Abstract

Segmented organisms present serially repeated homologous structures along the antero-posterior body axis. During development, Hox proteins control the segmental specialization of these structures transforming their morphology and function. Despite this divergence, the structures can still be clearly identified as homologous elements. We will present evidence showing a situation of extreme divergence where the original ectodermal structure has evolved into completely different structures: in one case, two mesenchymal endocrine components of the ring gland; and in the other, the internal epithelial respiratory tracheal network.

The development of trachea and endocrine organs initiates with the invagination of the polarized epithelial cells. The morphological divergence occurs shortly after invagination, when activation of Snail in the endocrine primordia induces an epithelial to mesenchymal transition. This is followed by the coalescence of the corpora allata and prothoracic gland primordia that migrate dorsally fusing, first, to the corpora cardiaca and, later, to the contralateral primordium in what becomes the ring gland. We show that the trachea can be transformed into gland and vice versa demonstrating their homologous character.

Our data are important because: (1) they uncover the genetic and developmental mechanisms for ring gland morphogenesis and (2) indicate that the respiratory tracheal organs and two main endocrine glands arose through a process of divergent evolution from an ectodermal repeated structure that lead to extremely different morphological and functional organs.