Developmental Critical Windows and Sensitive Periods as Three-Dimensional Constructs in Time and Space

Abstract

A critical window (sensitive period) represents a period during development when an organism's phenotype is responsive to intrinsic or extrinsic (environmental) factors. Such windows represent a form of developmental phenotypic plasticity and result from the interaction between genotype and environment. Critical windows have typically been defined as comprising discrete periods in development with a distinct starting time and end time, as identified by experiments following an on and an off protocol. Yet in reality, periods of responsiveness during development are likely more ambiguous that depicted. Our goal is to extend the concept of the developmental critical window by introducing a three-dimensional construct in which time during development, dose of the stressor applied, and the resultant phenotypic modification can be utilized to more realistically define a critical window. Using the example of survival of the brine shrimp (Artemia franciscana) during exposure to different salinity levels during development, we illustrate that it is not just stressor dose or exposure time but the interaction of these two factors that results in the measured phenotypic change, which itself may vary within a critical window. We additionally discuss a systems approach to critical windows, in which the components of a developing system--whether they be molecular, physiological, or morphological--may show differing responses with respect to time and dose. Thus, the plasticity of each component may contribute to a broader overall system response.

Salt sensitivity of the morphometry of Artemia franciscana during development: A demonstration of 3-D critical windows

“…Survival was previously shown to be significantly reduced following exposure to 40 and 50 ppt during days 1-6 and 7-9 (see fig. 7 in Burggren and Mueller, 2015), and should be considered when examining the salinity effects on morphometry during these exposure windows.…”

“…Of course, the more data available to construct a time-exposure-response 3D profile, the more 'smooth' are the contours that represent the phenotypic modifications. Burggren and Mueller (2015), in a largely theoretical treatment of the concept of the critical window as a 3D construct, provided smooth contours relating the timeexposure-magnitude responses of developing animals. The 3D contours formed in the present study are somewhat 'angular', even with 21 different treatments involving five salinities and four different potential critical windows.…”

“…The morphometric characteristics presented in this study must be examined with the caveat that there are differences in survival across the salinity treatments (Table 1; Burggren and Mueller, 2015). These survival differences and the salinity treatments may both contribute to the altered morphometric characteristics observed.…”

“…Survival immediately after each exposure window is presented in Table 1 and in fig. 7 in Burggren and Mueller (2015).…”

“…For example, during development, periods may exist when the emerging phenotype of an animal is particularly plastic or susceptible to an environmental stressor -these periods are often termed 'critical windows' or 'sensitive periods' (Pinkerton and Joad, 2000;Rice and Barone, 2000;van Aerle et al, 2002;Chan and Burggren, 2005;Hogan et al, 2008;Burggren and Reyna, 2011;Burggren et al, 2014;Burggren and Mueller, 2015). Across a range of animal taxa, from invertebrates to mammals, exposure to environmental, chemical or pharmacological stressors at certain time points of development is utilized to detect and understand critical windows during development (Green et al, 1986;Dzialowski et al, 2002;Maack and Segner, 2004;Chan and Burggren, 2005;Liu and Wong-Riley, 2010;Yuan et al, 2011;Tate et al, 2015).…”