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[
International Worm Meeting,
2019]
Cytokinesis occurs after mitosis, when the two daughter cells are physically separated. The last step of cytokinesis, termed abscission, involves numerous conserved regulators that coordinate all processes enabling resolution of the transient intercellular bridge, including midbody processing, cytoskeletal rearrangements membrane trafficking and scission. Certain cell types however undergo incomplete cytokinesis, generating cells that remain connected by a stable intercellular bridge, thus forming a syncytium. Stable intercellular bridges are a conserved feature during metazoan gametogenesis. In dividing mouse spermatocytes for instance, the protein TEX14 inhibits the recruitment of ESCRT-I regulators at the midbody ring, leading to a block of abscission and stabilization of the intercellular bridge. TEX14 is only found in vertebrates however, suggesting that this molecular mechanism of cytokinesis incompletion and syncytium formation is not conserved in most metazoans. The C. elegans syncytial gonad represents a relevant model to study how differential regulation of cytokinesis impacts syncytial organization. In C. elegans, all germ cells originate from the unique embryonic blastomere, P4, which divides incompletely to form two primordial germ cells that remain stably interconnected, Z2 and Z3. We hypothesize that an active mechanism prevents the completion of P4 cytokinesis during embryogenesis, and that this is required for proper syncytiogenesis of the larval and adult gonad. To define this mechanism and identify the step where abscission is blocked, we are using live-imaging approaches to monitor abscission regulators and compare their dynamics in P4 to those in neighboring somatic cells, which undergo complete cytokinesis. We found that the kinetics of Aurora B and midbody microtubules processing are identical in P4 and somatic cells. This indicates that the block of abscission in P4 is unlikely to be due to a defect in midbody maturation. We also found that the ESCRT-I regulator TSG-101 is loaded at the midbody ring with comparable timing in P4 and somatic cells, indicating that the mechanism of abscission block is different from that described in mouse spermatocytes. We are currently monitoring the dynamics of regulators that act at later steps in the process. Our work will help to define the mechanisms that enable differential cytokinetic regulation during gonad development.
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[
International Worm Meeting,
2011]
Pristionchus pacificus has been established as a model system in evolutionary developmental biology (evo-devo) and for comparison to C. elegans. Recent studies expanded the evo-devo work to ecology and population genetics. P. pacificus and related species have a well-defined association with scarab beetles: Sampling of more than 20,000 scarab beetles from around the world resulted in the isolation of more than 8,000 Pristionchus isogenic female lines. Mating experiments and molecular sequence analysis identified 26 species to date. We provide a molecular phylogenetic framework based on nearly 11,000 characters. Studies on La Reunion, an Island with a unique P. pacificus biodiversity hotspot, provide a case study to link population genetics with ecology and evo-devo. We obtained more than 400 wild isolates of P. pacificus and carried out microsatellite analysis of 21 markers. These studies indicate four major biogeographic clades of P. pacificus. La Reunion represents the only geographic area with P. pacificus strains being present in all four clades. This finding is best explained by P. pacificus invading the island independently with different beetle vectors, such as Oryctes, Amneidus and Maladera (Herrmann et al., 2010). We assessed the molecular phylogeny of P. pacificus and used mutation accumulation line approaches to provide divergence time estimates. Mutation rates were assessed first, by mitochondrial DNA analysis and second, from representative microsatellite markers. Mitochondrial DNA analysis suggests a minimal divergence time for P. pacificus of 105 to 106 generations (Molnar et al., 2011). Similarly, microsatellite markers are used to robustly provide minimal divergence time estimates for closely related strains from La Reunion. This includes a first attempt for a serious analysis of the population size of P. pacificus.
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[
International Worm Meeting,
2007]
The dauer stage is important in the ecology of Rhabditida nematodes. Therefore, evolutionary alterations in the regulation of dauer formation should play crucial roles in the adaptive radiation of nematodes and is an attractive subject in evolutionary developmental biology. For example, C. elegans is found primarily in compost heaps predominantly in the dauer stage (Barriere and Felix, 2005). In contrast, field studies indicate that Pristionchus nematodes live in close association with scarab beetles. Pristionchus dauer larvae rest on beetles, wait for their death to feed on the developing microbes on the carcass. This phenomenon is known as necromeny. The satellite species P. pacificus was shown to live on the Oriental beetle Exomala orientalis (Herrmann et al., 2007). These observations lead to the expectation that Pristionchus dauer larvae have evolutionary novelties that enable them to associate with beetles. To explore how the dauer stage is evolutionarily modified, we are investigating the dauer stage of P. pacificus. P. pacificus enters into the dauer stage after the second molt, if it is densely grown and starved. Dauers of P. pacificus have a thinner body shape compared with other stages and a closed mouth suggesting they are non-feeding. When put on a projection, they start the nictation behavior standing on the tip of the tail and wave their body. Apart from these features common to C. elegans, P. pacificus dauers have several features that are not common to C. elegans, such as prominent amphids, lack of alae, and the secretion of an oily substance on the body surface. The control of the entry into and the exit from the dauer stage is of utmost ecological importance because erroneous decisions in the dauer development lead to either less progeny or survival. Therefore, the control of dauer entry/exit might be subject to substantial evolutionary modifications in response to the environment each species faces. To reveal if there are differences in the control of dauer formation between P. pacificus and C. elegans, we first tested if P. pacificus dauer formation is controlled by a pheromone. Extracts prepared from the supernatant of P. pacificus liquid culture strongly induced dauer formation in P. pacificus. By contrast, C. elegans pheromone did not induce dauer formation in P. pacificus. We also mutagenized P. pacificus and screened for dauer formation abnormal mutants. In a pilot screen, we could obtain several lines of dauer constitutive and dauer defective mutants demonstrating that dauer formation of P. pacificus is amenable to genetic analyses. Taken together, these findings suggest that P. pacificus uses a dauer pheromone which is distinct from the C. elegans dauer pheromone.