Understanding temporal regulation of development remains an important challenge. Whereas average, species-typical timing of many developmental processes has been established, less is known about inter-individual variability and correlations in timing of specific events. Based on patterns of locomotor activity of freely moving Caenorhabditis elegans, we inferred durations of four larval stages (L1-L4) in over 100 individuals. Analysis of these data supports several conclusions. Individuals have consistently faster or slower rates of development because durations of L1 through L3 stages are positively correlated. The last larval stage, the L4, is less variable than the earlier stages and its duration is largely independent of the rate of early larval development, implying existence of two distinct larval epochs. These findings suggest that stage durations tend to scale relative to total developmental time.

Faerberg DF, Gurarie V, Ruvinsky I (2021) Inferring temporal organization of postembryonic development from high-content behavioral tracking. Dev. Biol. 475:54-64.

Key results

1. Behavioral tracking can be used to estimate transitions between larval stages of individual C. elegans to study aspects and structure of development.
2. Simple measures of activity such as average velocity and roaming fraction fail to predict developmental rates.
3. Empirical data is inconsistent with the model of uncorrelated stage durations.
4. C. elegans have a fraction of faster or slower developing worms due to consistently shorter or longer L1-L3 stages.
5. Fractional stage durations match across a number of previous publications despite varying experimental conditions.
6. Fractional stage duration CVs of L1-L3 are lower and less variable than the model of uncorrelated stage durations permits.
7. Mathematical analysis of relationship between absolute and fractional values further highlights the inconsistency with uncorrelated stage durations.

Conclusions

1. Behavioral tracking is an approach that can be used to automate generation of large datasets resolving developmental stages on an individual level. Such data is useful in studying the underlying structure of development in C. elegans and can provide insight into extremely behaving cohorts.
2. Post-embryonic development of C. elegans seems to be organized into two epochs: first comprised of fractionally scaling L1-L3 stages and second of L4. It is worthwhile to study the mechanisms imposing proportional scaling of L1-L3 (such as worm size) and it is intriguing that the two epochs may correspond to somatic and germline development, respectively.