Southern platyfish

Xiphophorus maculatus is a small (4–6 cm), colorful livebearer (family Poeciliidae) native to freshwater streams and ponds of the Atlantic slope of Mexico, Guatemala, and Honduras. It has a compact, deep body, a rounded caudal fin (lacking the "sword" extension seen in swordtails), and exhibits tremendous natural color polymorphism—wild populations display multiple genetically determined pigment patterns including spotted, striped, and various melanin-based dorsal and caudal markings. In the aquarium trade, extensive hybridization with X. hellerii and selective breeding have produced a vast array of color varieties (red, blue, gold, calico, tuxedo, etc.) commonly sold as "platies." Males possess a gonopodium and females are ovoviviparous, producing live fry every 4–6 weeks. X. maculatus is a key model organism in cancer biology: certain interspecific hybrids between X. maculatus and other Xiphophorus species develop spontaneous melanomas due to the interaction of a sex-linked oncogene (Xmrk, a mutant EGF receptor) with the loss of a tumor suppressor from the other species. The Xiphophorus melanoma system has been studied since the 1920s and remains foundational in understanding oncogene regulation. The genome of X. maculatus was sequenced in 2013 and has been extensively used in comparative genomics and pigmentation genetics.