A mechanism for anterior transposition of the anal fin and its appendicular support in the western mosquitofish, Gambusia affinis affinis [Baird and girard, 1854]

The interosseal and suspensory ligaments of the axial and appendicular skel eton of the Western Mosquitofish, Gambusia affinis affinis were investigate d in whole-mounted late embryonic and adult female and male G. a. affinis s tained with alcian blue and alizarin red S, cleared, and viewed using diffe rential interference contrast. The interosseal and suspensory ligaments of late embryonic female G. a. affinis are reduced prior to sexual differentia tion and continue reduced in adult females. However, in late embryonic male G. a. affinis the interosseal and suspensory ligaments are well developed prior to sexual differentiation and become robust in adult males. Treating late embryonic female G. a. affinis either with 30.0 or 40.0 mu g/diet of 1 7 alpha-methyltestosterone led to the premature calcification of the hemal spine of the 13th vertebrae. Treating late embryonic female G. a. affinis w ith 17 alpha-methyltestosterone did not lead to the remodeling of the hemal spines of the 14th-16th vertebrae. Thus, in all female G. a. affinis treat ed with 17 alpha-methyltestosterone, no anterior transposition of the anal fin and its appendicular support was observed. However, treating late embry onic male G. a. affinis with 30.0 mu g/g diet of 17 alpha-methyltestosteron e led to the premature calcification of the hemal spine of the 13th vertebr ae, blocking the anterior transposition in only 62.5% of the males treated, and treating late embryonic male G. a. affinis with 40.0 mu g/g diet of 17 alpha-methyltestosterone led to the premature calcification of the hemal s pine of the 13th vertebrae, blocking the anterior transposition in all (100 .0%) males treated. Treatment with either 30.0 or 40.0 mu g/g diet of 17 al pha-methyltestosterone appeared not to completely effect the interosseal an d suspensory ligaments of late embryonic male G. a. affinis as evident by t he male-specific remodeling of the hemal spines of the 14th-16th vertebrae. The results of this study implicate the interosseal and suspensory ligamen ts in the male-specific remodeling of the axial and appendicular skeletal e lements. Following the resorption of the 13th hemal spine, the interosseal and suspensory ligaments in late embryonic male G. a. affinis develop enoug h tension to cause directional growth, bending, and elongation of the 14-16 th hemal spines, thus providing a mechanism for the anterior transposition of the sexually dimorphic anal fin and its appendicular support. The data a lso support the need to revise the interosseal and suspensory ligament nome nclature by assigning the designation of 'female' and 'male' to these ligam ents.