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Hormones are key chemical messengers that regulate major physiological and developmental processes. They are extremely diverse and encompass peptides or small proteins encoded by genes as well as small molecules (e.g., steroids) that are the products of complex metabolic pathways. Their receptors are also diverse from membrane receptors acting via second messengers to nuclear hormone receptors that are ligand-activated transcription factors regulating directly target gene expression. If the mode of action of hormones is better and better known, the question of their origin and evolution is still enshrined in mystery. It is however possible now to draw some common principles underlying their evolution. At the heart of this question is the hormone/receptor couple that controls the hormonal response and whose evolution will be different depending on the nature of the ligand. Whatever this nature, the couple did evolve and recent results suggest that even small molecules such as steroids or thyroid hormones can change during evolution. Another important layer of evolutionary changes are all the mechanisms that exquisitely control hormone levels in peripheral organs. Lastly we can start to ask the question of the origins of some hormonal systems that are much more ancient than anticipated.

Publisher Summary This chapter discusses the evolution of nuclear receptors in insects. Nuclear receptors are metazoan transcription factors activated by small lipophilic ligands, such as steroid hormones, thyroid hormones, retinoids, vitamin D, or fatty acids. The availability of the ligand controls the transcriptional activity of nuclear receptors. Nuclear receptors are involved in a considerable diversity of developmental and physiological processes. Nuclear receptors are present in all metazoans, and only in metazoans. They are absent from the genome of the unicellular Monosiga brevicollis, which belongs to choanoflagellates, the sister group of animals. The set of nuclear receptors is strongly conserved in pterygote insects, ranging from 19 to 21 genes that are distributed into 22 groups. Unlike nematodes and chordates, insects did not experience any lineage-specific expansions within the nuclear receptors family.

Publisher Summary This chapter reviews the information that biologists interested in nuclear receptors can get by using a computer, including general resources and those specialized in nuclear receptors, whether for accessing data or analyzing it. Nuclear hormone receptors are one of the most abundant classes of transcriptional regulators in metazoans (animals), in which they regulate functions as diverse as reproduction, development, metabolism, or homeostasis. They function as ligand-activated transcription factors, thus, providing a direct link between signaling molecules that control these processes and transcriptional responses. The importance of nuclear receptors has prompted the accumulation of rapidly increasing data from a great diversity of fields of research—sequences, expression patterns, 3D structures, protein–protein interactions, target genes, physiological roles, mutations, etc. This has in turn highlighted the need for good bioinformatics. The present of bioinformatics for nuclear receptors is mostly composed of three specialized databases— The Nuclear Receptor Resource (NRR), NucleaRDB, and NUREBASE. which allow faster and easier access to relevant information, and more or less easy data-mining from other sources. The future lies in ‘‘postgenomic’’ bioinformatics: through the integration of experimental data into bioinformatic bases and the analysis of nonannotated sequences. This answer will in some cases be no more than a lead toward new experimental work and in others hopefully is almost definitive, as can already be done for phylogenies for example.