antagonistic Pleiotropy theory were developed under the traditional framework of population genetics, and therefore are logically centred on the ageing of individuals within a population. The Disposable Soma theory, based on principles of optimising physiology, mainly explains ageing within a species. Consequently, current leading evolutionary theories of ageing do not explicitly model the countless interspecific and ecological interactions, such as symbioses and host-microbiomes associations, increasingly recognized to shape organismal evolution across the Web of Life. Moreover, the development of network modelling supporting a deeper understanding on the molecular interactions associated with ageing within and between organisms is also bringing forward new questions regarding how and why molecular pathways associated with ageing evolved. Here, we take an evolutionary perspective to examine the effects of organismal interactions on ageing across different levels of biological organisation, and consider the impact of surrounding and nested systems on organismal ageing. We also apply this perspective to suggest open issues with potential to expand the standard evolutionary theories of ageing.

Some viruses (e.g., human immunodeficiency virus 1 and severe acute respiratory syndrome coronavirus 2) have been experimentally proposed to accelerate features of human aging and of cellular senescence. These observations, along with evolutionary considerations on viral fitness, raised the more general puzzling hypothesis that, beyond documented sources in human genetics, aging in our species may also depend on virally encoded interactions distorting our aging to the benefits of diverse viruses. Accordingly, we designed systematic network-based analyses of the human and viral protein interactomes, which unraveled dozens of viruses encoding proteins experimentally demonstrated to interact with proteins from pathways associated with human aging, including cellular senescence. We further corroborated our predictions that specific viruses interfere with human aging using published experimental evidence and transcriptomic data; identifying influenza A virus (subtype H1N1) as a major candidate age distorter, notably through manipulation of cellular senescence. By providing original evidence that viruses may convergently contribute to the evolution of numerous age-associated pathways through co-evolution, our network-based and bipartite network-based methodologies support an ecosystemic study of aging, also searching for genetic causes of aging outside a focal aging species. Our findings, predicting age distorters and targets for anti-aging therapies among human viruses, could have fundamental and practical implications for evolutionary biology, aging study, virology, medicine, and demography.

A molecular level perspective on how novel phenotypes evolve is contingent on our understanding of how genomes evolve through time, and of particular interest is how novel elements emerge or are lost. Mechanisms of protein evolution such as gene duplication have been well established. Studies of gene fusion events show they often generate novel functions and adaptive benefits. Identifying gene fusion and fission events on a genome scale allows us to establish the mode and tempo of emergence of composite genes across the animal tree of life, and allows us to test the repeatability of evolution in terms of determining how often composite genes can arise independently. Here we show that ∼5% of all animal gene families are composite, and their phylogenetic distribution suggests an abrupt, rather than gradual, emergence during animal evolution. We find that gene fusion occurs at a higher rate than fission (73.3% vs 25.4%) in animal composite genes, but many gene fusions (79% of the 73.3%) have more complex patterns including subsequent fission or loss. We demonstrate that nodes such as Bilateria, Euteleostomi, and Eutheria, have significantly higher rates of accumulation of composite genes. We observe that in general deuterostomes have a greater amount of composite genes as compared to protostomes. Intriguingly, up to 41% of composite gene families have evolved independently in different clades showing that the same solutions to protein innovation have evolved time and again in animals. Significance statement New genes emerge and are lost from genomes over time. Mechanisms that can produce new genes include, but are not limited to, gene duplication, retrotransposition, de novo gene genesis, and gene fusion/fission. In this work, we show that new genes formed by fusing distinct homologous gene families together comprise a significant portion of the animal proteome. Their pattern of emergence through time is not gradual throughout the animal phylogeny - it is intensified on nodes of major transition in animal phylogeny. Interestingly, we see that evolution replays the tape frequently in these genes with 41% of gene fusion/fission events occurring independently throughout animal evolution.

