Biology subjects

Launay, C.

Publications and source records attributed to Launay, C..

4 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↗

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↗

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↗