Summary Viable yet suboptimal (“loser”) cells can be recognized and selectively eliminated when in the presence of neighboring fitter (“winner”) cells through cell competition, thereby promoting optimal tissue fitness and homeostasis. One mechanism through which cells compare their relative fitness levels relies on isoforms of the conserved transmembrane protein Flower (Fwe). Despite the role of Fwe-dependent cell selection in several pathophysiological processes, little is known about downstream components of this pathway. In this study, we develop a versatile clonal interaction assay in Drosophila , the Easy Win Assay. By overexpressing human FWE1 Lose isoform to trigger cell competition, we perform an unbiased whole-genome RNAi screen and identify new pathway modulators. We show that the receptor Grindelwald/TNFR is necessary in either winner or loser cells to drive Eiger/TNF-α-independent elimination of losers, whereas the scaffolding protein Veli/LIN-7 is simultaneously required in both cell populations. We further demonstrate that Fwe, Grindelwald and Veli redistribute to establish a previously undescribed bilateral communication module at winner-loser interfaces, promoting intercellular communication and elimination of loser cells. These findings show that distinct cell elimination pathways converge on a common execution module while remaining independently regulated upstream, enabling flexible recognition and elimination of diverse damaged or dangerous cells.

Cellular interactions and mechanical forces are fundamental in shaping epithelial tissue architecture. In the Drosophila notum, tissue compression at the midline promotes epithelial cell elimination. Here, we conducted a multi-step RNAi screen and identified 47 diverse regulators of notum epithelial remodeling. We find that the two cell adhesion proteins Hibris (Hbs) and Roughest (Rst) show high expression in zones of cell survival versus low levels in areas of cell pruning. Notum-wide knock-down of hbs or rst or homogenous hbs overexpression disrupts cell death patterns and results in adult tissue malformations. Local suppression of Hbs and Rst in Hbshigh/Rsthigh territories triggers ectopic cell elimination indicating that Hbs/Rst can instruct cell removal. Interestingly, Hbs but not Rst is regulated by compaction-sensitive EGFR signaling, positioning Hbs as an integrator of both mechanical and cell property cues. These findings uncover a novel adhesive landscape that shapes the thorax midline and potentially other organs.

Live imaging has been instrumental in understanding cellular dynamics in Drosophila tissues, but technical limitations have prevented the long-term visualization of cell competition in adult brains. Here, we describe a simple ex vivo protocol that enables extended live imaging of adult Drosophila brains for up to 32 h. The method relies on non-supplemented Schneider's Drosophila medium and hydrophobic interactions to maintain brain stability during imaging, eliminating the need for complex culture conditions or embedding procedures. We validate this approach by studying cell competition in the optic lobes following traumatic brain injury, where cell competition is expected to occur with a peak at 48 h after damage. We demonstrate the value of this method by visualizing the expression of the fitness checkpoint Azot in a loser cell and its subsequent elimination. This protocol offers a versatile platform for studying cell competition and other cellular processes requiring extended observation of the adult Drosophila brain.

Cell competition, a conserved biological process in which cells compete for survival based on relative fitness, has emerged as a critical mechanism in diverse biological contexts. Here, we investigate the role of cell competition in traumatic brain injury (TBI) by characterizing the temporal expression pattern of Azot, a key downstream effector of fitness-based selection, following injury in Drosophila melanogaster . Our findings reveal a distinct temporal profile of Azot expression post-TBI, with TUNEL assays confirming that Azot-expressing cells undergo apoptotic elimination. We demonstrate that following injury, the proportion of dying cells marked as “losers” significantly increases compared to non-injured conditions, indicating that cell competition becomes a predominant elimination mechanism during acute post-injury phases. Contrary to previous findings in neurodegenerative disease models where competition was restricted to neurons, we show that following TBI, both neurons and glia are subject to competitive elimination. Furthermore, in azot knockout conditions, we observe an accumulation of cells attempting to express Azot, suggesting impaired clearance of suboptimal cells. These findings advance our understanding of cellular quality control mechanisms following brain injury and may inform the development of novel therapeutic approaches to enhance functional recovery after TBI. ### Competing Interest Statement The authors have declared no competing interest. * AD : After-damage Dpp : Decapentaplegic Hz : Homozygous JNK : c-Jun N-terminal kinase OL : Optic Lobe PBT : PBS-TritonX-1% ROL : Right Optic Lobe TBI : Traumatic Brain Injury Fundaéão para a Ciência e Tecnologia, https://ror.org/00snfqn58, SFRH/BD/138537/2018, UIDB/04443/2020 European Research Council, https://ror.org/0472cxd90, Active Mechanisms of Cell Selection: From Cell Competition to Cell Fitness Portuguese Platform of BioImaging, , LISBOA-01-0145-FEDER-022122

