Meeting Abstract

60.1  Sunday, Jan. 5 13:30  How the pilidium larva escapes an ancient constraint MASLAKOVA, S.A.*; VON DASSOW, G.; Oregon Institute of Marine Biology, University of Oregon svetlana@uoregon.edu

The pilidium is a planktotrophic larva with unique morphology, development and metamorphosis, found only in pilidiophoran nemerteans (an order-level taxon with ~ 450 species). Nemerteans, a.k.a. ribbon worms, share a conservative developmental program called spiral cleavage with several other phyla of marine invertebrates collectively referred to as spiralians (including the familiar annelids and mollusks). Primary trochoblasts are a set of four embryonic cells at the 16-cell stage – 1q2 in the scheme of spiral cleavage. In a typical spiralian these cells undergo one or two cell divisions, become multiciliated and contribute to the prototroch – the primary larval ciliated band, used for swimming, and, sometimes, feeding. But not in pilidiophorans. By labeling 1q2 cells in embryos of the pilidiophoran Micrura alaskensis and tracking their fate in larvae, we show that, unlike in typical spiral cleavage, primary trochoblasts in a pilidiophoran embryo continue to divide far more than just once or twice. Not only that, but their progeny contribute to the main growth zones of the pilidium, the axils, found in the recesses between the larval lobes and lappets. Earlier we demonstrated that the axils give rise to both the growing larval body (including the primary ciliated band) and the imaginal discs, which give rise to the juvenile body inside the pilidium. To sum it up, in a typical spiralian, the primary trochoblasts are quintessentially larval – they form the larval ciliated band, which is lost during metamorphosis, and they have no future in adulthood. In the pilidium larva, quite the opposite, the trochoblasts escape the constraint on cell division imposed by the multiciliated fate, and, instead, allow the larva to grow, and, also form the adult body.