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[
International Worm Meeting,
2009]
Many developmental processes generate invariant phenotypes despite environmental or mutational perturbations. Such robustness is a fundamental biological property, yet its extent, limits and adaptive significance have rarely been assessed empirically. Here we tested how environmental variation and accumulation of spontaneous random mutation impact the developmental system underlying vulval formation in Caenorhabditis nematodes. In different environments, a correct vulval pattern develops with high precision but rare deviant patterns reveal the system''s limits and how its mechanisms respond to environmental challenges. Key features of the apparent robustness are functional redundancy among vulval precursor cells and tolerance to quantitative variation in Ras, Notch and Wnt pathway activities. These environmental responses and the precision of the vulval patterning process further vary within and between Caenorhabditis species. To quantify how developmental precision responds to mutational perturbations, we used a set of mutation accumulation (MA) lines derived from two C. briggsae and two C. elegans genotypes. Developmental defects and variants increased after MA treatment for all tested genotypes, yet the type and proportion of the mutationally induced variation varied among genotypes. Thus, the mutability of this developmental system evolves, so that the mutationally induced phenotypic space is biased depending on the genetic background. Comparison of the standing genetic variance (VG) for deviant vulva phenotypes with the mutational variance (VM) leads to the conclusion that strong natural selection acts to maintain the robustness of this developmental process.
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[
European Worm Meeting,
2006]
We study C. elegans vulva development as an example of a biological process that is robust to stochastic noise and variations in the external environment. The vulva is formed by three precursor cells aligned in the ventral epidermis. P6.p usually adopts a 1 fate, P5.p and P7.p a 2 fate. In addition, P4.p and P8.p (and P3.p when it does not fuse to the epidermal syncytium) are competent to form vulval tissue but normally adopt a non-vulval 3 fate. The spatial fate pattern (332123) is organized around the gonadal anchor cell that emits a LIN-3/EGF signal. Cell fate patterning involves a well-characterized network of intercellular signaling pathways, including EGF/Ras, Notch and Wnt signaling. The cell fate pattern is quasi-invariant within and among wild isolates of different Caenorhabditis species.. . We measured the actual precision of this vulval fate patterning process and characterized deviant patterns in six laboratory environments (standard laboratory conditions at 15C, 20C and 25C; liquid; dauer; starvation in L2). Overall, we find that errors that result in a defective vulva occur at low frequency (<0.1%) in all environments and that the rates and patterns of variants depend on the environment and genetic background for different wild isolates of C. elegans and C. briggsae. For example, under the L2 starvation conditions, N2 individuals are prone to miscenter their vulva on P5.p (instead of P6.p), a variation rescued by P4.p competence, whereas the C. elegans isolate JU258 shows a dramatic increase in P4.p and P8.p fusion, but rarely miscenters the vulva, and C. briggsae AF16 is prone to miscenter its vulva on P7.p. In addition, P3.p fusion decreases upon starvation in AF16 while it increases in N2 and JU258. The interplay between variations in centering and competence is likely to be relevant to the evolution of the system.. Buffering of the system to environmental variations may result in the buffering of some genetic variation, thus allowing the evolution of developmental processes without phenotypic change (silent/cryptic evolution). To reveal cryptic variation within C. elegans, we introgressed vulva mutations into six divergent genotypes (CB4856, JU258, PS2025, AB1, PB303, PB306) and found that the phenotypic effect of a mutation varies greatly with the genotype. To reveal cryptic variation among Caenorhabditis species, we ablated the anchor cell at successive timepoints during vulval induction, which uncovers a temporal series of P(5-7).p cell fate patterns, starting from 333 and ending with the correct 212 pattern. P(5-7).p adopt an intermediate 222 fate pattern in C. briggsae and in ''basal Caenorhabditis species, as in many other nematode genera. Direct transition to the correct 212 fate pattern is found in a group of species including C. elegans, and a 232 intermediate pattern is found in C. remanei. We thus reveal candidate changes in the relative activities of different vulva signaling pathways, in the absence of change in the final phenotype.
