EVOLUTION OF VIVIPARITY IN REPTILES: INTRODUCTION TO THE SYMPOSIUM

Abstract

The Fifth World Congress of Herpetology, held in 2005 at the University of Stellenbosch (Republic of South Africa), provided an excellent opportunity to host a symposium on the evolution of viviparity in reptiles. Several symposia on viviparity have been held during the past 15 years, and these have yielded comprehensive reviews of squamates (e.g., Blackburn, 1993, 1998, 2000; Stewart, 1993; Stewart and Thompson, 2000, 2003; Thompson et al., 2000, 2004) and many other papers. However, most of these symposia were limited in scope to particular features (placental membranes, the oviduct) or aspects (morphology, physiology). The World Congress venue represented the first broad-based symposium to focus on viviparity in squamates, a taxon of special importance to biologists interested in viviparity. This symposium was multi-disciplinary, and integrated morphology, physiology, biochemistry, molecular biology, and evolutionary theory. The evolution of viviparity holds intrinsic interest for a number of reasons. First, humans are viviparous and have natural curiosity about their own reproduction. Second, mode of reproduction relates closely to a species’ ecology, physiology, and behavior, and is central to its life history strategy. Third, viviparity is a central, recurring theme in vertebrate history, and its existence raises fascinating functional and evolutionary questions. Squamate reptiles provide an excellent model system for studies on the evolution of viviparity because live-bearing reproduction has evolved in the Squamata at least 100 times (Blackburn, 1999), and complex placentae have evolved four or five times (Thompson and Speake, 2006). Many of the reptilian origins of viviparity have occurred at low taxonomic levels and in geologically recent times, potentially allowing a reconstruction of the transition to viviparity. Additionally, reptiles are amniotes, and their fetal membranes are homologous to those that contribute to mammalian placentae. Consequently, squamate reptiles arguably are the best available model for attempts to understand the evolution of mammalian reproduction. Nevertheless, squamates have a reptilian physiology as well as unique features (such as a yolk cleft and an isolated yolk mass), and the sequence by which viviparity and placentation evolved differs in squamates and mammals (Blackburn, 2006). Therefore, caution is required in applying conclusions from reptilian studies to mammalian systems. One mark of the growing maturity of the field is that studies on squamate viviparity are justified in their own right, aside from whatever insight they may give into mammals. This symposium addressed two related questions: how viviparity has evolved from oviparity, and how complex placentae have evolved from simple placentae. The symposium consisted of fifteen invited presentations, beginning with a review of the utility of reptiles to a more general model of the evolution of viviparity, followed by three papers on morphological details of extraembryonic and placental development and structure. Four papers described aspects of the physiology of maternal-embryonic interactions, including the occurrence of HoxA10 genes. The afternoon session began with a focus on eggs and oviparous species and the factors that may have led to or limited the evolution of viviparity, and concluded with a series of papers that covered diverse topics in ecology, morphology, and physiology. The symposium highlighted the rapid progress that has been made in understanding CORRESPONDENCE: e-mail, Mike.Thompson@bio.usyd. edu.au Herpetological Monographs, 20, 2006, 129–130 E 2006 by The Herpetologists’ League, Inc.