Meeting Abstract
S5.3 Monday, Jan. 5 Evolution of animal body plans evidence for early sophistication in sponge physiology and morphology LEYS, SP; University of Alberta sleys@ualberta.ca
One of the most dramatic transitions in the evolution of animals was becoming multicellular. Multicellularity required coordination among cells for feeding, growth and reproduction, which may initially have been via the secretion of signaling molecules. Multicellular fungi signal to organize polarity and structure, both with local messenger molecules and with molecules acting over longer distances. Responsiveness via electrical signaling was invented multiple times in protists (e.g., the bright flash emitted by Noticula or the avoidance response of Paramecium or Stentor). But in the evolution of multicellular animals cells presumably needed to retain their identity and position relative to one another to enable a unified (whole organism) response. This entailed evolution of an epithelium that regulated the internal ionic milieu, and permitted cell-cell signaling both during development and homeostasis. This level of organization is seen in modern sponges (Porifera). An evaluation of sponge integrity shows that during embryogenesis cells are organized by signaling molecules into differentiated tissues, the outermost of which form functional epithelia and is essential to the animals physiology. Sponges are specialized suspension feeders, superbly designed to extract minute particles (bacteria and flagellates) from a three dimensional environment. Despite this apparently simple lifestyle, the sponge body plan reflects innovations that are found in other filtration and gas exchange systems such as the kidney and lung, including alveolar-like extensions of the canal system, primary cilia that likely play a role in sensing water flow, and one-way valves that prevent backflushing during contractions to expel waste. Thus studies from sponges show that physiological and morphological integration arose early during the evolution of multicellular animals.
