Rebecca A. Green

Many proteins required for embryogenesis are expressed in multiple tissues and across developmental time, often with tissue- and stage-specific roles. To resolve where and when proteins function, we developed a tissue-targeted auxin-inducible degradation toolkit for C. elegans embryogenesis. We generated a plasmid set compatible with single-copy insertion or high-copy extrachromosomal array generation that employs tissue-specific promoters (active in the germline, intestine, muscle, pharynx, neuroblasts, neurons, sensory neurons, epidermis, and pan soma) to drive an operon expressing an optimized auxin-dependent plant F-box factor TIR1 and, in most cases, a fluorescent histone. Using a ubiquitous nuclear envelope marker as a test substrate, we identified single-copy insertions and arrays that exhibit efficient auxin-dependent, tissue-specific target degradation. We also establish a workflow that ensures all functionally important isoforms of a target are tagged and that degradation reproduces the loss-of-function phenotype prior to tissue-specific analysis. Applying the toolkit to the broadly expressed Rho kinase LET-502, we find that degradation in the epidermis, but not in other embryonic tissues, disrupts elongation. Unexpectedly, temporal analysis revealed that LET-502 is required for late as well as early elongation. SUMMARY STATEMENT: A toolkit for auxin-mediated tissue- and stage-specific degradation of essential targets during C. elegans embryogenesis reveals that the contractility regulator LET-502 is required in the epidermis for late as well as early elongation.

SUMMARY: Intrinsic reproductive isolation occurs when genetic divergence between populations disrupts hybrid development, preventing gene flow and enforcing speciation. ^1–4 Over the past two decades, researchers have identified molecular mechanisms underlying a few dozen cases of hybrid incompatibility in animals. ⁵ Much of this work has focused on mismatches in zygotic gene regulation, ^6–11 but other mechanisms have also emerged, including symbiont-driven incompatibilities, ¹² nucleoporin mismatches affecting nuclear-cytoplasmic transport, ¹³ and divergence in centromeric or heterochromatic regions and their regulatory proteins which can lead to the inability of the oocyte cytoplasm to segregate sperm-derived chromosomes. ^14–19 Since studies to date have focused on a limited number of species, uncovering mechanisms across diverse taxa will be important to understanding broader patterns of hybrid incompatibility. Here, we investigate the mechanistic basis of hybrid incompatibility in Caenorhabditis nematodes by leveraging the ability of C. brenneri females to produce embryos after mating with males from several other species. We find that incompatibilities emerge between fertilization and the onset of zygotic transcription, which begins at the 4-cell stage. ^20–23 In Caenorhabditis embryos, as in many animals, ^24,25 sperm deliver chromatin and centrioles into the oocyte. ^26–29 After remaining quiescent during oocyte meiosis, the sperm chromatin acquires a nuclear envelope, and centrioles initiate centrosome formation. ^30–32 Centrosomes remain tethered to the sperm pronucleus, which positions them near the cortex to establish anterior-posterior polarity. ^33,34 We identify two key processes that are destabilized in hybrids: (1) the ability of oocytes to control sperm-derived pronuclear expansion, and (2) successful polar body formation. When sperm pronuclear expansion is delayed, centrosomes detach, which leads to defects in polarity establishment. Hybrid embryos typically experience one or more stochastic failures of early developmental events that accumulate and eventually kill them.

Goldenhar Syndrome is a rare congenital disorder characterized by hemifacial microsomia. Although select mutations have been mapped for this disorder, the genetic etiologies in the majority of cases remain unknown. A recent clinical report of a Goldenhar Syndrome patient identified a homozygous missense mutation in FRK , a gene associated with various types of cancer. In this work, we precisely modeled the disease-associated missense mutation in the C. elegans FRK ortholog src-2 , using CRISPR/Cas9 gene editing, and investigated the physiological role of this mutation and the src-2 gene. In addition, we generated a conserved variant in src-1 ( FYN ortholog) to assess the functional redundancy of the conserved variant. The putative pathogenic variants src-1 (Val190Ile) or src-2 (Val170Ile) caused only subtle phenotypes, suggesting that these mutations alone are not sufficient to explain the facial deformities observed in the Goldenhar Syndrome patient. However, the src-2 (Val170Ile) mutant exhibited reduced brood size and moderately enhanced embryonic developmental phenotypes, including epidermal and neuronal patterning defects, in the src-1 (RNAi) condition, indicating that the src-2 (Val170Ile) locus could play a supportive role during developmental processes. Overall, however, these studies showed that src-1 /FYN is essential for regulating embryogenesis and morphogenesis, while src-2 /FRK is largely dispensable for normal embryonic development, suggesting FYN , not FRK , is the dominant non-receptor protein kinase during embryonic development in C. elegans .

