Quantifying Embryonic Lethality Rate in Caenorhabditis elegans

Summary

This protocol provides a simple, low-cost procedure to quantify the rate of embryonic lethality in Caenorhabditis elegans. It enables straightforward detection of embryonic abnormalities across populations without specialized equipment or worm-picking expertise. The protocol can be used to quantify the effects of genetic mutations, RNA interference-mediated gene knockdowns, and diet.

Abstract

Embryonic lethality is a cornerstone phenotype used in Caenorhabditis elegans research to characterize developmental defects arising from factors such as toxic exposures and genetic mutations. Therefore, accurately quantifying the penetrance of embryonic lethality is critical for understanding the mechanisms and impact of these perturbations on animal development. Although automated scoring platforms exist, these approaches often require substantial financial investment and specialized hardware or software. This article describes a low-tech, low-cost assay that quantifies embryonic lethality by measuring the percentage of embryos that successfully fail to hatch. Notably, this version of the assay eliminates the need for a worm pick, making it exceptionally suitable for beginners. An additional advantage of this approach is that each technical replicate assays progeny from multiple animals, avoiding the potential biases of single-worm assays. This protocol includes a discussion of critical parameters, such as the minimum number of progeny required to accurately capture phenotype penetrance and the rationale for the methodology. Finally, we provide guidance on adapting the assay to assess the effects of gene knockdown via RNA interference, pharmacological treatments, or nutritional supplementation, adding a versatile and accessible tool to the toolkit of C. elegans researchers.

Introduction

Embryonic lethality is a critical phenotype for assessing the roles of genes and environmental factors in early development. C. elegans is a powerful model for studying developmental processes because of its invariant cell lineage, transparent body, and rapid life cycle¹. C. elegans embryonic lethality can be used to assess developmental integrity under chemical, pharmacological, nutritional, and genetic perturbations^2,3,4,5. Therefore, accessible and versatile methods for accurately quantifying embryonic lethality rate would benefit the research community.

Advanced high-throughput and high-content methods that study embryogenesis rely on specialized imaging systems and software, often requiring substantial financial investment^4,6,7, whereas simpler high-throughput approaches provide primarily qualitative readouts^8,9. Classical methods, by contrast, use a manual, low-cost approach that typically involves repeated transfer of individual hermaphrodites with a worm pick to assess embryonic lethality of the whole brood^2,10. However, this method can be technically challenging for beginners and has limited scalability. In addition, classical brood-size-based assays quantify embryonic viability in the progeny of a single mother, which may introduce unintentional bias.

The overall goal of this method is to provide an alternative, simple, cost-effective, and versatile approach for quantifying embryonic viability at medium throughput. This method measures rates of embryonic lethality in the progeny of multiple animals per replicate, does not require the use of a worm pick, and avoids any specialized or costly equipment. The study also shows how the method can be applied to common perturbations, such as RNA interference and drug or nutrient testing.

Protocol

1. C. elegans maintenance

NOTE: Detailed guidance on C. elegans husbandry can be found in WormBook¹¹.

1. Prepare maintenance Nematode Growth Medium (NGM) plates.
   1. To make 1 L of NGM, combine 3 g NaCl, 2.5 g bacteriological peptone, and 17 g agar in a 2 L bottle. Add 975 mL of distilled water and a stir bar, then autoclave for 20 min at 121 °C.
   2. Allow the medium to cool to 55 °C in a water bath. Place the bottle on a stir plate and, using aseptic technique, add 25 mL of 1 M potassium phosphate buffer and 1 mL each of the following: 1 M MgSO₄, 1 M CaCl₂, and 5 mg/mL cholesterol. Pour approximately 25 mL into each 100 mm plate.
2. Prepare overnight E. coli OP50 cultures.
   1. Using aseptic technique, fill a 1 L Erlenmeyer flask to one-fifth of its volume (200 mL) with LB medium. Inoculate the medium with a single colony of E. coli OP50 using a sterile inoculating loop. Incubate overnight at 37 °C on an orbital shaker at 200 rpm.
3. Seed NGM maintenance plates with E. coli OP50 (Figure 1A).
   1. Transfer the E. coli OP50 culture into autoclaved centrifuge bottles and centrifuge for 30 min at 3,500 x g at room temperature. Resuspend the bacterial pellet in one-twentieth of the original volume. Use 1–1.5 mL of concentrated culture to seed each 100 mm NGM plate.
   2. Incubate the plates overnight at room temperature to allow the bacteria to grow.
      NOTE: These maintenance plates have a high density of bacteria to support up to 4,000 worms per plate, which is desirable when performing a large number of assays or working with a slow-growing strain of C. elegans. Seeded maintenance plates can be stored in the refrigerator at 4 °C for up to 1 month.
4. Establish a primary C. elegans culture (Figure 1A).
   1. Use a sterile pipette tip to cut out a small chunk of agar containing ~1,000 worms from a starved maintenance plate. Place the agar chunk onto a seeded NGM plate.
   2. Place the new plate in a plastic container and incubate at 20 °C for 3 days, until worms are gravid and laying eggs. Ensure that the bacterial lawn is sufficient, and that the worm population does not starve. At this point, the worm population is ready for the experiment.

