Predicted amphioxus endocrine pathways in which a red box identifies homologs found in the amphioxus genome. (A) Reproductive axis in which missing components include classical pituitary hormones (LH, luteinizing hormone; FSH, follicle stimulating hormone; Prl, prolactin) and their receptors. GPR54: glycoprotein receptor 54 (kisspeptin receptor); GnRH: gonadotropin-releasing hormone; E: estrogen; ER: estrogen receptor; SR: steroid receptor. (B) Thyroid metabolism showing that the thyroglobulin gene could not be identified in the amphioxus genome (hence, crossed out). CTHBP: cytosolic thyroid hormone binding protein; SIS: sodium iodide symporter; Tg: thyroglobulin; T2: diiodothyronine; T3: triiodothyronine; T4: thyroxine; TR: thyroid hormone receptor; RXR: retinoid X receptor; TRE: thyroid hormone response element. (C) Retinoid metabolism in which CRABP and AKR genes could not be identified in the amphioxus genome (hence, crossed out). ADH: alcohol dehydrogenase; AKR: aldo-keto reductase; ARAT: acylCoA:retinol acyltransferase; BCOX: beta,beta-carotene-15,15'-oxygenase; CRABP: cellular retinoic acid binding protein; CRBP: cellular retinol binding protein; CYP26: cytochrome P450–26; LRAT: lecithin:retinol acyltransferase; RALDH: retinaldehyde dehydrogenase; RDH: retinol dehydrogenase; REH: retinyl ester hydrolase; RPE65: retinal pigment epithelium-specific protein 65 kDa; RAR: retinoic acid receptor; RXR: retinoid X receptor; RARE: retinoic acid response element.