C. elegans is the premier system for the systematic analysis of cell fate specification and morphogenetic events during embryonic development. One challenge is that embryogenesis dynamically unfolds over a period of about 13 h; this half day-long timescale has constrained the scope of experiments by limiting the number of embryos that can be imaged. Here, we describe a semi-high-throughput protocol that allows for the simultaneous 3D time-lapse imaging of development in 80–100 embryos at moderate time resolution, from up to 14 different conditions, in a single overnight run. The protocol is straightforward and can be implemented by any laboratory with access to a microscope with point visiting capacity. The utility of this protocol is demonstrated by using it to image two custom-built strains expressing fluorescent markers optimized to visualize key aspects of germ-layer specification and morphogenesis. To analyze the data, a custom program that crops individual embryos out of a broader field of view in all channels, z-steps, and timepoints and saves the sequences for each embryo into a separate tiff stack was built. The program, which includes a user-friendly graphical user interface (GUI), streamlines data processing by isolating, pre-processing, and uniformly orienting individual embryos in preparation for visualization or automated analysis. Also supplied is an ImageJ macro that compiles individual embryo data into a multi-panel file that displays maximum intensity fluorescence projection and brightfield images for each embryo at each time point. The protocols and tools described herein were validated by using them to characterize embryonic development following knock-down of 40 previously described developmental genes; this analysis visualized previously annotated developmental phenotypes and revealed new ones. In summary, this work details a semi-high-throughput imaging method coupled with a cropping program and ImageJ visualization tool that, when combined with strains expressing informative fluorescent markers, greatly accelerates experiments to analyze embryonic development.

The Caenorhabditis elegans embryo is an important model for analyzing mechanisms of cell fate specification and tissue morphogenesis. Sophisticated lineage-tracing approaches for analyzing embryogenesis have been developed but are labor intensive and do not naturally integrate morphogenetic readouts. To enable the rapid classification of developmental phenotypes, we developed a high-content method that employs two custom strains: a Germ Layer strain that expresses nuclear markers in the ectoderm, mesoderm and endoderm/pharynx; and a Morphogenesis strain that expresses markers labeling epidermal cell junctions and the neuronal cell surface. We describe a procedure that allows simultaneous live imaging of development in 80-100 embryos and provide a custom program that generates cropped, oriented image stacks of individual embryos to facilitate analysis. We demonstrate the utility of our method by perturbing 40 previously characterized developmental genes in variants of the two strains containing RNAi-sensitizing mutations. The resulting datasets yielded distinct, reproducible signature phenotypes for a broad spectrum of genes that are involved in cell fate specification and morphogenesis. In addition, our analysis provides new in vivo evidence for MBK-2 function in mesoderm fate specification and LET-381 function in elongation.

Throughout metazoans, germ cells undergo incomplete cytokinesis to form syncytia connected by intercellular bridges. Gamete formation ultimately requires bridge closure, yet how bridges are reactivated to close is not known. The most conserved bridge component is centralspindlin, a complex of the Rho family GTPase-activating protein (GAP) CYK-4/MgcRacGAP and the microtubule motor ZEN-4/kinesin-6. Here, we show that oocyte production by the syncytial Caenorhabditis elegans germline requires CYK-4 but not ZEN-4, which contrasts with cytokinesis, where both are essential. Longitudinal imaging after conditional inactivation revealed that CYK-4 activity is important for oocyte cellularization, but not for the cytokinesis-like events that generate syncytial compartments. CYK-4’s lipid-binding C1 domain and the GTPase-binding interface of its GAP domain were both required to target CYK-4 to intercellular bridges and to cellularize oocytes. These results suggest that the conserved C1-GAP region of CYK-4 constitutes a targeting module required for closure of intercellular bridges in germline syncytia.

To ensure timely cytokinesis, the equatorial actomyosin contractile ring constricts at a relatively constant rate despite its progressively decreasing size. Thus, the per-unit-length constriction rate increases as ring perimeter decreases. To understand this acceleration, we monitored cortical surface and ring component dynamics during the first cytokinesis of the Caenorhabditis elegans embryo. We found that, per unit length, the amount of ring components (myosin, anillin) and the constriction rate increase with parallel exponential kinetics. Quantitative analysis of cortical flow indicated that the cortex within the ring is compressed along the axis perpendicular to the ring, and the per-unit-length rate of cortical compression increases during constriction in proportion to ring myosin. We propose that positive feedback between ring myosin and compression-driven flow of cortex into the ring drives an exponential increase in the per-unit-length amount of ring myosin to maintain a high ring constriction rate and support this proposal with an analytical mathematical model.