Gray anatomy: phylogenetic patterns of somatic gonad structures and reproductive strategies across the Bilateria

The last common ancestor of extant bilaterian animals is often referred to as "Urbilateria". Comparative studies of development in a variety of laboratory animals, both traditional model systems and newer "emerging" models, have resulted in many proposals as to the morphological and developmental genetic characteristics of Urbilateria. Most of these proposals are concerned with the development and emergence of external morphology, such as appendages, eyes, and ectodermal segmentation. Less attention has been paid to the evolutionary developmental biology of organogenesis. Arguably, one of the most important aspects of urbilaterian organogenesis would have been gonadogenesis, since Urbilateria must have successfully generated gametes and developed a strategy for extrusion and fertilization, in order to be the ancestor of all living Bilateria. This article considers what is known about gonadogenesis and reproductive strategies in extant metazoans, and searches for phylogenetic patterns that suggest what shared characteristics of these processes Urbilateria might have displayed. I conclude that the data presently available cannot suggest homologies of the somatic components of metazoan gonads, and that convergent evolution has resulted in many different morphological, and possibly molecular genetic, solutions to the various problems posed by sexual reproduction.