Patterns of scute development in turtle shell: Symmetry and asymmetry

Abstract

The scute mosaic (pholidosis) in the turtle shell is characterized by two phenomena: phylogenetic stability of the general body plan and wide individual variability. The study of turtle morphogenesis allowed this phenomenon to be explained. It is shown that scute rudiments are formed in embryogeny as local epidermal thickenings, placodes; the positions of placodes is strictly confined to certain parts of the body and their foundation is realized in strict sequence. In particular, in the carapace, placodes are formed exclusively opposite the transverse trunk myosepta, in septal depressions of the embryonic integument; at the same time, marginal placodes are formed in each septal depression (i.e., segment by segment); placodes of pleural and central series are formed in alternate position in the even and odd depressions, respectively. The stability of the scute pattern is connected with the basic mechanisms of morphogenesis, providing the maintenance of bilateral symmetry and determination of scute positions by the primary embryonic segmentation. The main sourse for pholidosis variability is the presence in turtle embryos of vacant septal depressions, in which extraordinary placodes could have developed in the case of minor disturbances of embryogenesis. These aberrations are rather frequent and usually result in asymmetry of the scutation. The breaking of turtle pholidosis symmetry is connected with deviations in ontogenesis, such as asymmetrical formation of epidermal placodes, with a shift to one body side for a segment or more, the formation of additional placodes in the vacant septal depressions, atypical fusion of neighboring epidermal placodes, the absence of regular epidermal placodes in typical body segments.