Seasonal Migration of Water Boatmen (Hemiptera: Corixidae) as a Wetland-River Ecosystem Linkage

Organisms that undertake seasonal migrations can act as important ecosystem linkages by subsidizing food webs. Such transfers of material mean that even food webs which seem isolated may be closely connected. One such linkage that has largely gone unstudied is the seasonal migration of a family of aquatic insects, water boatmen, or corixids (Hemiptera: Corixidae) that fly from geographically isolated wetlands into large rivers in the Prairie Pothole Region (PPR) of North America every fall, to overwinter. This thesis provides further documentation of the phenomenon of corixid migration in the North American prairies, while also investigating the ecological importance and drivers of this movement across the landscape. First, I quantified and recorded the shifts in abundance and species composition of corixids in wetland and river ecosystems in different seasons. I found that these migrations can lead to drastically increased riverine corixid densities as high as ~3,000 individuals/m2 within areas of standing or slow-moving water, with ~500 g of corixid material entering every meter of water immediately adjacent to the banks of rivers, where landings are concentrated. This movement shifts the corixid species assemblage in rivers to one dominated by wetland-breeding species, namely Callicorixa audeni, Sigara bicoloripennis, and Sigara decoratella. Stomach content analyses of riverine fishes revealed that goldeye (Hiodon alosoides), mooneye (Hiodon tergisus), longnose sucker (Catostomus catostomus), and white sucker (Catostomus commersoni) make heavy use of this forage subsidy, with corixids occurring in 97% to 100% of these fishes and accounting for 38% to 97% of stomach contents by weight during the corixid migration period in fall. This could have implications for the productivity and overwintering survival of corixid feeding fish, with the potential for cascading effects in riverine food webs. Across the landscape, I estimated that seasonal migrations could result in ~1500 metric tons of corixids entering the North and South Saskatchewan rivers (~12,000 river km) within Saskatchewan, and ~12,000 tons of biomass moving between wetlands and rivers across the entire PPR. Next, by studying changing patterns in abundance and evidence of flight into rivers, I designated different corixid species from my study area as being predominantly migratory (62% of encountered species), acting as cyclic colonizers between wetlands and rivers, or non-migratory residents of either habitat type (27% of encountered species). This information allows for the identification of the corixid assemblage that is driving the seasonal flux between the two habitat types, and helps to fill a knowledge gap which exists on the migratory abilities of corixids at the species level. Third, I examined the use of the stable isotope ratio of sulfur, δ34S, as a tracer of corixid movement and the incorporation of these insects as a dietary subsidy by riverine fish. I found that both corixids and other invertebrate taxa originating from wetland ecosystems exhibited lower δ34S values, with wetland taxa averaging -10.5 ± 5.8‰ overall, as opposed to riverine taxa at -4.1 ± 4.1‰, allowing the use of δ34S as a tracer of insects out of wetlands. Specifically, δ34S values of invertebrates from the South Saskatchewan River (-5. 1 ± 4.1‰) were more 34S depleted than those from the North Saskatchewan River (-1.4 ± 2.8‰). In the fall season, the corixid-feeding fish species goldeye, mooneye, and longnose sucker exhibited lower δ34S values in fast-turnover liver tissue than non-corixid feeding species, shorthead redhorse, northern pike, and walleye, with mixing models indicating that ~17 to 94% of liver tissue may be derived from wetland sources during this season. However, goldeye was the only species to exhibit a significant seasonal reduction in liver δ34S values in fall compared to summer. These findings indicate that δ34S has utility in tracing flows of energy between wetland and riverine food webs. Finally, I examined the overwintering strategy of corixids that do not migrate to rivers in the fall, documenting the little understood ability of these insects to survive in wetlands that freeze solid. I found that while multiple corixid species were present in wetlands at ice-over, those embedded within the ice were almost entirely composed of two non-migratory species, Cymatia americana and Dasycorixa hybrida, of which only the former revived after thawing. These findings indicate that migratory species are likely incapable of survival within the ice, driving the need to leave shallow waters in fall. The percent of C. americana that revived after being experimentally thawed out from the ice ranged from 4% to 10% in both winters of this study. The majority of corixids were grouped together within air pockets, which could enable them to limit direct contact with the surrounding ice. Other invertebrate taxa were also found overwintering within the ice, including adults and larvae of crawling water beetles (Coleoptera: Haliplidae) and adults of predaceous diving beetles (Dytiscidae) within air pockets alongside the corixids or on their own, as well as damselfly nymphs (Odonata: Coenagrionidae), caddisfly larvae (Trichoptera: Phryganeidae, Leptoceridae), midge larvae (Diptera: Chironomidae), and snails (Gastropoda: Physidae, Planorbidae) that appeared to be encased in solid ice. Taken together, this thesis has demonstrated an extensive cross-boundary flux that occurs between spatially separated wetland and river ecosystems, highlighting a need for conservation to ensure that this connection is maintained. By examining migratory patterns, I have identified which species drive this flux, which may allow for increased protection of habitats that these corixids require. δ34S was shown to have the potential to trace insect movement and consumer use between isotopically distinct freshwater systems in the prairies. The study of corixids overwintering in ice represents a little understood survival mechanism of aquatic invertebrates in shallow wetlands, knowledge of which could help predict how the abundance of these organisms might change in the face of altered overwintering conditions due to global warming. The seasonal flights of corixids between wetlands and rivers may represent one of the world’s great insect migrations, which has largely gone unnoticed, but could have important implications for ecosystem functioning and conservation in the North American prairies

