An experimentally tractable framework for quantitative and integrative analysis of aggregation in the origin of animals

Abstract

The origin of animals from their unicellular ancestors remains a fundamental biological question. Cell aggregation, a widespread eukaryotic behaviour, has been underappreciated as a potential pathway to multicellularity. Nevertheless, aggregation is a key mechanism present in extant animals and has been described in both choanoflagellates and filastereans, the closest unicellular relatives of animals. Sebe-Pedros et al. (2013) established that aggregative, filopodial and cystic cultures of Capsaspora owczarzaki have distinct transcriptomes. Here we extend this description by following serum-induced aggregation and disaggregation over 72 h using live imaging and RNA-seq at eight sampling times. Comparison with reanalysed 2013 data reveals changing correspondence to the established life-stage programs. The nominal 3-min sample shows an immediate RNA-abundance response distinct from the aggregative-versus-filopodial contrast. Aggregative-associated genes are enriched from 12 h onwards, while cystic-associated genes become enriched at 60-72 h alongside continued aggregative enrichment. Homologues of animal adhesion and signalling genes show dynamic expression. In parallel, we developed a novel quantitative mathematical model, calibrated with experimental data, that captures the full aggregation cycle, and identified a nonlinear threshold-like adhesion response to fetal bovine serum (chemical inducer) coupled to its local depletion, sufficient to explain and predict aggregate formation, growth, and disassembly. Together, the transcriptomic data and the mathematical model resolve temporal features missed by endpoint sampling in the previous study and provide an integrated experimental-theoretical framework to interrogate how facultative aggregation could have influenced the emergence of animal multicellularity.