2. Preparation of experimental media and bacterial cultures

NOTE: At this step, proceed with either Method 1 to test the effect of RNAi-mediated gene knockdown or Method 2 to investigate dietary supplements.

1. Method 1: Prepare experimental NGM plates for RNAi by feeding.
   1. Prepare 1 L of NGM as described in step 1.1, then, using aseptic technique, add 1 mL of 1 M IPTG and 1 mL of 50 mg/mL ampicillin. Pour approximately 5 mL into each 35 mm plate.
   2. Inoculate the desired number of E. coli HT115 RNAi feeding clones carrying L4440 plasmids targeting C. elegans genes of interest, together with the empty vector control. Pick a single colony with an inoculating loop and transfer it into 1.5 mL of LB containing 50 μg/mL ampicillin. Incubate for 18 h at 37 °C on an orbital shaker at 200 rpm.
   3. Seed 35 mm NGM plates containing ampicillin and IPTG with 100 µL of overnight E. coli HT115 culture, preparing two technical replicates per condition. Incubate the plates overnight at room temperature to allow bacterial growth and induce double-stranded RNA production. Store an equal number of unseeded plates at 4 °C for later use in egg transfer.
2. Method 2: Prepare experimental NGM plates with a nutritional or drug supplement.
   1. Prepare 1.5x concentrated NGM by following the instructions in step 1.1, but add 642 mL of distilled water instead of 975 mL. Keep the prepared medium in a 55 °C water bath.
   2. Prepare a water-based stock of the supplement of interest at 3x the target concentration (for example, to prepare plates containing 40 mM sodium formate, make a 120 mM stock). Calculate the required stock volume by multiplying the number of experimental plates by 1.666 mL (one-third of the plate volume). Use a syringe filter to filter-sterilize the stock, then warm it to 55 °C in a water bath.
   3. In a 50 mL conical tube, mix 2 parts 1.5x NGM with 1 part supplement stock. For no supplement control, mix 2 parts 1.5x NGM with 1 part hot sterile water. Gently mix by inverting the tube 10 times, then, using aseptic technique, quickly pour approximately 5 mL of medium into each 35 mm plate.
      NOTE: 1.5x NGM is used to compensate for medium dilution when large amounts of supplement are added. However, if the drug or metabolite stock is concentrated to 100× or more of the final concentration, regular NGM can be used instead of 1.5× NGM. When supplementing with an organic acid, using its sodium salt form will help prevent a drop in medium pH.
   4. Using aseptic technique, fill a 50 mL Erlenmeyer flask to one-fifth of its volume (10 mL) with LB medium. Inoculate with a colony of E. coli OP50 and incubate overnight at 37 °C on an orbital shaker at 200 rpm.
   5. Seed 35 mm NGM plates with 100 µL of overnight E. coli OP50 culture, preparing two technical replicates per condition. Incubate the plates overnight at room temperature. Store an equal number of unseeded plates at 4 °C for later use in egg transfer.