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[
International Worm Meeting,
2005]
Development generates stable and correct phenotypes despite variation in the environment. How is such developmental precision achieved and how does it evolve? The development of the C. elegans vulva provides an ideal study system to address these questions. The hermaphrodite vulva is the organ used for egg-laying and copulation. Its correct formation therefore appears to be crucial to maximize an individuals fitness. Vulval cell patterning involves a well-characterized, complex network of genetic pathways (LET-23-Ras-MAPK, LIN-12/Notch, WNT, SynMuv genes). The partially redundant nature and intricate regulation of these pathways seemingly ensure precision of vulval development. However, it is unclear how the interplay of these mechanisms may achieve precision when development is actually perturbed. We have measured the precision of vulval cell patterning in wild type animals of several wild isolates in six laboratory environments (NGM plates at 15C, 20C and 25C; liquid culture; passage through dauer; starvation). The precision of this developmental process varies with the environment, and errors leading to a defective vulva occur at a frequency of < 1% in most environments. Importantly, it is not only the frequency of error but also the type of error that varies with the environment. This result implies that environmental perturbations can trigger specific developmental responses (as opposed to an unpredictable de-buffering of development). In addition, frequency and type of error also vary among C. elegans wild isolates. This result demonstrates evolutionary variation in developmental precision. It further indicates that the molecular mechanisms governing vulva development are likely to differ among wild isolates (cf. contribution by Milloz et al.). Environmental conditions have previously been shown to modulate the penetrance of several vulva mutations (e.g. Ferguson & Horvitz 1985, Battu et al. 2003, Moghal et al. 2003). We have examined the development of various vulva mutants in six laboratory environments (see above). Many mutational effects (e.g. mutations in
lin-3,
let-60,
bar-1 or
sup-17) are strongly or subtly modified depending on the environment. Using this information, we are testing how the environment may affect specific interactions between different genetic pathways regulating vulval cell patterning. The integration of the above mentioned experiments will help clarifying how the different genetic pathways interact to maintain precision of vulval development. Furthermore, analyzing variation in the precision of vulval development among different wild isolates will provide insights into the microevolution of mechanisms underlying a robust developmental process.
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[
International Worm Meeting,
2013]
We are interested in how environmental variation affects the functioning of developmental systems and their underlying genetic networks. As a model system, we study the process of C. elegans vulval patterning - a mechanistically well-defined system involving Ras, Notch and Wnt pathways. We previously showed that vulval patterning generates an invariant phenotypic output across various environments although the underlying signalling pathways turned out to be highly environmentally sensitive. In particular, development in starvation conditions or dauer passage strongly suppresses the Vulvaless phenotype of the reduction-of-function mutations in the Egf/Ras/Mapk pathway. We are characterizing the mechanisms by which environmental signals alter activities and interaction of vulval signalling pathways, and how such changes impact the precision of developmental outcomes. We focus on the starvation suppression of
lin-3(rf), which cause a strong hypoinduced vulval phenotype under normal conditions. We present our current results indicating that (a) environmentally induced changes in signalling pathway activities are mediated by internal physiological cues rather than external sensory cues and (b) starvation effects suppressing the
lin-3(rf) mutations are unlikely due to the developmental delay caused by the starvation treatment. We are currently testing to what extent the relative contribution of Ras and Wnt pathways to the vulval inductive signal changes with the environment.
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[
International Worm Meeting,
2011]
We are interested in how environmental variation affects the functioning of developmental systems and their underlying genetic networks. As a model system, we study the process of C. elegans vulval patterning - a mechanistically well-defined system involving a signalling network of Ras, Notch and Wnt pathways. We previously showed that vulval patterning generates an invariant phenotypic output across various environments although the underlying signalling pathways turned out to be highly environmentally sensitive. In particular, our results and these of others suggest that development in starvation conditions or dauer passage strongly suppresses the Vulvaless phenotype of the reduction-of-function mutations in the Egf/Ras/Mapk pathway. These observations suggest an upregulation of the vulval inductive signal through Ras and/or Wnt pathways in these conditions. We are now characterizing the mechanisms by which environmental signals alter activities and interaction of vulval signalling pathways, and how such changes impact the precision of developmental outcomes. In particular, we focus on the starvation suppression of
lin-3(
e1417) and
lin-3(
n378), which cause a strong hypoinduced vulval phenotype under normal conditions. Our genetic analyses indicate that (a) internal physiological cues rather than externally perceived sensory cues are responsible for the observed starvation suppression, (b) dauer formation induced by mutation rather than by the environment is insufficient to suppress
lin-3(rf) mutations, and (c) starvation effects suppressing the penetrance of
lin-3 mutations are unlikely due to the extensive developmental delay (~48h) caused by the starvation treatment. Previous results further indicated that starvation significantly increases Ras pathway activity, which seems to partly explain
lin-3(rf) suppression. However, the Wnt pathway may be also involved because
lin-3(rf) starvation suppression is abolished in the double mutant
lin-3(rf);
bar-1(null). We are therefore currently testing to what extent the relative contribution of Ras and Wnt pathways to the vulval inductive signal changes with the environment. Ultimately, we aim to understand whether and how such environmental variability of molecular signals may contribute to phenotypic robustness.
