Coordination of size-control, reproduction and generational memory in freshwater planarians

Abstract

Uncovering the mechanisms that control size, growth, and division rates of organisms reproducing through binary division means understanding basic principles of their life cycle. Recent work has focused on how division rates are regulated in bacteria and yeast, but this question has not yet been addressed in more complex, multicellular organisms. We have, over the course of several years, assembled a unique large-scale data set on the growth and asexual reproduction of two freshwater planarian species, Dugesia japonica and Girardia tigrina, which reproduce by transverse fission and succeeding regeneration of head and tail pieces into new planarians. We show that generation-dependent memory effects in planarian reproduction need to be taken into account to accurately capture the experimental data. To achieve this, we developed a new additive model that mixes multiple size control strategies based on planarian size, growth, and time between divisions. Our model quantifies the proportions of each strategy in the mixed dynamics, revealing the ability of the two planarian species to utilize different strategies in a coordinated manner for size control. Additionally, we found that head and tail offspring of both species employ different mechanisms to monitor and trigger their reproduction cycles. Thus, we find a diversity of strategies not only between species but between heads and tails within species. Our additive model provides two advantages over existing 2D models that fit a multivariable splitting rate function to the data for size control: firstly, it can be fit to relatively small data sets and can thus be applied to systems where available data is limited. Secondly, it enables new biological insights because it explicitly shows the contributions of different size control strategies for each offspring type.

Mechanics dictate where and how freshwater planarians fission

“…Indeed, as illustrated by the examples in Fig.2A, the fission location varies and it is seemingly impossible to predict where an individual planarian fissions (Note: the dynamics of these fission events were not recorded). We then took advantage of a unique large-scale data set on the birth and division sizes, growth curves, and time between fissions ("reproductive waiting time" (RWT)) we had accumulated on D. japonica (18) and applied statistical analysis to assay whether those data would provide further insight. We found an asymmetric double-Gaussian distribution for the waist location based on imaging n=1335 specimen within 3 days after fission (see Fig.2B and Materials and Methods).…”

Let it rip: the mechanics of self-bisection in asexual planarians determines their population reproductive strategies

“…As previously shown [10], if given the relative pharynx location of the parent planarian, one can predict whether division will occur pre-or post-pharynx. This predictability allows for the development of theoretical growth models to explain the observed reproductive behavior [8].…”

“…Previous work has shown that Dugesia japonica (J-planarians), Schmidtea mediterranea (Splanarians) and Girardia tigrina (G-planarians) allocate resources differently among their offspring (table S1, area at birth). Interestingly, area at birth -not area at division -determines offspring survival outcomes and population growth dynamics (7,8). J-planarians reproduce ("fission") approximately once every month.…”

“…This rapid reproduction comes at the cost of small tail offspring with the highest death rates (table S1). Notably, the population reproductive strategies of J-and G-planarians have previously been described using mixed models of timer, adder, and sizer strategies (8). Moreover, while population growth rates are affected by feeding frequency (5), population density and competition between species (7), these parameters do not change the underlying mode of division or the relative biomass allocation among offspring.…”

“…Moreover, planarians will abort the division process upon any type of disturbance. Therefore, many studies have focused on the population level to gain insights into planarian reproductive dynamics [4][5][6][7][8][9].…”