3. Obtaining a synchronized population of L1 larvae

1. Prepare alkaline bleach solution by combining 7 mL of sterile water, 2 mL of 5% sodium hypochlorite solution, and 1 mL of 5 N NaOH.
2. Prepare M9 buffer by dissolving 3 g of KH₂PO₄, 6 g of Na₂HPO₄, and 5 g of NaCl in 1 L of distilled water, then adjust the pH to 6.5. Autoclave for 20 min at 121 °C, allow the solution to cool, and then aseptically add 1 mL of 1 M MgSO4.
3. Wash gravid adult worms (step 1.4.2) off the plate with M9 buffer and transfer them into a 15 mL conical tube. Centrifuge at 350 x g for 2 min, aspirate the supernatant, and fill the tube with fresh M9 buffer. Repeat the wash two times.
4. Add 2 mL of M9 buffer and 2 mL of alkaline bleach solution to the worm pellet. Vortex and shake vigorously, periodically checking under the microscope until nearly all worms are dissolved.
5. Fill the tube with M9 buffer and centrifuge at 350 x g for 2 min. Remove the supernatant by inverting the tube, then wash four times with M9 buffer. Make sure the first wash is performed immediately after the worms are lysed to prevent overbleaching. After the last wash, fill the tube with 8 mL of M9 buffer and incubate on a rocker for 18–20 h, until the eggs hatch.

4. Cultivating maternal populations

1. Place five 3 µL drops of worm suspension on a glass slide and count the number of hatched L1 larvae under a microscope. Divide the total count by 15 to obtain the concentration per microliter.
2. Plate 30 L1 larvae onto each seeded 35 mm plate and incubate at 20 °C for 72 h. If the experimental conditions induce developmental delay, extend the incubation time until the worms become gravid and begin laying eggs (Figure 1B).

5. Transferring embryos to hatching plates

1. Remove the unseeded 35 mm plates from the refrigerator and allow them to come to room temperature.
2. Examine the worm plates under a microscope and confirm that they contain more than 100 embryos, with no hatched L1 larvae present.
3. Carefully fill the plate with M9 buffer. Use a vacuum line to aspirate the liquid and remove the maternal population. Ensure that no adult worms remain on the plate.
4. Fill a micropipette tip with 200 µL of M9 buffer and pipette up and down forcefully to detach embryos from the bacterial lawn. While viewing through the microscope, guide the pipette tip toward embryo-dense areas.
5. Take an unseeded plate and pipette a drop of embryo suspension into the center. Examine the droplet under the microscope. If it does not contain 100–300 embryos, add more embryo suspension until this range is reached. Make sure that no worms from the parental population are transferred with the embryos.
6. Allow the liquid to dry, then incubate the transfer plates at 20 °C for at least 24 h.
   NOTE: If L1 larvae are already present on the experimental plate, use a vacuum line instead of an M9 wash to remove the entire maternal population one by one. This prevents washing away L1 hatchlings, which do not stick to the bacterial lawn as embryos do. Proceed with progeny transfer, which will now include both embryos and L1 larvae. Ensure that the same approach is applied to all conditions.

Figure 1: Experimental timeline and schematic of key steps in the embryonic lethality assay. (A) Flow chart timeline representing the full duration of the experimental protocol, from maintenance media preparation to scoring hatching plates. Key procedures are indicated for the corresponding days of the experiment. (B) Schematic of the embryonic lethality assay (steps 4–6). On Day 4, synchronized L1 larvae are added to the experimental RNAi or supplement plates to establish a maternal population. After 72 h, on Day 7, once worms have laid eggs, the maternal population is washed away while embryos remain attached to the bacterial lawn. Embryos are then washed off the lawn with a pipette and transferred onto a hatching plate, with at least 100 embryos per plate. After 24 h of incubation, hatched larvae and unhatched embryos are counted. Please click here to view a larger version of this figure.

6. Scoring plates

1. Use a permanent marker to draw a roughly 4 mm × 4 mm grid on the lid of a 35 mm Petri dish, then place the lid under the plate. Use the grid as a guide and, under a microscope, count the number of L1 larvae and unhatched embryos with a handheld click counter.
2. Combine counts from technical replicates to ensure appropriate weighting of each replicate based on its progeny count. Specifically, sum the numbers of unhatched embryos and hatched larvae across technical replicates.
3. Calculate one embryonic lethality rate per biological replicate by dividing the number of embryos (unhatched worms) by the total number of worms and embryos, then multiplying by 100 to express the value as a percentage.
   NOTE: If a plate contains only hatched larvae or only embryos, this counting step can be skipped, as the embryonic lethality rate is 0 (0%) or 1 (100%), respectively.

