Biology subjects

Simion, P.

Publications and source records attributed to Simion, P..

8 recordsLinked to original sources

bioRxiv2024

Transgenerational chromosome repair in the asexual bdelloid rotifer Adineta vaga

Homologous recombination is an essential DNA repair mechanism that promotes chromosome pairing and ensures allele segregation during meiosis in sexual organisms. Here, we explore the dual function of homologous recombination in the bdelloid rotifer Adineta vaga, an ancient asexual species known for its remarkable resilience to extreme genotoxic stresses. Genomic analyses reveal that A. vaga uses meiotic recombination, both to promote spontaneous crossovers and gene conversion during oogenesis and to repair the genome in response to DNA damage. Our study identifies a novel transgenerational DNA repair mechanism, termed break-induced homologous extension repair (BIHER), which operates on single DNA ends to repair fragmented chromosomes over successive generations. Our findings suggest that meiotic BIHER, coupled with the holocentric structure of chromosomes, represents a key adaptation of life in extreme environments.

evolutionary biology↗

bioRxiv2020

Haplotype divergence supports ancient asexuality in the oribatid mite Oppiella nova

Sex strongly impacts genome evolution via recombination and segregation. In the absence of these processes, haplotypes within lineages of diploid organisms are predicted to accumulate mutations independently of each other and diverge over time. This so-called Meselson effect is regarded as a strong indicator of the long-term evolution under obligate asexuality. Here, we present genomic and transcriptomic data of three populations of the asexual oribatid mite species Oppiella nova and its sexual relative Oppiella subpectinata. We document strikingly different patterns of haplotype divergence between the two species, strongly supporting Meselson effect like evolution and ancient asexuality in O. nova: (I) Variation within individuals exceeds variation between populations in O. nova but vice versa in O. subpectinata. (II) Two O. nova sub-lineages feature a high proportion of heterozygous genotypes and lineage-specific haplotypes, indicating that haplotypes diverged independently within the two lineages after their split. (III) The deepest split in gene trees generally separates haplotypes in O. nova, but populations in O. subpectinata. (IV) Tree topologies of the two haplotypes match each other. Our findings provide positive evidence for the absence of sex over evolutionary time in O. nova and suggest that asexual oribatid mites can escape the dead-end fate usually associated with asexual lineages.

evolutionary biology↗

bioRxiv2020

First annotated draft genomes of non-marine ostracods (Ostracoda, Crustacea) with different reproductive modes

ABSTRACTSOstracods are one of the oldest crustacean groups with an excellent fossil record and high importance for phylogenetic analyses but genome resources for this class are still lacking. We have successfully assembled and annotated the first reference genomes for three species of non-marine ostracods; two with obligate sexual reproduction (Cyprideis torosa and Notodromas monacha) and the putative ancient asexual Darwinula stevensoni. This kind of genomic research has so far been impeded by the small size of most ostracods and the absence of genetic resources such as linkage maps or BAC libraries that were available for other crustaceans. For genome assembly, we used an Illumina-based sequencing technology, resulting in assemblies of similar sizes for the three species (335-382Mb) and with scaffold numbers and their N50 (19-56 kb) in the same orders of magnitude. Gene annotations were guided by transcriptome data from each species. The three assemblies are relatively complete with BUSCO scores of 92-96%, and thus exceed the quality of several other published crustacean genomes obtained with similar techniques. The number of predicted genes (13,771-17,776) is in the same range as Branchiopoda genomes but lower than in most malacostracan genomes. These three reference genomes from non-marine ostracods provide the urgently needed basis to further develop ostracods as models for evolutionary and ecological research.

evolutionary biology↗

bioRxiv2020

Convergent consequences of parthenogenesis on stick insect genomes

The shift from sexual reproduction to parthenogenesis has occurred repeatedly in animals, but how the loss of sex affects genome evolution remains poorly understood. We generated de novo reference genomes for five independently evolved parthenogenetic species in the stick insect genus Timema and their closest sexual relatives. Using these references in combination with population genomic data, we show that parthenogenesis results in an extreme reduction of heterozygosity, and often leads to genetically uniform populations. We also find evidence for less effective positive selection in parthenogenetic species, supporting the view that sex is ubiquitous in natural populations because it facilitates fast rates of adaptation. Contrary to studies of non-recombining genome portions in sexual species, genomes of parthenogenetic species do not accumulate transposable elements (TEs), likely because successful parthenogens derive from sexual ancestors with inactive TEs. Because we are able to conduct replicated comparisons across five species pairs, our study reveals, for the first time, how animal genomes evolve in the absence of sex in natural populations, providing empirical support for the negative consequences of parthenogenetic reproduction as predicted by theory.