Abstract The gram-positive bacteria use quorum sensing secreted peptides to communicate and control critical bacterial phenotypes. All the bacterial peptides are produced in a longer inactive form that is secreted and processed into shorter active peptides. Recently, we discovered a new class of bacterial peptide in human pathogen Streptococcus pyogenes, a leaderless communication peptide (LCP), that is encoded as an 8 amino acid long active peptide without secretion signal sequence and processing sites. Nevertheless, LCP mediates intercellular communication and controls bacterial virulence. However, except for LCP from S. pyogenes, the occurrence of this unique class of intercellular signals in bacterial genomes and their contribution to bacterial biology remain unknown. Here, we report the widespread presence of LCPs among firmicutes encompassing broad taxonomic diversity and displaying diverse peptide communication codes. We further show that the newly identified LCPs act as bacterial intercellular signals and carry out gene regulation. Our results suggest that LCPs are more widespread than indicated by our study, and their characterization may uncover new signaling mechanisms and roles in coordinating diverse bacterial traits.

Gram-positive Firmicutes bacteria and their mobile genetic elements (plasmids and bacteriophages) encode peptide-based quorum-sensing systems (QSSs) that orchestrate behavioral transitions as a function of population densities. In their simplest form, termed "RRNPP", these QSSs are composed of two adjacent genes: a communication propeptide and its cognate intracellular receptor. RRNPP QSSs notably regulate social/competitive behaviors such as virulence or biofilm formation in bacteria, conjugation in plasmids, or lysogeny in temperate bacteriophages. However, the genetic diversity and the prevalence of these communication systems, together with the breadth of behaviors they control, remain largely underappreciated. To better assess the impact of density dependency on microbial community dynamics and evolution, we developed the RRNPP_detector software, which predicts known and novel RRNPP QSSs in chromosomes, plasmids, and bacteriophages of Firmicutes. Applying RRNPP_detector against available complete genomes of viruses and Firmicutes, we identified a rich repertoire of RRNPP QSSs from 11 already known subfamilies and 21 novel high-confidence candidate subfamilies distributed across a vast diversity of taxa. The analysis of high-confidence RRNPP subfamilies notably revealed 14 subfamilies shared between chromosomes/plasmids/phages, 181 plasmids and 82 phages encoding multiple communication systems, phage-encoded QSSs predicted to dynamically modulate bacterial behaviors, and 196 candidate biosynthetic gene clusters under density-dependent regulation. Overall, our work enhances the field of quorum-sensing research and reveals novel insights into the coevolution of gram-positive bacteria and their mobile genetic elements.

The human pathogen Streptococcus pyogenes secretes a short peptide (leaderless communication peptide, LCP) that mediates intercellular communication and controls bacterial virulence through interaction with its receptor, RopB. Here, we show that LCP and RopB homologues are present in other Firmicutes. We experimentally validate that LCPs with distinct peptide communication codes act as bacterial intercellular signals and regulate gene expression in Streptococcus salivarius , Streptococcus porcinus , Enterococcus malodoratus and Limosilactobacillus reuteri . Our results indicate that LCPs are more widespread than previously thought, and their characterization may uncover new signaling mechanisms and roles in coordinating diverse bacterial traits.

The genetic roots of the diverse paces and shapes of ageing and of the large variations in longevity observed across the tree of life are poorly understood. Indeed, pathways associated with ageing/longevity are incompletely known, both in terms of their constitutive genes/proteins and of their molecular interactions. Moreover, there is limited overlap between the genes constituting these pathways across mammals. Yet, dedicated comparative analyses might still unravel evolutionarily conserved, important pathways associated with longevity or ageing. Here, we used an original strategy with a double evolutionary and systemic focus to analyse protein interactions associated with ageing or longevity during the evolution of five species of Opisthokonta. We ranked these proteins and interactions based on their evolutionary conservation and centrality in past and present protein-protein interaction (PPI) networks, providing a big systemic picture of the evolution of ageing and longevity pathways that identified which pathways emerged in which Opisthokonta lineages, were conserved, and/or central. We confirmed that longevity/ageing-associated proteins (LAPs), be they pro- or anti-longevity, are highly central in extant PPI, consistently with the antagonistic pleiotropy theory of ageing, and identified key antagonistic regulators of ageing/longevity, 52 of which with homologues in humans. While some highly central LAPs were evolutionarily conserved for over a billion years, we report a clear transition in the functionally important components of ageing/longevity within bilaterians. We also predicted 487 novel evolutionarily conserved LAPs in humans, 54% of which are more central than mTOR, and 138 of which are druggable, defining new potential targets for anti-ageing treatments in humans.

