Getting back on track: exploiting canalization to uncover the mechanisms of developmental robustness

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Developing embryos can adapt dynamically to noise and variation to generate organs of incredible precision, a process termed ‘canalization’; however, the underlying robustness mechanisms are poorly understood. Technological developments, both in quantitative imaging and high precision perturbation, are now enabling targeted investigation into developmental robustness in vivo. Here, we will first distil the common design features of studies that have exploited the canalization behaviour of specific systems to interrogate developmental adaptation, to provide a general experimental framework for future investigations in other contexts. We will then highlight, using a selection of recent case studies, how this approach is revealing that tissues and embryos can fix themselves in unexpected ways.

Introduction

The ability to adapt robustly to change is one of the defining and most fascinating features of biological systems. For example, cells in developing embryos must adapt to genetic and environmental fluctuations to generate tissues and organs of remarkable precision and symmetry, in a process termed ‘canalization’ by CH Waddington [1,2]. The systems that ensure robust multicellular development likely also facilitate the regeneration of damaged tissues or help malignant tumours escape treatment. Yet we know very little about the nature of underlying adaptation mechanisms. One reason for this lack of knowledge is that the primary approach to address embryonic development, namely genetic screens, have selected for factors that give stable ‘endpoint’ phenotypes when depleted. While this ‘learning by breaking’ approach has been extremely effective at uncovering developmental mechanisms, robust systems are predicted to be ‘phenotype-less’ and are thus selected against.

Canalization approaches — where targeted perturbations are applied to shift tissues or organs from their normal development trajectory and ask if, and how, they recover — offer a promising, complementary approach to investigate robustness mechanisms directly. Here, the aim of the perturbation is not to identify factors that cause defects when depleted, rather to allow experimental investigation into how living systems ‘fix themselves’ in response to acute insult. Candidate robustness mechanisms, once identified, can be tested further using an array of techniques.

Section snippets

Defining the playing field: robustness mechanisms can be qualitative or quantitative

It is becoming clear that robustness mechanisms can be either qualitative in nature, resulting in the activation of dedicated factors and processes, or quantitative, resulting in the adaptive dialling up or down of factors and processes that are already active. Detection of the latter class of mechanism requires a priori quantitative knowledge of the normal physiological range of the specific behaviour under investigation. It is perhaps no surprise that some of the best studied developmental

Firing the starting gun: inducible perturbations allow ‘on demand’ canalization

A crucial step in identifying adaptation mechanisms is knowing when and where these mechanisms are at work. One complicating factor with chronic perturbations, such as loss of function mutations, is the difficulty in knowing when the change in activity (caused by systemic gene loss) first becomes detectable by the embryo. Thus, it is often unclear where and when adaptation mechanisms kick in. For this reason, experimental canalization approaches employ inducible ‘user-defined’ triggers that

Blink and you’ll miss it: towards quantitative, real-time measurement of system-level adaptation

Another crucial design feature for uncovering robustness mechanisms is the ability to carefully track the system’s reaction through time. Indeed, large-scale genetic screens have generally used ‘endpoint phenotype’ screening, this likely gave time for adaptation to rescue many defects by mechanisms that went unnoticed. For example, a potentially widespread ‘transcriptional adaptation’ mechanism has recently been identified in the zebrafish [35•]. Here, the recognition of premature stop-codons

The many routes back on track

We will now demonstrate, using recent case studies, how canalization approaches are currently being used to reveal different ways in which developmental adaptation can be achieved.

Robust tissue migration through perfect (or near-perfect) adaptation

Cell migration is a fundamental driving force in the positioning and shaping of tissues and organs. This is a process where active adaptation to change usually plays a central role, as cells migrating up chemotactic gradients are, by definition, exposed to constantly changing attractant levels. In order to address how migrating tissues maintain robust directionality in the presence of fluctuating guidance cues in vivo, Wong et al. [9••] directly exploited the zebrafish posterior lateral line

Adaptation through ‘catch-up’ growth

Further key developmental processes where the importance of robustness is self-evident is coordinated growth and shape control. Several elegant studies have addressed the adaptive response of growing systems to acute perturbations, such as temperature-shifts [50, 51, 52, 53]. In the case of paired organs, the origin of bilateral symmetry can be addressed by acutely interfering with the developmental trajectory of one organ, using spatially restricted genetic or mechanical interventions, and

Shifting to a new equilibrium: no organ left behind

The application of a similar experimental approach in Drosophila has revealed an alternative logic for growth coordination [56]. In this organism, retarding the growth of single imaginal discs, either by using temperature-sensitive mutations [57,58] or radiation [59], is sufficient to cause a systemic arrest of larval development. Here, the adaptation response is that the development of the entire organism is delayed until all organs reach the expected size, thus rather than one process

Developmental robustness through ‘Lockdown’

The case studies described above all address how developing systems dynamically respond to change via active adaptation mechanisms. However, another even less explored source of robustness is when differentiating tissues and organs become refractory to the same developmental pathways that once shaped them. During differentiation cells must transition to a state where they are no longer receptive to potent signals, such as chemoattractants and morphogens, as responding to fluctuations in these

Conclusions and outlook: outwitting canalization

Canalization approaches can uncover the mechanistic basis of robust developmental adaptation. It is likely that such mechanisms serve also to maintain homeostasis by buffering fluctuations that occur normally, thus increasing organismal fitness. This will become clearer through further investigation of these activities in the unperturbed state. Conversely, lessons learnt should enable us to ‘outwit canalization’ [10••] in situations where it works against us, such as during the adaptation of

Conflict of interest statement

Nothing declared.

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

We are grateful to Francesca Peri and Marios Chatzigeorgiou for useful discussions and feedback. This work was supported by Swiss National Science Foundation, grant 31003A_176235 (to D.G.).

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