The Developmental Basis for Evolutionary Changes in the Larval Biology of Brachiopods

University Of Texas At Austin, Austin TX

Investigators

Abstract

Invertebrate animals in most extant phyla have a complex life history with larval and adult phases that are separated by a metamoprhic transition. It is thought that a great deal of evolutionary change has occurred in the context of this transition. Phylogenetic studies on brachiopods indicate that an ancestor of two subphyla: Linguliformea and Craniiformea gave rise to the more derived subphylum Rhynchonelliformea. Extant members of the Linguliformea have long lived planktotrophic larvae that eat and grow in the plankton, while extant Craniiformea have short lived lecithotrophic larvae that do not eat. Both the Linguliformea and Craniiformea have an excellent fossil record that extends back to the Lower Paleozoic. By analyzing the apex of the shells of these fossils one can infer when the mantle tissue that laid down the first shell was formed. All of the families in these two subphyla initially had planktotrophic larvae. The extant members of the Rhynchonelliformea all have lecithotriophic larvae. Since this subphylum originated from an ancestor with planktotrophic larvae, lecithotrophy is a derived state. One aim of this proposal is to do a study of fossils in the Rhynchonelliformea that show traces of an embryonic or larval mantle in order to trace the evolution of larval type. The other aim of this proposal is to examine mantle formation in the context of metamorphosis in extant brachiopods. 1) A comparative survey will be done in order to find out to what extent representatives of extant subphyla use the same signal transduction mechansisms to trigger metamorphosis following exposure to an appropriate external cue. 2) Experiments will be done where larvae are reared well past the normal time of metamorphosis in order to see if continuing developmental events lead to the formation of post-larval structures such as a feeding apparatus, alimentary system and shell in the absence of metamorphosis. 3) There is a candidate gene , brachiopod "engrailed" that may play a role in mantle differentiation. A comparative study will be done to examine tissue specific expression of this gene in representatives of each subphylum.