Visualization of dynein-dependent microtubule gliding at the cell cortex: implications for spindle positioning

Distinct dynein–microtubule interactions are used for asymmetric spindle-positioning tasks in the C. elegans embryo

Automated tracking and analysis of centrosomes in early Caenorhabditis elegans embryos

Motivation: The centrosome is a dynamic structure in animal cells that serves as a microtubule organizing center during mitosis and also regulates cell-cycle progression and sets polarity cues. Automated and reliable tracking of centrosomes is essential for genetic screens that study the process of centrosome assembly and maturation in the nematode Caenorhabditis elegans

Evolutionary comparisons reveal a positional switch for spindle pole oscillations in Caenorhabditis embryos.

International audienceDuring the first embryonic division in Caenorhabditis elegans, the mitotic spindle is pulled toward the posterior pole of the cell and undergoes vigorous transverse oscillations. We identified variations in spindle trajectories by analyzing the outwardly similar one-cell stage embryo of its close relative Caenorhabditis briggsae. Compared with C. elegans, C. briggsae embryos exhibit an anterior shifting of nuclei in prophase and reduced anaphase spindle oscillations. By combining physical perturbations and mutant analysis in both species, we show that differences can be explained by interspecies changes in the regulation of the cortical Gα-GPR-LIN-5 complex. However, we found that in both species (1) a conserved positional switch controls the onset of spindle oscillations, (2) GPR posterior localization may set this positional switch, and (3) the maximum amplitude of spindle oscillations is determined by the time spent in the oscillating phase. By investigating microevolution of a subcellular process, we identify new mechanisms that are instrumental to decipher spindle positioning

Microtubule-severing enzymes: From cellular functions to molecular mechanism.

Microtubule-severing enzymes generate internal breaks in microtubules. They are conserved in eukaryotes from ciliates to mammals, and their function is important in diverse cellular processes ranging from cilia biogenesis to cell division, phototropism, and neurogenesis. Their mutation leads to neurodegenerative and neurodevelopmental disorders in humans. All three known microtubule-severing enzymes, katanin, spastin, and fidgetin, are members of the meiotic subfamily of AAA ATPases that also includes VPS4, which disassembles ESCRTIII polymers. Despite their conservation and importance to cell physiology, the cellular and molecular mechanisms of action of microtubule-severing enzymes are not well understood. Here we review a subset of cellular processes that require microtubule-severing enzymes as well as recent advances in understanding their structure, biophysical mechanism, and regulation

