Edie, S. M. | Search bioRxiv

bioRxiv2024

Origin of division of labor is decoupled from polymorphism in colonial animals

Division of labor, the specialization of sometimes phenotypically divergent cell types or group members, is often associated with ecological success in eukaryotic colonial organisms. Despite its many independent evolutionary origins, how division of labor originates remains unclear. Conventional hypotheses tend towards an "economic" model, so that biological division of labor may reflect a partitioning of pre-existing tasks and morphologies into specialized colony members. Here, we present an alternative model of the origin of division of labor, which can explain the evolution of new functions within a colony. We show that in colonies of the Cretaceous aged (103-96 Ma) fossil bryozoan of the genus Wilbertopora, the first cheilostome bryozoan to evolve polymorphism, new member morphologies were not a simple partitioning of pre-existing morphologies, but instead expanded into novel morphospace as they lost functions, specifically feeding. This expansion into new morphologies occurred primarily during two pulses of heightened morphological disparity, suggesting that the evolution of polymorphism corresponded to relaxed constraints on morphology and perhaps to the exploration of novel functions. Using a simple model of physiological connections, we show that regardless of the functionality of these new colony members, all non-feeding members could have been supported by neighboring feeding members. This suggests that the geometric constraints and physiological connectedness could be prerequisites for evolving both polymorphism and division of labor in modular organisms, and that a classic partitioning model of specialization cannot be broadly applied to biological systems. One Sentence summaryIn cheilostome bryozoans, polymorphism evolved through the loss of preexisting functions, rather than the gain of new functions, suggesting that polymorphism can evolve through drift rather than division of labor.

paleontology↗

bioRxiv2022

Specimen alignment with limited point-based homology: 3D morphometrics of disparate bivalve shells (Mollusca: Bivalvia)

1.Comparative morphology fundamentally relies on the orientation and alignment of specimens. In the era of geometric morphometrics, point-based homologies are commonly deployed to register specimens and their landmarks in a shared coordinate system. However, the number of point-based homologies commonly diminishes with increasing phylogenetic breadth. These situations invite alternative, often conflicting, approaches to alignment. The bivalve shell (Mollusca: Bivalvia) exemplifies a homologous structure with few universally homologous points--only one can be identified across the Class, the shell beak. Here, we develop an axis-based framework, grounded in the homology of shell features, to orient shells for landmark-based, comparative morphology. As the choice of homologous points for alignment can affect shape differences among specimens, so can the choice of orientation axes. Analysis of forty-five possible alignment schemes finds general conformity among the shape differences of typical equilateral shells, but the shape differences among atypical shells can change considerably, particularly those with distinctive modes of growth. Each alignment implies a hypothesis about the ecological, developmental, or evolutionary basis of morphological differences, but we recognize one alignment in particular as a continuation of the historical approaches to morphometrics of shell form: orientation via the hinge line. Beyond bivalves, this axis-based approach to aligning specimens facilitates the comparison of continuous differences in shape among many other phylogenetically broad and morphologically disparate samples.

evolutionary biology↗