Academic literature on the topic 'Non-Clonal asexuality'

Meiosis in triploids faces the seemingly insuperable difficulty of dividing an odd number of chromosome sets by two. Triploid vertebrates usually circumvent this problem through either asexuality or some forms of hybridogenesis, including meiotic hybridogenesis that involve a reproductive community of different ploidy levels and genome composition. Batura toads ( Bufo baturae ; 3 n = 33 chromosomes), however, present an all-triploid sexual reproduction. This hybrid species has two genome copies carrying a nucleolus-organizing region (NOR+) on chromosome 6, and a third copy without it (NOR−). Males only produce haploid NOR+ sperm, while ova are diploid, containing one NOR+ and one NOR− set. Here, we conduct sibship analyses with co-dominant microsatellite markers so as (i) to confirm the purely clonal and maternal transmission of the NOR− set, and (ii) to demonstrate Mendelian segregation and recombination of the NOR+ sets in both sexes. This new reproductive mode in vertebrates (‘pre-equalizing hybrid meiosis’) offers an ideal opportunity to study the evolution of non-recombining genomes. Elucidating the mechanisms that allow simultaneous transmission of two genomes, one of Mendelian, the other of clonal inheritance, might shed light on the general processes that regulate meiosis in vertebrates.

SUMMARYTrematodes form clonal colonies in their first intermediate host. Individuals are, depending on species, rediae or sporocysts (which asexually reproduce) and cercariae (which develop within rediae or sporocysts and infect the next host). Some species use a division of labour within colonies, with 2 distinct redial morphs: small rediae (non-reproducing) and large rediae (individuals which produce cercariae). The theory of optimal caste ratio predicts that the ratio of caste members (small to large rediae) responds to environmental variability. This was tested in Philophthalmus sp. colonies exposed to host starvation and competition with the trematode, Maritrema novaezealandensis. Philophthalmus sp. infected snails, with and without M. novaezealandensis, were subjected to food treatments. Reproductive output, number of rediae, and the ratio of small to large rediae were compared among treatments. Philophthalmus sp. colonies responded to host starvation and competition; reproductive output was higher in well-fed snails of both infection types compared with snails in lower food treatments and well-fed, single infected snails compared with well-fed double infected snails. Furthermore, the caste ratio in Philophthalmus sp. colonies was altered in response to competition. This is the first study showing caste ratio responses to environmental pressures in trematodes with a division of labour.

Organisms adapt to different environments by selection of the most suitable phenotypes from the standing genetic variation or by phenotypic plasticity, the ability of single genotypes to produce different phenotypes in different environments. Because of near genetic identity, asexually reproducing populations are particularly suitable for the investigation of the potential and molecular underpinning of the latter alternative in depth. Recent analyses on the whole-genome scale of differently adapted clonal animals and plants demonstrated that epigenetic mechanisms such as DNA methylation, histone modifications and non-coding RNAs are among the molecular pathways supporting phenotypic plasticity and that epigenetic variation is used to stably adapt to different environments. Case studies revealed habitat-specific epigenetic fingerprints that were maintained over subsequent years pointing at the existence of epigenetic ecotypes. Environmentally induced epimutations and corresponding gene expression changes provide an ideal means for fast and directional adaptation to changing or new conditions, because they can synchronously alter phenotypes in many population members. Because microorganisms inclusive of human pathogens also exploit epigenetically mediated phenotypic variation for environmental adaptation, this phenomenon is considered a universal biological principle. The production of different phenotypes from the same DNA sequence in response to environmental cues by epigenetic mechanisms also provides a mechanistic explanation for the “general-purpose genotype hypothesis” and the “genetic paradox of invasions”.

Genetic diversity can influence resilience and adaptative capacity of organisms to environmental change. Genetic diversity within populations is largely structured by reproduction, with the prevalence of asexual versus sexual reproduction often underpinning important diversity metrics that determine selection efficacy. Asexual or clonal reproduction is expected to reduce genotypic diversity and slow down adaptation through reduced selection efficacy, yet the evolutionary consequences of clonal reproduction remain unclear for many natural populations. Here, we examine the genomic consequences of sympatric sexual (haplodiplontic) and clonal morphs of the kelp Ecklonia radiata that occur interspersed on reefs in Hamelin Bay, Western Australia. Using genome-wide single nucleotide polymorphisms, we confirm significant asexual reproduction for the clonal populations, indicated by a significantly lower number of multi-locus lineages and higher intra-individual diversity patterns (individual multi-locus heterozygosity, MLH). Nevertheless, co-ancestry analysis and breeding experiments confirmed that sexual reproduction by the clonal morph and interbreeding between the two morphs is still possible, but varies among populations. One clonal population with long-term asexuality showed trends of decreased selection efficacy (increased ratio non- vs. synonymous gene diversities). Yet, all clonal populations showed distinct patterns of putative local adaptation relative to the sexual morph, possibly indicating maladaptation to local environmental conditions and high vulnerability of this unique clonal morph to environmental stress.

