Abstract Transitions from obligate sex to obligate parthenogenesis have occurred repeatedly across the tree of life. Whether these transitions occur abruptly or via a transient phase of facultative parthenogenesis is rarely known. We discovered and characterised facultatively parthenogenetic populations of the North American stick insect Timema douglasi , a species in which only obligately parthenogenetic populations were known so far. These populations comprised three genetic lineages. Females from all lineages were capable of parthenogenesis (with variable efficiency) but their propensity to reproduce sexually after mating varied extensively. In all three lineages, parthenogenesis resulted in the complete loss of heterozygosity in a single generation. Obligately parthenogenetic Timema have also lost all heterozygosity, suggesting that the transition to obligate parthenogenesis did not require a modification of the proximate mechanism, but rather involved a gradual increase in frequency. We speculate that facultative parthenogenesis may often be transient and be replaced by obligate strategies (either sex or parthenogenesis) because of a trade-off between the efficiency of the two reproductive modes. Such a trade-off could help explain why facultative parthenogenesis is rare among animals, despite its potential to combine the known benefits of sex and parthenogenesis.
Transitions from obligate sex to obligate parthenogenesis have occurred repeatedly across the tree of life. Whether these transitions occur abruptly or via a transient phase of facultative parthenogenesis is rarely known. We discovered and characterised facultatively parthenogenetic populations of the North American stick insect Timema douglasi, a species in which only obligately parthenogenetic populations were known so far. These populations comprised three genetic lineages. Females from all lineages were capable of parthenogenesis (with variable efficiency) but their propensity to reproduce sexually after mating varied extensively. In all three lineages, parthenogenesis resulted in the complete loss of heterozygosity in a single generation. Obligately parthenogenetic Timema have also lost all heterozygosity, suggesting that the transition to obligate parthenogenesis did not require a modification of the proximate mechanism, but rather involved a gradual increase in frequency. We speculate that facultative parthenogenesis may often be transient and be replaced by obligate strategies (either sex or parthenogenesis) because of a trade-off between the efficiency of the two reproductive modes. Such a trade-off could help explain why facultative parthenogenesis is rare among animals, despite its potential to combine the known benefits of sex and parthenogenesis.
Understanding the evolutionary dynamics underlying herbivorous insect mega-diversity requires investigating the ability of insects to shift and adapt to different host plants. Feeding experiments with nine related stick insect species revealed that insects retain the ability to use ancestral host plants after shifting to novel hosts, with host plant shifts generating fundamental feeding niche expansions. These expansions were, however, not accompanied by expansions of the realised feeding niches, as species on novel hosts are generally ecologically specialised. For shifts from angiosperm to chemically challenging conifer hosts, generalist fundamental feeding niches even evolved jointly with strong host plant specialisation, indicating that host plant specialisation is not driven by constraints imposed by plant chemistry. By coupling analyses of plant chemical compounds, fundamental and ecological feeding niches in multiple insect species, we provide novel insights into the evolutionary dynamics of host range expansion and contraction in herbivorous insects.
The factors contributing to the maintenance of sex over asexuality in natural populations remain unclear. Ecological divergences between sexual and asexual lineages could help to maintain reproductive polymorphisms, at least transiently, but the consequences of asexuality for the evolution of ecological niches are unknown. Here, we investigated how niche breadths change in transitions from sexual reproduction to asexuality. We used host plant ranges as a proxy to compare the realized feeding niche breadths of five independently derived asexual Timema stick insect species and their sexual relatives at both the species and population levels. Asexual species had systematically narrower realized niches than sexual species, though this pattern was not apparent at the population level. To investigate how the narrower realized niches of asexual species arise, we performed feeding experiments to estimate fundamental niche breadths but found no systematic differences between reproductive modes. The narrow realized niches found in asexual species are therefore probably a consequence of biotic interactions such as predation or competition, that constrain realized niche size in asexuals more strongly than in sexuals.
