Geometric morphometrics of symmetry and allometry in Micrasterias rotata (Zygnemophyceae, Viridiplantae)

Algal cells show many types of complex symmetry. For instance, Micrasterias rota to cells are symmetric relative to two perpendicular axes of symmetry. Due to the mode of cell division of Micrasterias rotata cells, the symmetry axes can be interpreted as a left-right axis and a juvenile-adult axis. Here, we analyze symmetry and allometry in Micrasterias rotata cells. We apply a new general approach for shape analysis of structures with any type of symmetry. Our method can separate a component of symmetric variation among individuals from one or more components of asymmetry, depending on the type of symmetry under study, and thus is appropriate for the study of symmetry in these cells. Our results suggest that almost two-thirds of the shape variation in our example (62.3% of the total variance) is explained by the asymmetric component relative to the juvenile-adult axis. Therefore, most morphological variation occurs between juvenile and adult semicells. Given that these shape changes are associated with the size of the semicells, they indicate a type of allometry between semicells. We also compared these patterns of allometry within cells to allometry among cells by using a multivariate regression of shape averaged by individuals onto size of the whole cells. A permutation test shows a highly significant association between size and shape of the whole cells.