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 skeleton of the Western Mosquitofish, Gambusia affinis affinis were investigated in whole-mounted late embryonic and adult female and male G. a. affinis stained with alcian blue and alizarin red S, cleared, and viewed using differential interference contrast. The interosseal and suspensory ligaments of late embryonic female G. a. affinis are reduced prior to sexual differentiation 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 with either 30.0 or 40.0 μg/diet of 17α-methyltestosterone led to the premature calcification of the hemal spine of the 13th vertebra. Treating late embryonic female G. a. affinis with 17α-methyltestosterone did not lead to the remodeling of the hemal spines of the 14th-16th vertebrae. Thus, in all female G. a. affinis treated with 17α-methyltestosterone, no anterior transposition of the anal fin and its appendicular support was observed. However, treating late embryonic male G. a. affinis with 30.0 μg/g diet of 17α-methyltestosterone led to the premature calcification of the hemal spine of the 13th vertebra, blocking the anterior transposition in only 62.5% of the males treated, and treating late embryonic male G. a. affinis with 40.0 μg/g diet of 17α- methyltestosterone led to the premature calcification of the hemal spine of the 13th vertebra, blocking the anterior transposition in all (100.0%) males treated. Treatment with either 30.0 or 40.0 μg/g diet of 17α- methyltestosterone appeared not to completely effect the interosseal and suspensory ligaments of late embryonic male G.a. affinis as evident by the male-specific remodeling of the hemal spines of the 14th-16th Vertebrae. The results of this study implicate the interosseal and suspensory ligaments in the male-specific remodeling of the axial and appendicular skeletal elements. Following the resorption of the 13th hemal spine, the interosseal and suspensory ligaments in late embryonic male G. a. affinis develop enough tension to cause directional growth, bending, and elongation of the 14th- 16th hemal spines, thus providing a mechanism for the anterior transposition of the sexually dimorphic anal fin and its appendicular support. The data also support the need to revise the interosseal and suspensory ligament nomenclature by assigning the designation of 'female' and 'male' to these ligaments.