7. Statistical analysis

1. Repeat the entire experiment at least three times to obtain independent biological replicates. For each biological replicate, use two technical replicate plates per condition, with a minimum of 100 embryos per technical replicate.
2. Determine whether embryonic lethality rates differ significantly among conditions using one-way ANOVA, followed by post hoc pairwise Student’s t-tests with multiple-testing correction, such as the Benjamini-Hochberg method.
   NOTE: Combining two technical replicates yields a minimum of 200 embryos per biological replicate. This satisfies the sample size requirement (~184 embryos/condition) estimated by the R base function power.prop.test to detect a 15 percent difference in lethality (e.g., 20% vs. 35%) with 90% power and a significance level of α = 0.05.

Results

The embryonic lethality assay was performed in two strains: wild-type N2 and the RNAi-hypersensitive strain rrf-3(pk1426). Using RNAi by feeding, we knocked down genes previously reported to cause embryonic lethality in high-throughput screens^9,12. Figure 2 provides a representative demonstration of assay performance. T04G9.4 knockdown produced nearly fully penetrant embryonic lethality in both rrf-3(pk1426) and N2 animals (Figure 2A,B) and therefore may serve as a reliable positive control for a strong embryonic lethality phenotype. Because embryonic lethality is measured as a proportion, it falls into the category of binomial outcomes, for which variance is expected to be maximal near the middle of the range and lower near the boundaries¹³. Accordingly, phenotypes approaching the extremes of 0% lethality, such as vector, or 100% lethality, such as T04G9.4 and K08E3.5, were less variable. In contrast, RNAi knockdowns with intermediate penetrance, such as B0272.3 and fat-6, displayed higher variance (Figure 2B). Thus, additional biological replicates would be required to more precisely estimate the true mean difference in embryonic lethality for intermediate-penetrance RNAi conditions. Overall, the results confirmed previously reported embryonic lethal phenotypes, and, importantly, revealed a broad range in the severity of embryonic lethality that is not apparent from screening results alone (Figure 2A,B).

Figure 2: Representative results of the embryonic lethality assay. (A,B) Embryonic lethality following RNAi of the indicated genes in rrf-3(pk1426) mutants (A) and wild-type N2 (B). Each dot represents a biological replicate obtained by pooling at least two corresponding technical replicates. At least four independent biological replicates were performed (n ≥ 4). Statistical significance was assessed by one-way ANOVA followed by post hoc unpaired Student’s t-tests to compare each RNAi treatment with the vector control. P-values were adjusted for multiple comparisons using the Benjamini-Hochberg (FDR) method. ***p < 0.001. Please click here to view a larger version of this figure.

Discussion

Several steps are critical for assay reproducibility and accuracy. First, the bleaching step must be carefully timed to dissolve adult worms without damaging the embryos. Another important point is transferring a sufficient number of embryos for counting: too few will reduce the precision, whereas too many will make counting unnecessarily tedious. Next, embryo transfer should be performed before the embryos begin hatching. Otherwise, hatchlings will be washed away together with adults, which can skew hatching rates toward higher embryonic lethality. Finally, to maintain consistency in embryo and L1 counts, novice experimenters must be trained to accurately identify embryos and detect all hatchlings. This seemingly straightforward task may be complicated by debris and changes in the texture of the bacterial lawn, which can occur with certain supplements.

It is also important to distinguish true embryonic lethality from developmental delay. Under standard conditions, ex utero embryonic development of wild-type embryos is completed within 9 h^14,15. However, some mutant strains and treatment conditions can markedly delay embryonic development^16,17,18,19. The proposed 24-h incubation period provides a buffer to account for modest delays in embryonic development, such as those observed in a non-N2 wild-type C. elegans strain²⁰ or in mutants deficient in yolk provisioning²¹. More severe delays can be addressed by extending the incubation time of hatching plates. For example, mutations in the clk-1 gene can prolong embryonic development to 22.8 h ± 5 h and therefore require an extended observation period for accurate documentation¹⁷. Another extreme example of delayed hatching was described for frm-1 mutant embryos, which appeared 23% unhatched at the 24-h mark and required a 72-h observation period to reveal a true embryonic lethality rate of only 3%¹⁶.

This protocol can be readily adapted to investigate diverse factors affecting embryonic development. The use of RNAi by feeding and nutritional or drug supplements was described in the protocol, though using different mutants, environmental factors, or other perturbations is also possible. This method also supports the addition of chemical supplements, enabling rescue experiments to be conducted. As previously shown, screening different dietary supplements can not only identify potential mechanisms of action, but also highlight potential interventions^22,23 , maximizing the utility of C. elegans models of disease.

