The profound changes in form that occur during metamorphosis are accompanied buy equally profound changes in behavior. We have been interested in how the nervous system copes with controlling the behavior of different body forms through metamorphosis. We have focused on the metamorphosis of higher insects, which transform from a rather sedentary, feeding larval stage to an active adult devoted to dispersal and reproduction.

The insect CNS is comprised of a brain and series of ventral segmental ganglia. Each region comes from a stereotyped array of neuronal stem cells, the neuroblasts. The number and position of neuroblasts in each array are highly conserved through insect evolution, as are the basic classes of neurons that each produces. We have focused on these lineages in trying to understand the cellular processes that allowed metamorphosis to evolve from direct developing ancestors.

The frog, Eleutherodactylus coqui, develops on land from a large egg directly to a froglet with no tadpole stage. Despite this apparent morphological continuity, thyroid hormone acts on skin, muscle, and other tissues to generate the froglet. While these activities are also present in frogs with tadpoles, a novel activity of embryo-produced thyroid hormone in E. coqui is to stimulate the utilization of yolk, stored as nutritional endoderm, for growth of the froglet. This late activity raises the question as to whether early yolk utilization in amphibian embryos depends on maternal supplies of thyroid hormone, present in the egg.

Paleontological evidence is now solid that bacteria first appeared on earth more than 3.5 billion years ago and rapidly evolved to create dense communities that coated sea bottoms with a great diversity of phylotypes. The first eukaryotes joined these communities about 1.5 billion years ago, and from them, less than 900 million years ago, the first animals evolved. Thus the history of animal life on earth is one of evolution within a densely populated microbial world. It is not surprising that animals have continued to have intimate relationships with bacteria throughout their lives. We should have expected such relationships to occur during settlement and metamorphosis of marine invertebrate animals, when minute larval stages descend to a benthos densely coated by microbial films. And, it should not be surprising that we find the stimulus for attachment and metamorphosis to arise from benthic bacteria for many - probably most -- marine invertebrate animals. This distinguishes them from vertebrates and insects whose metamorphic transitions are triggered by hormonal or neuroendocrine factors whose appearance is developmentally timed. A gene set has been identified in a specific bacterial species that encodes products upon which larvae of a sessile marine polychaete, probably a coral and perhaps larvae from other phyla, depend to trigger benthic attachment and metamorphosis. Marine-invertebrate larvae must detect these signals on external receptors, probably born on cilia or flagella, and rapidly transduce them into internal processes that, in most marine invertebrates, consist principally of loss; larva-specific tissues and organs rapidly break down and disappear, liberating already present tissues and organs that make up the body of the juvenile. The massive and complex structures that must rapidly appear during metamorphosis in insects and many vertebrates present a strong contrast with the transformation of invertebrates.