DEVELOPMENT OF THE ANAL FIN APPENDICULAR SUPPORT IN THE WESTERN MOSQUITOFISH, GAMBUSIA-AFFINIS AFFINIS (BAIRD AND GIRARD, 1854) - A REINVESTIGATION AND REINTERPRETATION

Development of the sexually dimorphic anal fin appendicular support of an internal fertilizing bony fish Gambusia affinis affinis was invest igated by staining whole-mounted embryos, immature, and adult female a nd male G. a. affinis with alizarin red S and alcian blue. The tissue was examined histologically to assess, development of the amphicelous centrum and to verify specificity of the stains. Our data confirm earl ier claims about the development of the male and female characteristic s in this species, and we provide for the first time direct embryonic evidence suggesting that development of the sexually dimorphic anal fi n appendicular support is biphasic: (1) anteriorization of the most an terior caudal segments, and (2) growth and elongation of hemal arches of vertebrae 14-16. The first process involves a sequential homeotic t ransformation of hemal arches of vertebrae 11-13 through resorption of mineralized connective tissue, thus forming parapophyses that bear pl eural ribs. This process begins in undifferentiated embryos and procee ds similarly in postnatal males and females. During the same period, t he second process, likely induced by male gonadal hormones, causes the addition of mineralized connective tissue at the hemal arches of vert ebrae 14-16. This second process, which occurs only in males, elongate s the hemal arches of vertebrae 14-16 anteriorly. This elongation appa rently translocates the anal fin appendicular support (including parts of the hemal spine of the hemal arch of vertebra 13) to the level of vertebra 11. It appears that the developmental programs of both female and male G. a. affinis create an area of 6 vertebrae which are marked ly different from any vertebrae anterior to 11 and posterior to 16. We propose to term the area including these vertebrae and the associated anal fin, the genital area. We also propose that the first process, h omeotic transformation of caudal into precaudal segments, is regulated by differential expression of control genes, such as homeobox genes, whereas the second process is regulated by gene expression under the c ontrol of male gonadal hormones. Conflicting data in the literature ca n be resolved with this model. Appropriate tests of the model are prop osed.