The integron is a new type of mobile element which has evolved by a site-specific recombinational mechanism. Integrons consist of two conserved segments of DNA separated by a variable region containing one or more genes integrated as cassettes. Oligonucleotide probes specific for the conserved segments have revealed that integrons are widespread in recently isolated clinical bacteria. Also, by using oligonucleotide probes for several antibiotic resistance genes, we have found novel combinations of resistance genes in these strains. By using PCR, we have determined the content and order of the resistance genes inserted between the conserved segments in the integrons of these clinical isolates. PCR mapping of integrons can be a useful epidemiological tool to study the evolution of multiresistance plasmids and transposons and dissemination of antibiotic resistance genes.
‘Reproduction’ is one of the key characteristic of life. Despite this, our knowledge of the evolution of reproductive systems is still incomplete. In particular, the reasons for why the vast majority of eukaryotes use sex, and thus take a complicated and costly detour to reproduction, when straightforward routes, such asexuality, are available, remains a central and largely unanswered question in evolutionary biology. The aim of my thesis is to contribute to the understanding of this evolutionary mystery, and for that I use stick insects of the genus Timema as a study system. This small group of herbivorous insects, endemic to Western United States is ideal for studying and comparing sexual and asexual reproduction as seven asexual lineages have been identified in this group, each with a sexual sister species, allowing us to make multiple independent comparisons between sexual and asexual lineages.
The perhaps most broadly accepted theoretical argument is that sex allows selection to work efficiently, which would ultimately favor the adaptive potential of populations. My objective during this thesis was to test two theories directly related to this, but working each time in two successive steps: i) I started by clarifying the ecological and evolutionary aspects and mecanisms concerned by these theories in Timema focusing only on sexual species and thus independently of the reproductive mode, ii) and I then empirically tested these theories. Specifically, I first investigated whether sexuals are able to exploit more ecological niches than asexuals, which would give them an advantage in fluctuating or heterogeneous environments. From this first investigation, I overall found that sexual species are systematically using a larger portion of their environment than their asexual relatives, but I did not find this pattern regarding their intrinsic and physiologic abilities to use their environment. The reduced portion used by asexuals is thus likely a consequence of external and biotic interactions that affect asexuals more strongly than sexuals. I secondly aimed to empirically test if sex confer an advantage when the allele combinations that are favored by selection vary over time, as it is the case in context of coevolution with parasites. My work suggests that parasites are indeed contributing to the maintenance of sex in Timema. In the last part of the thesis, I finally present some preliminary results regarding new Timema populations that I discovered by chance, that feature unusual reproductive strategies with a mixture of sexual, asexual ad facultatively asexual individuals. These populations will be very profitable for future research concerning the evolution of reproduction in Timema. Overall this thesis work contributes to a better understanding of several aspects of the ecology and evolution of Timema stick insects in partiular, and more generally contribute to give novel insights in the understanding of the maintenance of sex in the living world.
--
L’une des caracteristiques essentielles d’un etre vivant est sa capacite a se reproduire. Malgre cela, notre connaissance et comprehension de l'evolution de la reproduction est encore tres partielle. En particulier, les raisons pour lesquelles la grande majorite des eucaryotes utilise un mode de reproduction aussi complique et raffine que le sexe, alors que des manieres beaucoup plus simples de se reproduire existent reste une veritable enigme de la biologie evolutive. Le but de ma these est de contribuer a la resolution de ce mystere evolutif. Pour cela j’etudie les phasmes du genre Timema, un petit groupe d'insectes herbivores endemique de l'ouest des Etats-Unis. C’est un systeme d’etude ideal pour comparer les couts et benefices de la reproduction sexuee et de la reproduction asexuee car sept lignees asexuees ont ete identifiees au sein de ce groupe, chacune avec une espece ancestrale soeur sexuee. Cela nous permet de faire des comparaisons independantes entre lignees sexuees et asexuees.