Alzheimer’s Disease (AD) is a neurodegenerative disorder characterised by Amyloid-beta 42 (Aβ42) plaque accumulation and cognitive decline, with current treatments focused on symptomatic relief. Emerging therapeutics, such as dietary interventions, can modulate cognitive decline and delay AD progression. Our previous work in Drosophila melanogaster identified cell competition as a key mechanism that eliminates unfit neurons in an AD model, improving locomotion by removing the unfit neurons expressing flowerLoseB and ahuizotl (azot). Here, we explored how diet influences azot-dependent cell competition and locomotion in the AD model. Flies were fed with either a yeast-based diet (YBD) or a synthetic (SAA) diet for up to 28 days. In contrast to YBD, SAA delayed cell competition activation until day 21, coinciding with locomotion improvement and delayed Aβ formation. The overexpression of the human Flower (hFWE) isoforms in a Drosophila neuronal context revealed functional conservation: hFWE1 acted as the sole loser isoform, and hFWE2 as a winner isoform. With the YBD, forcing cell competition by expressing hFWE2 in the AD model led to an accumulation of unfit cells and promoted worse locomotion phenotypes over time compared to with the SAA diet. Our data highlights the complex interaction between diet, cell competition, and Aβ toxicity, offering new therapeutic insights.

Cell competition is a conserved phenomenon spanning from arthropods to humans. It involves the elimination of viable yet suboptimal “loser” cells when juxtaposed with their fitter “winner” counterparts. This process has received increased attention for its implications in cancer initiation and progression, neurodegeneration, and ageing.This study investigates the presence of the loser fitness fingerprint Flower LoseB (Fwe LB) and the fitness checkpoint Azot in the optic lobes over a period of 28 days. Notably, the absence of Azot is conventionally linked to the accumulation of loser cells over time. However, our investigation reveals that this accumulation is not perpetual and, intriguingly, Azot is not required for loser cell elimination in this context. Furthermore, we estimate that fewer than 50% of Fwe LB-expressing cells also express Azot and undergo apoptosis. Remarkably, our calculations also demonstrate that over 50% of cells undergoing apoptosis at any given time point are positive for the loser markers Fwe LB and Azot.This comprehensive analysis of fitness marker dynamics over a 28-day timeframe sheds new light on the intricate mechanisms governing Flower-dependent cell competition.### Competing Interest StatementThe authors have declared no competing interest.* Fwe LB : Flower LoseB KI : knockin KO : knockout

Cell competition and fitness comparison between cancer and tumor microenvironment (TME) cells determine oncogenic fate. Our previous study established a role for human Flower isoforms as fitness fingerprints, where the expression of Flower Win isoforms in tumor cells leads to growth advantage over TME cells expressing Lose isoforms. Here we demonstrate that the expression of Flower Lose and reduced microenvironment fitness is not a pre-existing condition but, rather, a cancer-induced phenomenon. Cancer cells actively reduce TME fitness by the exosome-mediated release of a cancer-specific long non-coding RNA, Tu-Stroma, which controls the splicing of the Flower gene in the TME cells and expression of Flower Lose isoform, which leads to reduced fitness status. This mechanism controls cancer growth, metastasis and host survival in ovarian cancer. Targeting Flower protein with humanized monoclonal antibody (mAb) in mice significantly reduces cancer growth and metastasis and improves survival. Pre-treatment with Flower mAb protects intraperitoneal organs from developing lesions despite the presence of aggressive tumor cells.

Tissues encompass a quality control mechanism that promotes their optimal state. This mechanism, designated cell competition, is characterised by the elimination of suboptimal yet viable cells when they are near healthier cells within the same tissue compartment. This study explores Flower-dependent cell competition and introduces Ikebana as a novel player. The differential expression of the flower isoforms labels cells as winners or losers, influencing their fate in diverse contexts, including eye development, traumatic brain injury, and Alzheimer’s disease. Ikebana, ubiquitously produced in wing imaginal discs and adult brains, modulates loser cell elimination. Reduction of ikebana expression correlates with an increased number of loser cells, while its overexpression in the Alzheimer’s disease model reduces the number of Flower LoseB-positive cells. We suggest that Ikebana protects loser cell elimination, particularly when excessive elimination of loser cells can compromise tissue function. Thus, Ikebana might be a potential therapeutic target for modulating Flower LoseB expression. ### Competing Interest Statement The authors have declared no competing interest. * Aβ42 : Amyloid beta 42 ACI : After Clone Induction bp : base pair ctr : control Diap1 : Death-associated inhibitor of apoptosis 1 KI : knockin KO : knockout LAPTM : Lysosomal Protein Transmembrane