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[
International Worm Meeting,
2011]
To better understand the role of gene-environment interactions in development and evolution, we study quantitative natural genetic variation and plasticity of the Caenorhabditis germline and reproductive system in different environments. Although the basic germline organization and processes are conserved among Caenorhabditis species, reproductive output and schedules vary both within and between species. We are interested in understanding how differences in offspring number and quality are coupled to the underlying properties and processes of the germline (e.g. sperm number, germ cell number, proliferation, apoptosis) and to what extent they are plastic, i.e. vary across different (ecologically relevant) environments. In an initial analysis, we have quantified germline and reproductive phenotypes in 15 isolates of the three hermaphroditic species (C. elegans, C. briggsae and C. sp. 11). Overall, sperm number, germ cell number and offspring number are positively correlated; however, sperm number does not always closely match offspring number, indicating that isolates and species may differ in sperm fertility or efficiency of sperm use. The most striking observation is that many C. sp. 11 isolates show a highly reduced offspring, sperm, germ cell number and mitotic zone relative to C. elegans. To carry out an integrative analysis of germline and reproductive plasticity, we have characterized how e.g. germ cell proliferation, entry into meiosis and apoptosis are modified in N2 animals exposed to diverse conditions (such as liquid, starvation, different bacterial food sources, ethanol, acetic acid, temperature shifts, osmotic or hypoxia). Our results confirm that diverse germline processes are highly environmentally sensitive. We also show that stressful conditions may reduce offspring number through reduction of either sperm fertility or number, as well as defects in germline progression. The plastic responses in reproductive features of C. elegans N2 may differ greatly from the ones observed in other wild isolates of C. elegans, C. briggsae or C. sp. 11, revealing considerable genotype-by-environment interactions. We will discuss these and other results in the context of how such differential plasticity of the reproductive system contributes to germline integrity and reproductive success in variable environments.
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[
International Worm Meeting,
2013]
Under stressful environmental conditions, such as starvation, Caenorhabditis hermaphrodites and females increasingly retain developing embryos in the uterus ("bagging"). As a result, embryos will hatch and continue larval development internally, causing the mother's premature death. Retention of embryos and internal hatching potentially represent adaptive strategies to provide offspring with resources provided by the mother in the absence of food in the external environment. To elucidate the evolutionary significance and genetic mechanisms of worm bagging, we study variation in embryo retention in different C. elegans wild isolates and different Caenorhabditis species in response to variable environmental conditions (e.g. temperature, food quantity and quality). Although bagging occurs to some extent in all Caenorhabditis species, isolates and species may strongly differ in their bagging response, revealing significant genotype-by-environment interactions. Moreover, certain isolates may show a very high frequency of bagging, even under food-rich conditions. Making use of an F2 RIL mapping population we have now started to characterize the genetic differences between isolates that show divergent bagging behaviour, which may lead to the identification of genetic factors involved in the regulation of embryo retention and matricide in Caenorhabditis nematodes.
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[
International Worm Meeting,
2011]
Many Caenorhabditis species proliferate in a variety of rotting vegetal substrates: fruits, flowers, plant stems. We will report on wide sampling of such substrates and possible carrier invertebrates, focusing on 1) French Guiana; 2) mainland France.
In French Guiana, we sampled mostly in wild tropical forest around the Nouragues CNRS Biological Station. Besides finding three new species, C. sp. 12, 17 and 18 (see abstract by Kiontke et al.), we determined the spatial distribution of six Caenorhabditis species at different scales and constituted a frozen isolate collection. Caenorhabditis species often co-occur in the same individual fruit or flower. Their spatial distribution over the location at one timepoint appears inhomogeneous.