L’un des arguments theoriques le plus largement propose pour expliquer la predominance du sexe, est qu’il permet a la selection naturelle de fonctionner plus efficacement, ce qui favoriserait le potentiel adaptatif des populations. Au cours de cette these, j’avais pour objectif de tester deux theories s’incrivant dans ce contexte. J’ai travaille en deux etapes successives : J’ai tout d'abord etudie et clarifie les aspects et processus ecologiques et evolutifs concernes par ces theories chez les Timema en se concentrant exclusivement sur les expeces sexuees, et donc independemment du mode de reproduction, puis, dans un second temps, j’ai teste empiriquement ces theories. Premierement, j’ai verifie si les sexues sont capables d'exploiter plus de niches ecologiques que les asexues, ce qui leur confererait un avantage au sein des environnements fluctuants ou heterogenes. J'ai trouve que les especes sexuees utilisent systematiquement une plus large portion de leur environnement que les especes asexuees, mais je n’ai pas retrouve un tel pattern en ce qui concerne leurs capacites intrinseques et physiologiques a utiliser cet environnement. Cette utilisation de l’environnement reduite des asexues compare aux sexues indique que les pressions externes et biotiques affectent plus fortement la capacite des asexues a exploiter leur environnement que celle des sexues. Deuxiemement, j'ai verifie empiriquement si le sexe confere un avantage lorsque les combinaisons d'alleles favorisees par la selection varient au cours du temps, comme c’est le cas lors d’une coevolution hotes-parasites. Mon travail suggere que les pressions parasitaires contribuent effectivement au maintien du sexe chez leurs hotes Timema. Dans la derniere partie de cette these, je presente des resultats preliminaires concernant de nouvelles populations de Timema que j’ai decouvert par chance au cours du doctorat. Ces populations ont des strategies reproductives inhabituelles comprenant une mixture d’individus sexues et asexues, et seront tres utiles lors des futures recherches concernant l'evolution de la reproduction. Dans l'ensemble, ma these contribue a une meilleure connaissance de l'ecologie et de l'evolution des phasmes Timema, et contribue plus generalement a comprendre pourquoi le sexe est le mode de reproduction predominant au sein du monde vivant.
Abstract Host–parasite coevolution stems from reciprocal selection on host resistance and parasite infectivity, and can generate some of the strongest selective pressures known in nature. It is widely seen as a major driver of diversification, the most extreme case being parallel speciation in hosts and their associated parasites. Here, we report on endoparasitic nematodes, most likely members of the mermithid family, infecting different Timema stick insect species throughout California. The nematodes develop in the hemolymph of their insect host and kill it upon emergence, completely impeding host reproduction. Given the direct exposure of the endoparasites to the host's immune system in the hemolymph, and the consequences of infection on host fitness, we predicted that divergence among hosts may drive parallel divergence in the endoparasites. Our phylogenetic analyses suggested the presence of two differentiated endoparasite lineages. However, independently of whether the two lineages were considered separately or jointly, we found a complete lack of codivergence between the endoparasitic nematodes and their hosts in spite of extensive genetic variation among hosts and among parasites. Instead, there was strong isolation by distance among the endoparasitic nematodes, indicating that geography plays a more important role than host‐related adaptations in driving parasite diversification in this system. The accumulating evidence for lack of codiversification between parasites and their hosts at macroevolutionary scales contrasts with the overwhelming evidence for coevolution within populations, and calls for studies linking micro‐ versus macroevolutionary dynamics in host–parasite interactions.
'%3e%3cpath fill='%23012169' d='M0 0v30h60V0z'/%3e%3cpath stroke='%23fff' stroke-width='6' d='m0 0 60 30m0-30L0 30'/%3e%3cpath stroke='%23C8102E' stroke-width='4' d='m0 0 60 30m0-30L0 30' clip-path='url(%23b)'/%3e%3cpath stroke='%23fff' stroke-width='10' d='M30 0v30M0 15h60'/%3e%3cpath stroke='%23C8102E' stroke-width='6' d='M30 0v30M0 15h60'/%3e%3c/g%3e%3c/svg%3e)