Biology subjects

Delattre, M.

Publications and source records attributed to Delattre, M..

9 recordsLinked to original sources

bioRxiv2025

Somatic Programmed DNA Elimination is widespread in free-living Rhabditidae nematodes

All cells of a multicellular organism usually share an identical genome, faithfully transmitted through successive divisions. Yet, a number of animal species deviate from this dogma, as parts of their DNA are systematically eliminated in all their somatic nuclei, in a process called Programmed DNA Elimination (PDE). PDE leads to the unexpected reorganisation of the genome at every generation in all somatic cells but its molecular mechanism, evolutionary origins, and functional significance remain unknown. This lack of understanding partially stems from limitations in genetically tractable model species. PDE can target an entire chromosome, or involve chromosome fragmentation followed by selective fragment retention and elimination, raising further questions on genome stability, genome integrity and mechanisms of DNA repair. PDE by chromosome fragmentation has been described in parasitic nematodes in the family Ascarididae, copepods in the genus Cyclops and unicellular ciliates. More recently, PDE has been discovered in three non-parasitic, lab-tractable nematode species from the Rhabditidae family, opening new perspectives. In this study, we used cytological approaches to screen 25 new Rhabditidae species for PDE. We found evidence of PDE in 17 species. Our work reveals that PDE is present in 12 out of 17 tested genera, demonstrating its widespread presence in Rhabditidae nematodes, with the notable exception of C. elegans. Genetic tools have already been established for some species. This work provides a collection of lab-tractable species that can be used to test many aspects of somatic Programmed DNA Elimination by chromosome fragmentation in animals.

evolutionary biology↗

bioRxiv2025

Emergence and stabilisation of a neo-Y chromosome in nematode species with rare males

How do sex chromosomes evolve in the transition to asexuality? So far, species that depart from canonical sexual reproduction - for example, parthenogens with rare sex, or species where the paternal genome is set aside - have been found to carry either no sex chromosomes, or sex chromosomes but no male-specific sex chromosome (i. e., no Y). Here we reveal that, in Mesorhabditis nematodes, a new Y chromosome emerged once, from sexual ancestors, in species that have transitioned into an unconventional mode of reproduction called autopseudogamy. In this reproductive system, females produce clonal females plus rare ([~]10%) males that are needed for fertilisation, but that do not contribute to the female genome. Analysing the Y chromosomes of two autopseudogamous species, we found high levels of degeneration, most likely due to loss of recombination, and two additional conserved features: i) they accumulated male-beneficial genes, and ii) they display a strong fertilisation drive, in that mainly Y-bearing sperm fertilise female oocytes. Both features are likely evolutionarily favourable in the context of autopseudogamy. Our results suggest that male-specific chromosomes can still be maintained in systems with rare, and possibly genetically useless, males.

evolutionary biology↗

bioRxiv2025

Disentangling the sources of variation in functional responses: between-individuals variability, measurements errors and inherent stochasticity of the prey-predator interaction process

The consumption rate of prey by predators, or functional responses, are known to be highly variable even within a single population. Identifying and estimating the different sources of variation of functional responses is a long-standing challenge. We develop here a statistical framework derived from a mechanistic stochastic process model that explicitly accounts for different sources of variation. We apply it to disentangle and estimate in particular 1) residual variance due to measurement errors and model misspecification, 2) between-predator variability, and 3) the interaction stochasticity, i.e. the intrinsic and mechanistic variability due to interactions processes between prey and predators. We show that it is possible to estimate these sources of variation under realistic experimental conditions. Our results also show that model fitting can compensate by overestimating residual source of variation, leading to biased parameter estimates when interaction stochasticity is misspecified. Applied to empirical data, the model reveals that standard assumptions, such as prey renewal and lack of spatial structure, fail to capture observed variability. We also show how experimental design affects parameter identifiability, highlighting the trade-off between the number of individuals and repeated observations.