La majorité des espèces parthénogétiques sont souvent perçues comme clonales. La clonalité est coûteuse à long terme, car elle peut entraîner l'accumulation de mutations délétères et une moins bonne capacité d’adaptation. Cependant, les cas d’espèces asexuées non clonales s'accumulent. L’asexualité non-clonale génère des conséquences génomiques et de fitness très différentes de la clonalité, et pourraient représenter une étape-clé dans la transition du sexe vers l’asexualité. De plus, l’asexualité peut être souvent non-obligatoire, avec des événements de sexe cryptiques. Ces évènements peuvent aussi façonner le génome et l'évolution des lignées asexuées. Dans cette thèse, j'ai étudié le mode de reproduction d'Artemia parthenogenetica, et son rôle dans la transition du sexe vers l'asexualité et l'évolution des lignées asexuées. En particulier, j'ai utilisé la capacité des mâles produits par voie asexuée (“mâles rares”) à se croiser avec des femelles sexuées et à transmettre l’asexualité à leurs descendants (asexualité contagieuse), pour générer expérimentalement de nouvelles lignées. J’ai montré que les Artemia asexués diploïdes ont un mode de reproduction non-clonal, dans lequel la recombinaison entraîne une perte d'hétérozygotie (LOH, pour “loss of heterozygosity”) chez les descendants. Le LOH est coûteux car il peut révéler des mutations délétères récessives. Peut-être en raison de la sélection causée par les conséquences délétères du LOH, le taux de recombinaison chez les Artemia asexués était plus faible que chez une espèce sexuée apparentée. J'ai également constaté que les hybrides sexués avaient une reproduction mixte sexuée et asexuée, et que les femelles asexuées issues de populations naturelles étaient capables de sexe rare. Cela signifie que des événements rares de sexe chez les Artemia asexués pourraient se produire entre un mâle rare et une femelle asexuée se reproduisant sexuellement. En effectuant une revue de la façon don t les modes de reproduction asexués sont identifiés dans la littérature, j'ai constaté que l'identification et la perception générale des asexués étaient biaisées en faveur de la clonalité, car une grande partie des espèces asexuées examinées étaient en fait non-clonales, et les preuves de la clonalité étaient souvent insuffisantes. En outre, la majorité des asexués non-clonaux avaient des modes de reproduction qui entraînaient de faibles taux de LOH. Cela suggère que les asexués non-clonaux évoluent souvent secondairement vers une reproduction plus clonale. Ainsi, même les espèces clonales pourraient ne pas avoir été clonales au cours de leur histoire évolutive. Enfin, avec une analyse génomique sur de nouvelles lignées générées par contagion, j'ai démontré que chez Artemia, les mâles rares sont produits asexuellement par recombinaison et donc LOH sur les chromosomes sexuels ZW. Nous savons que l'asexualité contagieuse, et peut-être des croisements entre lignées, ont eu lieu au cou rs de l'histoire évolutive d'A. parthenogenetica. L'asexualité contagieuse et/ou des événements sexuels chez les asexués constituent peut-être des opportunités pour que le(s) gène(s) contrôlant l'asexualité s'échappe(nt) des lignées en déclin vers de nouvelles lignées. Dans ce cas, l'asexualité contagieuse par le biais de mâles rares pourrait être la raison pour laquelle la recombinaison persiste chez les Artemia asexués. Chez de nombreuses espèces, l’identification de l’asexualité non clonale et des événements de sexe n'est toujours pas claire et nécessite une étude approfondie. Théoriquement, il y a un fort besoin de modèles prenant en compte les conséquences génomiques de l'asexualité non-clonale et non-obligatoire, et leur rôle dans la transition du sexe vers l'asexualité et la maintenance du sexe
The majority of parthenogenetic species are often thought to be clonal. Clonality is costly in the long term, as it can result in accumulation of deleterious mutations and lower adaptability. However, cases reporting non-clonal asexuals are accumulating. Non-clonal asexuality has very different genomic and fitness consequences compared to clonality, and may be a key intermediate step in the transition from sex to asexuality. Additionally, asexuality may be often non-obligate, with events of cryptic sex. These events may also shape the genome and evolution of asexual lineages. In this PhD, I investigated the reproductive mode of Artemia parthenogenetica and its role in the transition from sex to asexuality and the evolution of asexual lineages. Specifically, I used the capacity of asexually produced males (“rare males”) to cross with sexual females and transmit asexuality to their offspring (contagious asexuality), to experimentally generate new lineages. I showed that diploid asexual Artemia have a non-clonal reproductive mode, in which recombination results in loss of heterozygosity (LOH) in the offspring. LOH is costly as it can reveal recessive deleterious mutations. Perhaps due to selection caused by the deleterious consequences of LOH, the recombination rate in these asexuals was lower than in a closely related sexual species. I also found that sex-asex hybrids had a mixed sexual and asexual reproduction, and that asexual females from natural populations were capable of rare sex. This means that rare events of sex in asexual Artemia could occur between a rare male and an asexual female reproducing sexually. In a review of how asexual reproductive modes were identified in the literature, I found that there was a bias in the identification and general perception of asexuals toward clonality, as an important part of the asexual species reviewed were in fact non-clonal, and evidence for clonality was often missing. Furthermore, the maj ority of non-clonal asexuals had reproductive modes that resulted in low LOH. This suggests that non-clonal asexuals often evolve secondarily toward a more clonal-like reproduction, so that even clonal species may not have been clonal throughout their evolutionary history. Finally, using genomics on contagion-generated lineages, I found that in Artemia, rare males are produced asexually through recombination and thus LOH on the ZW sex chromosomes. We know that contagious asexuality, and possibly between-lineages crosses, occurred in the evolutionary history of A. parthenogenetica. Perhaps, contagious asexuality and/or within asexual sex events provide opportunities for the gene(s) controlling asexuality to escape declining lineages into new ones. In this case, contagious asexuality through rare males may be the reason why recombination persists in asexual Artemia. Whether non-clonal asexuality and sex events occur in many parthenogenetic species is still unclear, and requires thorou gh investigation. Theoretically, there is a strong need for models taking into account the genomic consequences of non-clonal and non-obligate asexuality, and their role in the transition from sex to asexuality and the maintenance of sex