LEYS, SP; University of Alberta: Sponge development: using microscopy to understand key features of the metazoan body plan.

Sponges (Porifera) are enigmatic animals whose unusual body plans make interpreting phylogenetic relationships within the group and with other basal metazoan taxa a difficult task. Although molecular approaches have offered new insights, some questions can only be answered with detailed ultrastructural and/or confocal studies on developing embryos and larvae. I have used these techniques to examine two problems in sponge evolution and development. First I confirm that glass sponges (Hexactinellida) are cellular until the 5th cleavage stage when syncytial tissues first form; thereafter all cleavages are incomplete and the major tissue component of the larvae is syncytial, as in the adult sponge. This pattern is vastly different from any cellular sponge; however, common features of spiculogenesis in hexactinellid and demosponge larvae suggest a shared ancestry. Second, I show that representative demosponges and calcareous sponges undergo gastrulation by ingression to form a multi-layered larva that is sensitive to its environment. Though ciliated larval cells do transdifferentiate to form choanocytes at metamorphosis in both Reniera (Demospongiae) and Sycon (Calcarea), these cell movements constitute the reorganization of a fully differentiated larva, much as is seen in other non-feeding invertebrate larvae. The data confirm that key elements of the metazoan body plan can be recognized during early sponge development.