Several limitations of this method should be considered. The RNAi-adapted version inherits the known limitations of feeding RNAi in C. elegans, including variable knockdown across different genes, tissues, and individuals and occasional off-target or vector-related effects^24,25,26. Maternal age may also influence assay outcomes, because progeny from young (day 1) and older (day 2 and day 3) mothers differ in several early-life phenotypes²⁷; in such cases, age-stratified or brood-based assays may be preferable. In addition, bleach-based synchronization may not be suitable for bleach-sensitive strains, for which non-bleach embryo isolation methods such as size-based filtration or sorting can be used instead²⁸.

Despite these limitations, this method introduces a practical and versatile approach to quantifying embryonic lethality. Current methods tend to fall at opposite ends of a spectrum: some sacrifice throughput but provide a complete account of brood viability per animal^2,10,29, whereas others analyze only a limited number of embryos while enabling the assay of hundreds of perturbations^12,30. The current approach fills this gap by offering a methodological middle ground. On the one hand, this assay analyzes hundreds of embryos laid by a population of worms; on the other hand, it remains scalable enough to quantify embryonic lethality across dozens of conditions.

In previous studies, we have demonstrated the utility of similar methods ^22,23, but this is the first step-by-step protocol with critical discussion. The approach is also useful in developmental biology, genetics, and toxicology for studying gene function and evaluating chemical effects on embryonic lethality. Its simplicity allows researchers with limited C. elegans or laboratory experience to learn the assay quickly and move rapidly to biological questions. Overall, this protocol provides an adaptable, low-cost, low-tech method for assessing embryonic lethality in C. elegans.

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Ampicillin sodium salt	Sigma-Aldrich	A9518
Bacteriological peptone	Fischer Scientific	DF0118-17-0
C. elegans N2	Caenorhabditis Genetics Center	N2	wild type
C. elegans NL2099	Caenorhabditis Genetics Center	NL2099	rrf-3(pk1426) II. RNAi hypersensitive
Calcium chloride, anhydrous	Sigma-Aldrich	746495
Cholesterol	Sigma-Aldrich	C3045
Conical tubes, 15 mL	VWR	60818-725
Culture tubes, 14 mL	VWR	470150-966
Diaphragm vacuum pump	Amazon	TC-100
Dissecting microscope	AmScope	SM-2
Escherichia coli HT115	Caenorhabditis Genetics Center		RNAse III-deficient, used for RNAi by feeding experiments
Escherichia coli OP50	Caenorhabditis Genetics Center
Granulated agar	Fischer Scientific	BP97445
Handheld tally counter	LabDepot	HTCP01
IPTG, dioxane free (Isopropyl-b-D-thiogalactoside)	US Biological	I8500
LB Broth (Miller)	US Biological	L1520	To make LB medium dissolve 25 g of LB powder in 1 L of distilled water. Autoclave for 20 min at 121.1 °C.
Luer Lok disposable syringe	BH Supplies	BH20LL
Magnesium sulfate	Sigma-Aldrich	M2643
Microscopy slides	Sail Brand	7101
Petri dishes, 100 mm	Tritech Research	T3371
Petri dishes, 35 mm	Tritech Research	T3501
Potassium Phosphate Buffer (PPB)			To make potassium phosphate buffer (1 M), dissolve 98 g of potassium phosphate monobasic and 48 g of potassium phosphate dibasic in 1 L of distilled water. Adjust the pH to 6. Autoclave for 20 min at 121 °C.
Potassium phosphate dibasic	Sigma-Aldrich	795496
Potassium phosphate monobasic	Sigma-Aldrich	529568
Rocker (nutating mixer)	Corning LSE	6720
Serological pipettes, 25 mL	Corning	4489
Serological pipettes, 5 mL	Celltreat	229235
Shaking incubator Infors Multitron	INFORS HT	NA
Sodium chloride	Sigma-Aldrich	S9888
Sodium hydroxide solution, 5 N	Fisher Scientific	SS256-500
Sodium hypochlorite, 5%	LabDepot	LC246302
Sodium phosphate dibasic	Sigma-Aldrich	S9763
Sorvall ST1 Plus Centrifuge	Thermo Scientific	75016030
Stir plate	ONilab	ONi28-S
Syringe filters	Semmerfeld	JQP033022QE1
Water bath	FOUR E's Scientific	WB401