ecology↗

bioRxiv2024

Dynamics of recombination, X inactivation and centromere proteins during stick insect spermatogenesis

In eukaryotes, the cellular processes contributing to gamete formation form the building blocks of genetic inheritance across generations. While traditionally viewed as conserved across model organisms, emerging studies reveal significant variation in meiotic and post-meiotic processes. Extending our knowledge to non-model organisms is therefore critical to improve our understanding of the evolutionary origin and significance of modifications associated with gamete formation. We describe the cytological patterns underlying chromosome segregation, recombination, and meiotic sex chromosome inactivation during male meiosis in the stick insect group Timema. Our results provide a detailed description of centromere protein loading dynamics during spermatogenesis, and further reveal that 1) recombination initiates before synapsis (unlike Drosophila meiosis), and 2) that the X remains actively silenced despite two waves of transcriptional activation in autosomes during spermatogenesis. Together, our observations help understand the evolutionary significance of key cellular events related to spermatogenesis and shed light on the diversity of their associated molecular processes among species, including Timema stick insects.

cell biology↗

bioRxiv2023

Co-segregation of recombinant chromatids maintains genome-wide heterozygosity in an asexual nematode

In asexual animals, female meiosis is modified to produce diploid oocytes. Associated with recombination, this is expected to lead to a rapid loss of heterozygosity, with adverse effects on fitness. Many asexuals, however, have a heterozygous genome, the underlying mechanisms being most often unknown. Cytological and population genomic analyses in the nematode Mesorhabditis belari revealed another case of recombining asexual being highly heterozygous genome-wide. We demonstrated that heterozygosity is maintained because the recombinant chromatids of each chromosome pair co-segregate during the unique meiotic division. A theoretical model confirmed that this segregation bias is necessary to account for the observed pattern and likely to evolve under a wide range of conditions. Our study uncovers a new type of cell division involving Directed Chromatid Assortment. One sentence summaryGenome wide heterozygosity in the asexual nematode Mesorhabditis belari is achieved by directed assortment of recombinant chromatids during female meiosis

evolutionary biology↗

bioRxiv2022

Programmed-DNA Elimination in the free-living nematodes Mesorhabditis

In most species, elaborate programs exist to protect chromatin and maintain its integrity over cell cycles and generations. However some species systematically undergo excision and elimination of portions of their genome in somatic cells in a process called programmed-DNA elimination (PDE). PDE involves the elimination of mainly repeated elements but also protein-coding genes. PDE has been described in approximately 100 species from very distinct phyla, and more extensively in the parasitic nematodes Ascaris and in the unicellular Ciliates. In Ciliates, where PDE is pervasive, the underlying mechanisms have been studied and involve small RNA-guided heterochromatinization. In striking contrast, chromatin recognition and excision mechanisms remain mysterious in Metazoans, because the study species are not amenable to functional approaches. Above all, the function of such a mechanism, which has emerged repeatedly throughout evolution, is unknown. Answering these questions will provide significant insights into our understanding of chromatin regulation and genome stability. We fortuitously discovered the phenomenon of PDE in all species of the free-living nematode genus Mesorhabditis. Mesorhabditis, which belong to the same family as C. elegans, have a small [~]150 Mb genome and offer many experimental advantages to start elucidating the elimination mechanisms in Metazoans. In this first study, we have used a combination of cytological observation and genomic approaches to describe PDE in Mesorhabditis. We found that the dynamics of chromosome fragmentation and loss are very similar to those described in Ascaris. Elimination occurs once in development, at the third embryonic cell division in all 5 presomatic blastomeres. Similar to other species, Mesorhabditis eliminate repeated elements but also about a hundred unique sequences. Most of the eliminated unique sequences are either pseudogenes or poorly conserved protein-coding genes. Our results raise the possibility that PDE has not been selected for a gene regulatory function in Mesorhabditis but rather mainly is a mechanism to irreversibly silence repeated elements in the soma.

