Crustacean Pigmentary-Effector Hormones: Chemistry and Functions of RPCH, PDH, and Related Peptides1

Abstract

Integumental color changes and eye pigment movements in crustaceans are regulated by pigmentary-effector hormones. The identified hormones include: an octapeptide RPCH (red pigment-concentrating hormone) and several forms of octadecapeptide PDH (pigment-dispersing hormone: α-PDH, β-PDH). RPCH-related peptides (AKHs, adipokinetic hormones) and PDH-related peptides (PDFs, pigment-dispersing factors) occur in insects, and are recognized as members of AKH/RPCH and PDH/PDF peptide families. The domain for mature peptide is located between the signal peptide and precursor-related peptide in AKH/RPCH precursors, and at the C-terminal end in the PDH/PDF precursors. The precursor-related (associated) peptides in RPCH and PDH precursors in Crustacea show little or no similarity to corresponding domains of AKH and PDF precursors in insects. Although the functions of precursor-related peptides are unknown, the mature peptides are shown to serve diverse functions. RPCH's actions in crustaceans include: pigment concentration in one or more types of chromatophores, dark-adaptational screening pigment movement in distal eye pigment cells, increase of retinal sensitivity, and neuromodulation. The related AKHs largely influence metabolism in insects, although they serve additional functions. PDHs trigger pigment dispersion in chromatophores and induce light-adaptational screening pigment movements in extraretinular eye pigment cells. The related PDFs appear to serve as a transmitter of circadian signals in the regulation of biological rhythms in insects. Evolutionary relationships among the PDH/PDF peptides and directions for future research are discussed.