Dynamic regulation of the melanocortin 4 receptor system in body weight homeostasis and reproductive maturation in fish Puberty is an important period of life with physiological changes to enable animals to reproduce. Xiphophorus fish exhibit polymorphism in body size puberty timing and reproductive tactics. These phenotypical polymorphisms are controlled by the Puberty (P) locus. In X. nigrensis and X. multilineatus the P locus encodes the melanocortin 4 receptor (Mc4r) with high genetic polymorphisms. Mc4r is a member of the melanocortin receptors belonging to class A G protein coupled receptors. The Mc4r signaling system consists of Mc4r the agonist Pomc (precursor of various MSH and of ACTH) the antagonist Agrp and accessory protein Mrap2. In humans MC4R has a role in energy homeostasis. MC4R and MRAP2 mutations are linked to human obesity but not to puberty. Mc4rs in X. nigrensis and X. multilineatus are present in three allele classes A B1 and B2 of which the X linked A alleles express functional receptors and the male specific Y linked B alleles encode defective receptors. Male body sizes are correlated with B allele type and B allele copy numbers. Late maturing large males carry B alleles in high copy number while early maturing small males carry B alleles in low copy number or only A alleles. Cell culture co expression experiments indicated that B alleles may act as dominant negative receptor mutants on A alleles. In this study the main aim was to biochemically characterize the mechanism of puberty regulation by Mc4r in X. nigrensis and X. multilineatus whether it is by Mc4r dimerization and or Mrap2 interaction with Mc4r or other mechanisms. Furthermore Mc4r in X. hellerii (another swordtail species) and medaka (a model organism phylogenetically close to Xiphophorus) were investigated to understand if the investigated mechanisms are conserved in other species. In medaka the Mc4r signaling system genes (mc4r mrap2 pomc agrp1) are expressed before hatching with agrp1 being highly upregulated during hatching and first feeding. These genes are mainly expressed in adult brain and the transcripts of mrap2 ...