genomics↗

bioRxiv2021

Evolutionary divergence of anaphase spindle mechanics in nematode embryos constrained by antagnonistic pulling and viscous forces

Cells are the basic unit of biological organization, and their division is remarkably conserved across phyla. However from an evolutionary perspective, it remains unclear how much cellular parameters can diverge, without altering the basic function they sustain. We address the mechanics of asymmetric mitotic spindle positioning during the first embryonic division of six nematode species. We propose a viscoelastic model of spindle positioning and mobility that can provide a physical explanation of why in C. elegans it undergoes oscillations during elongation, whereas most others lack oscillations. To test this model, we measured the pulling forces and opposing cytoplasmic drag by a combination of laser ablation of the anaphase spindle and tracking of intracellular granules. While centrosomes of all species recoil on spindle cutting, quantitative differences correlate with the cytoplasmic viscosity. In fact, increased viscosity correlates with decreased oscillation speeds of intact spindles across species. However, the absence of oscillations despite low viscosity in some species, can only be explained by smaller pulling forces. Consequently, we find that spindle mobility across the species analyzed here is characterized by a tradeoff between cytoplasmic viscosity and pulling forces. Our work provides a framework for understanding mechanical constraints on evolutionary diversification of spindle mobility.

biophysics↗

bioRxiv2018

Why would parthenogenetic females systematically produce males who never transmit their genes to females?

SummaryIn pseudogamous species, females use the sperm of males from another species to activate their oocytes and produce females, without using the sperm DNA. Here we report a novel reproductive strategy found in the pseudogamous nematode Mesorhabditis belari, which produces its own males at low frequency. We find that the 8% of M. belari males are necessary to fertilize all oocytes but pass on their genes only to males, and never to females. Thus, the production of males has no impact on the genetic diversity of females. Using game theory, we show that the production of males at low frequency constitutes an efficient strategy only if sons are more likely to mate with their sisters. We validate this prediction experimentally by revealing a mating preference between siblings. We uncover the remarkable reproductive strategy of parthenogenetic females that pay the cost of producing males while males do not spread their genes.

evolutionary biology↗

bioRxiv2017

Astral microtubule dynamics regulate anaphase * oscillation onset and set a robust final position * of the C. elegans zygote spindle.

During the asymmetric division of the Caenorhabditis elegans nematode zygote, the polarity cues distribution and daughter cell fates depend on the correct positioning of the mitotic spindle, which results from both centering and cortical pulling forces. Revealed by anaphase spindle rocking, these pulling forces are regulated by the force generator dynamics, which are in turn consequent of mitotic progression. We found a novel, additional, regulation of these forces by the spindle position. It controls astral microtubule availability at the cortex, on which the active force generators can pull. Importantly, this positional control relies on the polarity dependent LET-99 cortical band, which restricts or concentrates generators to a posterior crescent. After delaying anaphase onset, we detected this positional pulling force regulation in C. elegans as a precocious spindle rocking with respect to anaphase onset. We ascribed this control to the microtubule dynamics at the cortex. Indeed, in mapping the cortical contacts, we found a correlation between the centrosome-cortex distance and the microtubule contact density. In turn, it modulates pulling force generator activity. We modelled this control, predicting and experimentally validating that the posterior crescent extent controlled where the anaphase oscillations started, in addition to mitotic progression. We found in particular that the oscillation onset position resists changes in cellular geometry and moderate variations of active force generator count. Finally, we propose that spatially restricting force generator to a posterior crescent sets the spindles final position, reflecting polarity through the LET-99 dependent restriction of force generators to a posterior crescent. This regulation superimposes that of force generator processivity. This novel control confers a low dependence on microtubule and active force generator exact numbers or dynamics, provided that they exceed the threshold needed for posterior displacement. Interestingly, this robustness originates in cell mechanics rather than biochemical networks.

biophysics↗