Here we review the literature of a male poecillid's sexually dimorphic body plan, behavior, and nervous system, including work dating from the mid 1800s to the mid 1990s as well as work in press or in preparation for publication. Rosa-Molinar described the remodeling of the sexually dimorphic anal fin appendicular support, confirmed earlier claims about the development of the male and female secondary sex characteristics in the Western Mosquitofish, Gambusia affinis and provided for the first time direct embryonic evidence suggesting that remodeling of the sexually dimorphic anal fin appendicular support is biphasic. The first process begins in embryos and proceeds similarly in immature males and females; the second process occurs only in males and results in the anterior transposition of the anal fin and its appendicular support to the level of vertebra 11 [Rosa-Molinar E, Hendricks SE, Rodriguez-Sierra JF, Fritzsch B. 1994. Development of the anal fin appendicular support in the western mosquitofish, Gambusia affinis (Baird and Girard, 1854): a reinvestigation and reinterpretation. Acta Anat 151:20-35.] and the formation of a gonopodium used for internal fertilization. Studies using high-speed video cameras confirmed and extended Peden's and others' observations of copulatory behavior. The cameras showed that circumduction is a complex movement combining in a very fast sequence abduction, extension and pronation, S-start-type fast-start (defined as torque-thrust), and adduction movements. Recent work on the nervous system demonstrated dye-coupling between motor neurons and interneurons via gap junctions, suggesting an attractive substrate for the rapid motions involved in poecillid copulatory reflexes.

The western mosquitofish (Gambusia affinis affinis) is a useful model for the study of sexual dimorphism and the neural circuits associated with sexual differentiation. This is largely because of its anal fin, which undergoes radical postnatal transformation in males. Understanding the neural mechanisms involved in this process may also help elucidate basic principles of the nervous system. The authors describe the mosquitofish as a model for research and present guidelines for the care and use of this species.

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