Competitive interactions between tumor cells and surrounding healthy cells are constantly present during the progression of a solid tumor, and their outcome has been proposed to affect the clinical behavior. Previous studies have described various mechanistic and molecular aspects that characterize this process, overall indicating that cancer cells behave as supercompetitors, which eliminate neighboring healthy cells to gain vital space for growth and infiltration of the tissue. Nevertheless, there is a lack of systematic characterization of these competitive interactions, particularly in the context of cancer in mammals. Furthermore, previous studies in the field of cell competition have primarily focused on homotypic cell competition, involving different clones of the same cell or cells deriving from the same tissue. Data are scarce regarding heterotypic cell competition between two unrelated cell types, which is particularly critical for the understanding of metastatic tumors. In this research, we study cell competition in the context of liver metastases, providing a broad characterization of this process in different relevant scenarios, including cells growing in vitro in 2D and 3D, and in vivo . Results show that in vitro , only a subset of cancer cell lines are coherently strong or moderate competitors against hepatocytes, while the remaining demonstrate poor competitiveness. The competitive proficiency can vary depending on the experimental growth system that is employed, and often predicts the phenotype of liver metastases in terms of aggressiveness and morphology. Finally, our data point towards an involvement of mechanical competition in determining the supercompetitor trait of cancer cells. Altogether, our research provides the first comprehensive characterization of heterotypic cell competition, and indicates that cancer cells possess heterogeneous competitive proficiency towards hepatocytes which can be affected by the growth conditions.### Competing Interest StatementThe authors have declared no competing interest.

Cell competition is a conserved phenomenon spanning from arthropods to humans. It involves the elimination of viable yet suboptimal "loser" cells when juxtaposed with their fitter "winner" counterparts. This process has received increased attention for its implications in cancer initiation and progression, neurodegeneration, and ageing. This study investigates the presence of the loser fitness fingerprint Flower LoseB (Fwe LB) and the fitness checkpoint Azot in the optic lobes over a period of 28 days. Notably, the absence of Azot is conventionally linked to the accumulation of loser cells over time. However, our investigation reveals that this accumulation is not perpetual and, intriguingly, Azot is not required for loser cell elimination in this context because loser cells are still eliminated by apoptosis in its absence. Furthermore, we wanted to clarify the percentage of loser cells that are eliminated, and the percentage of dying cells identified as loser during cell competition. We estimate that fewer than 50% of Fwe LB-expressing cells also express Azot and undergo apoptosis. Remarkably, our calculations also demonstrate that over 50% of cells undergoing apoptosis at any given time point are positive for the loser markers Fwe LB and Azot, stressing the relevant role of cell competition in promoting the elimination of suboptimal cells. This comprehensive analysis of fitness marker dynamics over a 28-day timeframe sheds new light on the intricate mechanisms governing Flower-dependent cell competition.

Development of multicellular organisms is orchestrated by persistent cell-cell communication between neighboring partners. Direct interaction between different cell types can induce molecular signals that dictate lineage specification and cell fate decisions. Current single-cell RNA-seq technology cannot adequately analyze cell-cell contact-dependent gene expression, mainly due to the loss of spatial information. To overcome this obstacle and resolve cell-cell contact-specific gene expression during embryogenesis, we performed RNA sequencing of physically interacting cells (PIC-seq) and assessed them alongside similar single-cell transcriptomes derived from developing mouse embryos between embryonic day (E) 7.5 and E9.5. Analysis of the PIC-seq data identified gene expression signatures that were dependent on the presence of specific neighboring cell types. Our computational predictions, validated experimentally, demonstrated that neural progenitor (NP) cells upregulate Lhx5 and Nkx2-1 genes, when exclusively interacting with definitive endoderm (DE) cells. Moreover, there was a reciprocal impact on the transcriptome of DE cells, as they tend to upregulate Rax and Gsc when in contact with NP cells. Using individual cell transcriptome data, we formulated a means of computationally predicting the impact of one cell type on the transcriptome of its neighboring cell types. We have further developed a distinctive spatial-t-distributed stochastic neighboring embedding to display the pseudospatial distribution of cells in a 2-dimensional space. In summary, we describe an innovative approach to study contact-specific gene regulation during embryogenesis.