All genomes include gene families with very limited taxonomic distributions that potentially represent new genes and innovations in protein-coding sequence, raising questions on the origins of such genes. Some of these genes are hypothesized to have formed de novo, from noncoding sequences, and recent work has begun to elucidate the processes by which de novo gene formation can occur. A special case of de novo gene formation, overprinting, describes the origin of new genes from noncoding alternative reading frames of existing open reading frames (ORFs). We argue that additionally, out-of-frame gene fission/fusion events of alternative reading frames of ORFs and out-of-frame lateral gene transfers could contribute to the origin of new gene families. To demonstrate this, we developed an original pattern-search in sequence similarity networks, enhancing the use of these graphs, commonly used to detect in-frame remodeled genes. We applied this approach to gene families in 524 complete genomes of Escherichia coli. We identified 767 gene families whose evolutionary history likely included at least one out-of-frame remodeling event. These genes with out-of-frame components represent ∼2.5% of all genes in the E. coli pangenome, suggesting that alternative reading frames of existing ORFs can contribute to a significant proportion of de novo genes in bacteria.

Diet has been suggested to be an important driver of variation in microbiota composition in mammals. However, whether this is a more general phenomenon and how fast changes in gut microbiota occur with changes in diet remains poorly understood. Forty-nine years ago, ten lizards of the species Podarcis siculus were taken from the island of Pod Kopište and introduced onto the island of Pod Mrčaru (Croatia). The introduced population underwent a significant dietary shift, and their descendants became omnivorous (consuming up to 80% plant material during summer). Variation in their gut microbiota has never been investigated. To elucidate the possible impact on the gut microbiota of this rapid change in diet, we compared the microbiota (V4 region of the 16S rRNA gene) of P. siculus from Pod Mrčaru, Pod Kopište, and the mainland. In addition, we explored other drivers of variation in gut microbiota including insularity, the population of origin, and the year of sampling. Alpha-diversity analyses showed that the microbial diversity of omnivorous lizards was higher than the microbial diversity of insectivorous lizards. Moreover, omnivorous individuals harbored significantly more Methanobrevibacter. The gut microbial diversity of insectivorous lizards was nonetheless more heterogeneous. Insectivorous lizards on the mainland had different gut microbial communities than their counterparts on the island of Pod Kopište. Bacillus and Desulfovibrio were more abundant in the gut microbiota from insular lizards compared to mainland lizards. Finally, we showed that the population of origin was also an important driver of the composition of the gut microbiota. The dietary shift that occurred in the introduced population of P. siculus has had a detectable impact on the gut microbiota, but other factors such as insularity and the population of origin also contributed to differences in the gut microbial composition of these lizards, illustrating the multifactorial nature of the drivers of variation in gut microbiota. Overall, our data show that changes in gut microbiota may take place on ecological timescales. Yet, diet is only one of many factors driving variation in gut microbiota across populations.

