Meeting Abstract

S9.4  Wednesday, Jan. 6  The invention of the pilidium larva in an otherwise perfectly good spiralian phylum Nemertea. MASLAKOVA, S.A.; Oregon Institute of Marine Biology, U. of Oregon svetlana@uoregon.edu

The nemertean pilidium is one of the most unusual larval forms among the spiralians. It is restricted to the monophyletic clade Pilidiophora. All other nemerteans have juvenile-like planuliform larvae, some of which possess a vestigial prototroch, which relates them to the classical spiralian trochophore. The invention of the pilidium larva is associated with several major developmental innovations, starting with the formation of an extensive gel-filled blastocoel which supports the delicate larval frame. Meanwhile, the trochoblast cell lineage must escape cleavage-arrest, forming a growing, elaborated ciliary band instead of a prototroch. It appears that the hoplonemerteans and at least some palaeonemerteans also evaded the typical spiralian fate of the trochoblast lineage, possibly setting the scene for the origin of the pilidium larva. The most remarkable innovation of the pilidium is the development of the juvenile worm inside the larva from a series of isolated rudiments, called imaginal discs. The paired cephalic, cerebral organ and trunk discs originate as invaginations of larval epidermis and subsequently grow and fuse to form the juvenile. I review recent data on development of palaeo- and hoplonemerteans and speculate about the possible homology between pilidial imaginal discs and invaginated rudiments observed in planuliform larvae. Comparative analysis of cell lineage and rudiment-specific genetic markers will help to test this hypothesis. I describe preliminary results from a global search of pilidial developmental transcriptomes for differentially expressed genes that will identify such markers.