Because of ongoing climate change, populations of organisms are being subjected to stressful temperatures more often. This is especially problematic for ectothermic organisms, which are likely to be more sensitive to changes in temperature. Therefore, we need to know if ectotherms have adapted to environmental temperature and, if so, what are the evolutionary mechanisms behind such adaptation. Here, we use the nematode Pristionchus pacificus as a case study to investigate thermal adaptation on the Indian Ocean island of La Réunion, which experiences a range of temperatures from coast to summit. We study the evolution of high-temperature tolerance by constructing a phylogenetic tree of strains collected from many different thermal niches. We show that populations of P. pacificus at low altitudes have higher fertility at warmer temperatures. Most likely, this phenotype has arisen recently and at least twice independently, consistent with parallel evolution. We also studied low-temperature tolerance and showed that populations from high altitudes have increased their fertility at cooler temperatures. Together, these data indicate that P. pacificus strains on La Réunion are subject to divergent selection, adapting to hot and cold niches at the coast and summit of the volcano. Precisely defining these thermal niches provides essential information for models that predict the impact of future climate change on these populations.

Cell competition is a process by which suboptimal cells are eliminated to the benefit of cells with higher fitness. It is a surveillance mechanism that senses differences in the fitness status by several modes, such as expression of fitness fingerprints, survival factor uptake rate and resistance to mechanical stress. Fitness fingerprints-mediated cell competition recognizes isoforms of the transmembrane protein Flower, and translates the relative fitness of cells into distinct fates through the Flower code. Impairments in cell competition potentiate the development of diseases like cancer and ageing-related pathologies. In cancer, malignant cells acquire a supercompetitor behaviour, killing the neighbouring cells and overtaking the tissue, thus avoiding elimination. Neurodegenerative disorders affect millions of people and are characterized by cognitive decline and locomotor deficits. Alzheimer's disease is the most common form of dementia, and one of the largely studied diseases. However, the cellular processes taking place remain unclear. Drosophila melanogaster is an emerging neurodegeneration model due to its versatility as a tool for genetic studies. Research in a Drosophila Alzheimer's disease model detected fitness markers in the suboptimal and hyperactive neurons, thus establishing a link between cell competition and Alzheimer's disease. In this Review, we overview cell competition and the new insights related to neurodegenerative disorders, and discuss how research in the field might contribute to the development of new therapeutic targets for these diseases.

Risk stratification of COVID-19 patients is essential for pandemic management. Changes in the cell fitness marker, hFwe-Lose, can precede the host immune response to infection, potentially making such a biomarker an earlier triage tool. Here, we evaluate whether hFwe-Lose gene expression can outperform conventional methods in predicting outcomes (e.g., death and hospitalization) in COVID-19 patients. We performed a post-mortem examination of infected lung tissue in deceased COVID-19 patients to determine hFwe-Lose's biological role in acute lung injury. We then performed an observational study (n = 283) to evaluate whether hFwe-Lose expression (in nasopharyngeal samples) could accurately predict hospitalization or death in COVID-19 patients. In COVID-19 patients with acute lung injury, hFwe-Lose is highly expressed in the lower respiratory tract and is co-localized to areas of cell death. In patients presenting in the early phase of COVID-19 illness, hFwe-Lose expression accurately predicts subsequent hospitalization or death with positive predictive values of 87.8-100% and a negative predictive value of 64.1-93.2%. hFwe-Lose outperforms conventional inflammatory biomarkers and patient age and comorbidities, with an area under the receiver operating characteristic curve (AUROC) 0.93-0.97 in predicting hospitalization/death. Specifically, this is significantly higher than the prognostic value of combining biomarkers (serum ferritin, D-dimer, C-reactive protein, and neutrophil-lymphocyte ratio), patient age and comorbidities (AUROC of 0.67-0.92). The cell fitness marker, hFwe-Lose, accurately predicts outcomes in COVID-19 patients. This finding demonstrates how tissue fitness pathways dictate the response to infection and disease and their utility in managing the current COVID-19 pandemic.

Recent advances in rapid medical detection and diagnostic technology have extended both human health and life expectancy. However, ageing remains one of the critical risk factors in contributing to major incapacitating and fatal conditions, including cancer and neurodegeneration. Therefore, it is vital to study how ageing attributes to (or participates in) endangering human health via infliction of age-related diseases and what must be done to tackle this intractable process. This review encompasses the most recent literature elaborating the role of cell competition (CC) during ageing. CC is a process that occurs between two heterogeneous populations, where the cells with higher fitness levels have a competitive advantage over the neighbouring cells that have comparatively lower fitness levels. This interaction results in the selection of the fit cells, within a population, and elimination of the viable yet suboptimal cells. Therefore, it is tempting to speculate that, if this quality control mechanism works efficiently throughout life, can it ultimately lead to a healthier ageing and extended lifespan. Furthermore, the review aims to collate all the important state of the art publications that provides evidence of the relevance of CC in dietary restriction, stem cell dynamics, and cell senescence, thus, prompting us to advocate its contribution and in exploring new avenues and opportunities in fighting age-related conditions.