In mainland France, rotting fruit and stem habitats were found to be shared by C. elegans and C. briggsae. Both species were isolated throughout most of France. In addition, C. remanei or C. sp. 13 were found each in one location. We sampled a systematic spatio-temporally structured set of rotting apples in an apple variety orchard in Orsay. We scored the prevalence of each species in different parts of the same apple, in different apples below a given tree, throughout the orchard and at 19 timepoints over three years. C. elegans and C. briggsae were abundantly found and may co-occur in the same apples. However, their temporal distribution did not coincide. C. briggsae was found alone in summer; both species co-occurred in early fall and C. elegans remained alone in late fall. This temporal sharing of the habitat coincides with their temperature preference in the lab. In relatively natural habitats of France (wood, heath), rotting stems of several plant species yielded Caenorhabditis. Both C. elegans and C. briggsae were found in the same plant species and sometimes in the same individual stem.
Most populations were analyzed on the day of sampling, which enabled us to determine the census and the stage of worms. Population sizes spanned a range of 1 to over 10,000 Caenorhabditis individuals in one fruit, flower or stem. Some populations of intermediate size contained all non-dauer stages expected from a proliferating population. Samples with high census always contained some L2d and dauers; some did not contain L3 and L4s at all, indicative of a population entering the dauer stage at the end of a proliferative stage.
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[
International Worm Meeting,
2013]
We are interested in understanding how environmental variation modulates developmental processes and resulting phenotypic variation. We focus on the C. elegans germline, a molecularly well-tractable system that shows a high degree of environmental sensitivity. The basic germline organization is conserved among Caenorhabditis species, however reproductive schedules and fecundity are very variable between species and isolates. Yet, it remains unclear how evolution of such life history traits is explained by changes in germline properties, e.g. sperm number, oocyte quality or the sperm-oocyte switch. We aim to link environmental and evolutionary variability of the Caenorhabditis germline to better understand genotype-by-environment interactions for developmental traits underlying fitness-related characters. Our first aim was to characterize how germline proliferation and differentiation as well as fecundity are modified in animals exposed to experimental conditions mimicking ecologically relevant environments. Here we present how one specific environment -high temperature exposure (27 deg C)- affects germline and reproductive properties. In addition to previously reported deleterious effects on sperm function, we found that high temperature significantly impacts germline proliferation and differentiation, apparently through modifications of Distal Tip Cell morphology and signalling through the Delta/Notch pathway. Our second aim was to quantify genotype-by-environment interactions by analysing how isolates of the three hermaphroditic species (C. elegans, C. briggsae and C. sp. 11) differ in germline plasticity across different environments. We uncover significant genotype-by-environment interactions for germline and fecundity in response to high temperature. More specifically, we find that the thermal limits of reproduction are species-specific and may be associated with particular germline defects. We are now using this experimental paradigm to explore how environmental factors impact the Caenorhabditis reproductive system and how germline integrity is maintained in variable environments.
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[
International Worm Meeting,
2021]
A fundamental goal of evolutionary biology is to understand the divergence of genetic and developmental mechanisms underlying the great diversity of organismal life histories. Comparison among distant taxa cannot easily address this issue due to high complexity; however, it is possible to investigate the quantitative genetic architecture underlying life history and developmental phenotypes within species. Here we show that C. elegans wild isolates from around the globe display extensive size variation of the germline mitotic zone (MZ), indicative of variation in germline proliferative activity. To learn more about the molecular genetic differences underlying such natural variation in the C. elegans germ stem cell niche, we focused on two wild isolates, JU1200 and JU751, with strong differences in the size of total germline and mitotic compartment. Quantification of MZ size in ~70 F2 recombinant inbred lines (RILs) derived from the parental cross between these two isolates and subsequent quantitative trait locus (QTL) mapping identified a large-effect QTL on chromosome II (~7.25 Mb) that acts additively with a QTL on chromosome V (~2.6Mb). Together, these two loci explain 32% of the observed phenotypic variation, suggesting that there are other small effect loci in the genetic background important in determining MZ size. While efforts to identify causal variants in the chromosome II QTL region were unsuccessful, we were able to investigate a promising candidate variant within the chromosome V QTL. Through CRISPR-Cas9 gene editing, we demonstrate that this variation, a 150bp deletion upstream of
lag-2 containing a BHLH-2 binding site, strongly affects MZ size in the JU1200 genetic background but not in the JU751 background. We find a similar but weaker interaction between this locus and the chromosome II QTL. Finally, we demonstrate surprisingly complex three-way interactions between the genetic background, the chromosome II QTL, and the 150 bp deletion upstream of
lag-2. Together, our results identify a specific molecular variant affecting a cellular process that ultimately regulates reproductive potential, and they shed light on the complex, quantitative genetic architecture underlying natural variation in a germ stem cell niche.