Cytokinesis in bloodstream stage Trypanosoma brucei requires a family of katanins and spastin

Microtubule severing enzymes regulate microtubule dynamics in a wide range of organisms and are implicated in important cell cycle processes such as mitotic spindle assembly and disassembly, chromosome movement and cytokinesis. Here we explore the function of several microtubule severing enzyme homologues, the katanins (KAT80, KAT60a, KAT60b and KAT60c), spastin (SPA) and fidgetin (FID) in the bloodstream stage of the African trypanosome parasite, Trypanosoma brucei. The trypanosome cytoskeleton is microtubule based and remains assembled throughout the cell cycle, necessitating its remodelling during cytokinesis. Using RNA interference to deplete individual proteins, we show that the trypanosome katanin and spastin homologues are non-redundant and essential for bloodstream form proliferation. Further, cell cycle analysis revealed that these proteins play essential but discrete roles in cytokinesis. The KAT60 proteins each appear to be important during the early stages of cytokinesis, while downregulation of KAT80 specifically inhibited furrow ingression and SPA depletion prevented completion of abscission. In contrast, RNA interference of FID did not result in any discernible effects. We propose that the stable microtubule cytoskeleton of T. brucei necessitates the coordinated action of a family of katanins and spastin to bring about the cytoskeletal remodelling necessary to complete cell divisio

HURP permits MTOC sorting for robust meiotic spindle bipolarity, similar to extra centrosome clustering in cancer cells

Similar to clustering of extra centrosomes in cancer cells, HURP promotes microtubule stability and sorts MTOCs into distinct poles during meiosis

The Role of γ-Tubulin in Centrosomal Microtubule Organization

As part of a multi-subunit ring complex, γ-tubulin has been shown to promote microtubule nucleation both in vitro and in vivo, and the structural properties of the complex suggest that it also seals the minus ends of the polymers with a conical cap. Cells depleted of γ-tubulin, however, still display many microtubules that participate in mitotic spindle assembly, suggesting that γ-tubulin is not absolutely required for microtubule nucleation in vivo, and raising questions about the function of the minus end cap. Here, we assessed the role of γ-tubulin in centrosomal microtubule organisation using three-dimensional reconstructions of γ-tubulin-depleted C. elegans embryos. We found that microtubule minus-end capping and the PCM component SPD-5 are both essential for the proper placement of microtubules in the centrosome. Our results further suggest that γ-tubulin and SPD-5 limit microtubule polymerization within the centrosome core, and we propose a model for how abnormal microtubule organization at the centrosome could indirectly affect centriole structure and daughter centriole replication

Self-Organization of Anastral Spindles by Synergy of Dynamic Instability, Autocatalytic Microtubule Production, and a Spatial Signaling Gradient

Assembly of the mitotic spindle is a classic example of macromolecular self-organization. During spindle assembly, microtubules (MTs) accumulate around chromatin. In centrosomal spindles, centrosomes at the spindle poles are the dominating source of MT production. However, many systems assemble anastral spindles, i.e., spindles without centrosomes at the poles. How anastral spindles produce and maintain a high concentration of MTs in the absence of centrosome-catalyzed MT production is unknown. With a combined biochemistry-computer simulation approach, we show that the concerted activity of three components can efficiently concentrate microtubules (MTs) at chromatin: (1) an external stimulus in form of a RanGTP gradient centered on chromatin, (2) a feed-back loop where MTs induce production of new MTs, and (3) continuous re-organization of MT structures by dynamic instability. The mechanism proposed here can generate and maintain a dissipative MT super-structure within a RanGTP gradient

LET-99 inhibits lateral posterior pulling forces during asymmetric spindle elongation in C. elegans embryos

GPR-1/2 (regulators of Gα signaling necessary for asymmetric cell division) receives a positional cue from Let-99, resulting in its appropriate distribution around the posterior cortex