Tumors are complex cellular and acellular environments within which cancer clones are under continuous selection pressures. Cancer cells are in a permanent mode of interaction and competition with each other as well as with the immediate microenvironment. In the course of these competitive interactions, cells share information regarding their general state of fitness, with less-fit cells being typically eliminated via apoptosis at the hands of those cells with greater cellular fitness. Competitive interactions involving exchange of cell fitness information have implications for tumor growth, metastasis, and therapy outcomes. Recent research has highlighted sophisticated pathways such as Flower, Hippo, Myc, and p53 signaling, which are employed by cancer cells and the surrounding microenvironment cells to achieve their evolutionary goals by means of cell competition mechanisms. In this review, we discuss these recent findings and explain their importance and role in evolution, growth, and treatment of cancer. We further consider potential physiological conditions, such as hypoxia and chemotherapy, that can function as selective pressures under which cell competition mechanisms may evolve differently or synergistically to confer oncogenic advantages to cancer.

During adult life, damaged but viable neurons can accumulate in the organism, creating increasingly heterogeneous and dysfunctional neural circuits. One intriguing example is the aberrant increased activity of cerebral networks detected in vulnerable brain regions during preclinical stages of Alzheimer's disease. The pathophysiological contribution of these early functional alterations to the progression of Alzheimer's disease is uncertain. We found that a unique cell selection mechanism based on relative fitness comparison between neurons is able to target and remove aberrantly active neurons generated by heterologous human amyloid-β in Drosophila. Sustained neuronal activity is sufficient to compromise neuronal fitness and upregulate the expression of the low fitness indicators FlowerLoseB and Azot in the fly. Conversely, forced silencing of neurons restores brain fitness and reduces amyloid-β-induced cell death. The manipulation of this cell selection process, which was already proved to be conserved in humans, might be a promising new avenue to treat Alzheimer's.

Within heterogeneous tumors, cancer cells are constantly interacting. Cell competition (CC) is a fitness-based selection mechanism that results in increased proliferation of discrete populations at the expense of their less fit neighbors. CC-based selection of fit cells may also drive selection of the most aggressive cancer cells.

Abstract Cell competition is a conserved mechanism by which unfit cells are outcompeted by fitter neighbours. This quality control mechanism allows the maintenance and propagation of fitter cells, by potentiating the elimination of unfit cells from tissues. Cell competition plays a central role during embryogenesis and in adulthood, contributing to proper development and preservation of tissues homeostasis. The absence of fitness control impacts organ function, which ultimately results in shorten lifespan and disease development. In fact, impairment of cell competition or the appropriation of this process by pre‐malignant cells can allow the expansion of potentially dangerous cells leading to tumorigenesis. Different molecular mechanisms and strategies have been described to be required for the elimination of unfit cells depending on the tissue context and genetics. Key Concepts Cell competition actively takes place since embryonic stages, throughout development and in the adult. Cell competition is a conserved mechanism in multicellular animals. When in the presence of fitter cells, unfit cells are eliminated from tissues by cell competition. The context and genes involved determine the mechanism by which loser cells are eliminated. Selection of fit cells to be maintained in tissues promote homeostasis and improves lifespan. Accumulation of viable but less fit cells is detrimental for the organism. Pre‐malignant cells may hijack the cell competition mechanism, leading to tumour initiation.

Cancer is a complex disease with high incidence and mortality rates. The important role played by the tumor microenvironment in regulating oncogenesis, tumor growth, and metastasis is by now well accepted in the scientific community. SPARC is known to participate in tumor-stromal interactions and impact cancer growth in ambiguous ways, which either enhance or suppress cancer aggressiveness, in a context-dependent manner. p53 transcription factor, a well-established tumor suppressor, has been reported to promote tumor growth in certain situations, such as hypoxia, thus displaying a duality in its action. Although both proteins are being tested in clinical trials, the synergistic relation between them is yet to be explored in clinical practice. In this review, we address the controversial roles of SPARC and p53 as double agents in cancer, briefly summarizing the interaction found between these two molecules and its importance in cancer.