Debates between proponents of a tree of life and that of a web of life [ 1. Blais C. Archibald J.M. The past, present and future of the tree of life. Curr. Biol. 2021; 31: R314-R321 Abstract Full Text Full Text PDF PubMed Scopus (7) Google Scholar ] encouraged a substantial rethink about which events genuinely compose the history of microbial lineages. Admittedly, the reconstruction of patterns of vertical descent alone cannot recapitulate the entire evolutionary history of lineages, which share some gene families due to introgressive processes [ 2. Dagan T. et al. Modular networks and cumulative impact of lateral transfer in prokaryote genome evolution. Proc. Natl. Acad. Sci. U. S. A. 2008; 105: 10039-10044 Crossref PubMed Scopus (274) Google Scholar , 3. Doolittle W.F. Bapteste E. Pattern pluralism and the Tree of Life hypothesis. Proc. Natl. Acad. Sci. U. S. A. 2007; 104: 2043-2049 Crossref PubMed Scopus (349) Google Scholar , 4. Koonin E.V. et al. Evolution of microbial genomics: conceptual shifts over a quarter century. Trends Microbiol. 2021; 29: 582-592 Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar ]. Thus, both vertical and lateral inheritances of genetic material are recognized to contribute to the history of a lineage. Moreover, recent works by Koonin et al. [ 4. Koonin E.V. et al. Evolution of microbial genomics: conceptual shifts over a quarter century. Trends Microbiol. 2021; 29: 582-592 Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar , 5. Iranzo J. et al. Gene gain and loss push prokaryotes beyond the homologous recombination barrier and accelerate genome sequence divergence. Nat. Commun. 2019; 10: 5376 Crossref PubMed Scopus (22) Google Scholar , 6. Wolf Y.I. et al. Two fundamentally different classes of microbial genes. Nat. Microbiol. 2016; 2: 16208 Crossref PubMed Scopus (24) Google Scholar ] highlight how, during evolution, genomes are significantly (re)-invented through processes of gene gains and losses, and how our knowledge of microbial evolution is shifting. Consistently, we build upon these works to propose a further, deeper rethink on what makes up a lineage’s genuine history. Specifically, as a consequence of the shifts characterized by [ 4. Koonin E.V. et al. Evolution of microbial genomics: conceptual shifts over a quarter century. Trends Microbiol. 2021; 29: 582-592 Abstract Full Text Full Text PDF PubMed Scopus (12) Google Scholar ], we argue that the genesis of phyla should explain more than the origins of shared genes: it should also explain the origins of lineage-specific gene families, at all levels of the biological classification, and include all historical genetic events of descent and modification that shape lineages.

Many separate fields and practices nowadays consider microbes as part of their legitimate focus. Therefore, microbiome studies may act as unexpected unifying forces across very different disciplines. Here, we summarize how microbiomes appear as novel major biological players, offer new artistic frontiers, new uses from medicine to laws, and inspire novel ontologies. We identify several convergent emerging themes across ecosystem studies, microbial and evolutionary ecology, arts, medicine, forensic analyses, law and philosophy of science, as well as some outstanding issues raised by microbiome studies across these disciplines and practices. An 'epistemic revolution induced by microbiome studies' seems to be ongoing, characterized by four features: (i) an ecologization of pre-existing concepts within disciplines, (ii) a growing interest in systemic analyses of the investigated or represented phenomena and a greater focus on interactions as their root causes, (iii) the intent to use openly multi-scalar interaction networks as an explanatory framework to investigate phenomena to acknowledge the causal effects of microbiomes, (iv) a reconceptualization of the usual definitions of which individuals are worth considering as an explanans or as an explanandum by a given field, which result in a fifth strong trend, namely (v) a de-anthropocentrification of our perception of the world.

How, when and why do organisms, their tissues and their cells age remain challenging issues, although researchers have identified multiple mechanistic causes of aging, and three major evolutionary theories have been developed to unravel the ultimate causes of organismal aging. A central hypothesis of these theories is that the strength of natural selection decreases with age. However, empirical evidence on when, why and how organisms age is phylogenetically limited, especially in natural populations. Here, we developed generic comparisons of gene co-expression networks that quantify and dissect the heterogeneity of gene co-expression in conspecific individuals from different age-classes to provide topological evidence about some mechanical and fundamental causes of organismal aging. We applied this approach to investigate the complexity of some proximal and ultimate causes of aging phenotypes in a natural population of the greater mouse-eared bat Myotis myotis, a remarkably long-lived species given its body size and metabolic rate, with available longitudinal blood transcriptomes. Myotis gene co-expression networks become increasingly fragmented with age, suggesting an erosion of the strength of natural selection and a general dysregulation of gene co-expression in aging bats. However, selective pressures remain sufficiently strong to allow successive emergence of homogeneous age-specific gene co-expression patterns, for at least seven years. Thus, older individuals from long-lived species appear to sit at an evolutionary crossroad: as they age, they experience both a decrease in the strength of natural selection and a targeted selection for very specific biological processes, further inviting to refine a central hypothesis in evolutionary aging theories.