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

## TL;DR

This study investigates how freshwater planarians regulate size and reproduction, revealing diverse strategies and the importance of generational memory, through a new additive modeling approach that captures complex biological behaviors.

## Contribution

The paper introduces a novel additive model that combines multiple size control strategies and accounts for generational memory in planarians, providing deeper biological insights.

## Key findings

- Planarians utilize different size control strategies for reproduction.
- Generation-dependent memory effects influence reproduction timing.
- The model accurately fits limited data and reveals strategy contributions.

## Abstract

Uncovering the mechanisms that control size, growth, and division rates of systems 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 acquired a unique large-scale data set on the growth and asexual reproduction of two freshwater planarian species, Dugesia japonica and Dugesia 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 also 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.