evolutionary biology↗

bioRxiv2020

Homologous chromosomes in asexual rotifer Adineta vaga suggest automixis

The several hundreds of species of bdelloid rotifers are notorious because they represent an ancient clade comprising only asexual lineages1. Moreover, most bdelloid species have the ability to withstand complete desiccation and high doses of ionizing radiation, being able to repair their DNA after massive genome breakage2. To better understand the impact of long-term asexuality and DNA breakage on genome evolution, a telomere-to-tolemere reference genome assembly of a bdelloid species is critical3, 4. Here we present the first, high quality chromosome-scale genome assembly for the bdelloid A. vaga validated using three complementary assembly procedures combined with chromosome conformation capture (Hi-C) data. The different assemblies reveal the same genome architecture and using fluorescent in situ hybridization (FISH), we demonstrate that the A. vaga genome is composed of six pairs of homologous chromosomes, compatible with meiosis. Moreover, the synteny between homoeologous (or ohnologous) chromosomes is also preserved, confirming their paleotetraploidy. The diploid genome structure of A. vaga and the presence of very long homozygous tracts show that recombination between homologous chromosomes occurs in this ancient asexual scandal, either during DSB repair or during meiotic pairing. These homozygosity tracts are mainly observed towards the chromosome ends in the clonal A. vaga suggesting signatures of a parthenogenetic mode of reproduction equivalent to central fusion automixis, in which homologous chromosomes are not segregated during the meiotic division.

evolutionary biology↗

bioRxiv2019

Is adaptation limited by mutation? A timescale dependent effect of genetic diversity on the adaptive substitution rate in animals.

Whether adaptation is limited by the beneficial mutation supply is a long-standing question of evolutionary genetics, which is more generally related to the determination of the adaptive substitution rate and its relationship with the effective population size Ne. Empirical evidence reported so far is equivocal, with some but not all studies supporting a higher adaptive substitution rate in large-Ne than in small-Ne species.\n\nWe gathered coding sequence polymorphism data and estimated the adaptive amino-acid substitution rate {omega}a, in 50 species from ten distant groups of animals with markedly different population mutation rate {theta}. We reveal the existence of a complex, timescale dependent relationship between species adaptive substitution rate and genetic diversity. We find a positive relationship between {omega}a and {theta} among closely related species, indicating that adaptation is indeed limited by the mutation supply, but this was only true in relatively low-{theta} taxa. In contrast, we uncover a weak negative correlation between {omega}a and {theta} at a larger taxonomic scale. This result is consistent with Fishers geometrical model predictions and suggests that the proportion of beneficial mutations scales negatively with species long-term Ne.

evolutionary biology↗

bioRxiv2018

A phylogenetic framework of the legume genus Aeschynomene for comparative genetic analysis of the Nod-dependent and Nod-independent symbioses

SUMMARYO_LISome Aeschynomene legume species have the property of being nodulated by photosynthetic Bradyrhizobium lacking the nodABC genes. Knowledge of this unique Nod (factor)-independent symbiosis has been gained from the model A. evenia but our understanding remains limited due to the lack of comparative genetics with related taxa using a Nod-dependent process.\nC_LIO_LITo fill this gap, this study significantly broadened previous taxon sampling, including in allied genera, to construct a comprehensive phylogeny. This backbone tree was matched with data on chromosome number, genome size, low-copy nuclear genes and strengthened by nodulation tests and a comparison of the diploid species.\nC_LIO_LIThe phylogeny delineated five main lineages that all contained diploid species while polyploid groups were clustered in a polytomy and were found to originate from a single paleo-allopolyploid event. In addition, new nodulation behaviours were revealed and Nod-dependent diploid species were shown to be tractable.\nC_LIO_LIThe extended knowledge of the genetics and biology of the different lineages in the legume genus Aeschynomene provides a solid research framework. Notably, it enabled the identification of A. americana and A. patula as the most suitable species to undertake a comparative genetic study of the Nod-independent and Nod-dependent symbioses.\nC_LI

plant biology↗

bioRxiv2017

A phylogenomic framework and timescale for comparative genomics and evolutionary developmental biology of tunicates

BackgroundTunicates are the closest relatives of vertebrates and are widely used as models to study the evolutionary developmental biology of chordates. Their phylogeny, however, remains poorly understood and to date, only the 18S rRNA nuclear gene and mitogenomes have been used to delineate the major groups of tunicates. To resolve their evolutionary relationships and provide a first estimate of their divergence times, we used a transcriptomic approach to build a phylogenomic dataset including all major tunicate lineages, consisting of 258 evolutionarily conserved orthologous genes from representative species.\n\nResultsPhylogenetic analyses using site-heterogeneous CAT mixture models of amino acid sequence evolution resulted in a strongly supported tree topology resolving the relationships among four major tunicate clades: 1) Appendicularia, 2) Thaliacea + Phlebobranchia + Aplousobranchia, 3) Molgulidae, and 4) Styelidae + Pyuridae. Notably, the morphologically derived Thaliacea are confirmed as the sister-group of the clade uniting Phlebobranchia + Aplousobranchia within which the precise position of the model ascidian genus Ciona remains uncertain. Relaxed molecular clock analyses accommodating the accelerated evolutionary rate of tunicates reveal ancient diversification (~450-350 million years ago) among the major groups and allow comparing their evolutionary age with respect to the major vertebrate model lineages.\n\nConclusionsOur study represents the most comprehensive phylogenomic dataset for the main tunicate lineages. It offers a reference phylogenetic framework and first tentative timescale for tunicates, allowing the direct comparison with vertebrate model species in comparative genomics and evolutionary developmental biology studies.

evolutionary biology↗