Homology and Developmental Transformation: Models for the Origins of the Staminodes of Loasaceae Subfamily Loasoideae

Abstract

Hypotheses of homology for staminodes of Loasaceae subfamily Loasoideae are evaluated and a new model for their evolutionary origin is presented. Grau and Weigend have suggested that staminodes of Loasoideae are homologous to stamens of Mentzelia that have distally forked filaments. The results of this study are inconsistent with their proposal. Neither the scales nor inner staminodes of Loasoideae are phylogenetically homologous to stamens of Mentzelia that have distally forked filaments. Scales and inner staminodes also lack distinctive attributes that would be necessary to support hypotheses that they are structurally homologous to stamens that have distally forked filaments. Staminode evolution in Loasoideae has been characterized by a set of developmental transformations. The institution of zonal growth unifying the initial outer whorl antesepalous stamens was central to the origin of complex staminodial scales. Another important innovation of scales in Loaseae was the origin of an early developmental adaxial thickening that can be seen as analogous to radial thickening processes in ensiform, unifacial leaves. Inner staminode evolution in Loasoideae is distinguished primarily by the formation of a basal outgrowth, which is similar to the adaxial thickening of the scales and has been highly elaborated in Loasoideae such as Kissenia and Presliophytum. The scales and inner staminodes of Loasoideae are serial homologues of fertile stamens that have largely conventional forms. Scales and inner staminodes share structural homology with stamens of other Loasaceae at the peglike stage of primordium development. From that early ontogenetic state onward, developmental evolutionary novelties of scales and inner staminodes have led to their morphological divergence from more conventional stamen forms.