Evolutionary transformations of fetal membranes and reproductive strategies

Abstract

Fetal membranes (such as the chorioallantois and yolk sac) are essential to embryonic development, and have contributed importantly to the evolutionary and ecological diversity of vertebrates. Since the mid-19th century, many scientific careers have been devoted to investigations of their structure, function, and development. However, significant gaps remain in our understanding of the diversity and evolution of fetal membranes. This symposium volume focuses on the use of cladistic principles and phylogenetic relationships to reconstruct the evolutionary morphology of fetal membranes. The main goal of the present paper is to introduce the application of such methods to the evolution of vertebrate fetal membranes, as well as to provide the reader with background information on relevant concepts and terminology. Contributions within this journal issue draw upon studies of metatherian and eutherian mammals, as well as sauropsid reptiles (notably squamates). Particular attention is given to historical transformations of fetal membranes associated with the evolution of such phenomena as placentation and matrotrophy, and reproductive strategies such as viviparity. What emerges from the contributed papers is a broad sampling of contemporary research on fetal membranes, and an overview of how these membranes have evolved to support embryonic life in diverse terrestrial and intra-uterine environments.

Evolution of placental specializations in viviparous African and South American lizards

“…Organisms are not grouped by overall similarity, but rather, taxa represent clades that are characterized by the possession of shared derived features. Thus, for example, amniotes can be classified as a clade by the presence of their derived fetal membranes, the chorion, amnion, and allantois (Mess et al, 2003). In contrast, the absence of these membranes does not characterize any vertebrate taxon; ''anamniotes'' are a diverse, heterogeneous group of organisms that have no shared derived features not found among amniotes.…”

Placental Evolution within the Supraordinal Clades of Eutheria with the Perspective of Alternative Animal Models for Human Placentation

“…Groups such as mammals, birds, and other sauropsids evolved by evolutionary radiations within amniotes. Mammalia was systematically divided into the oviparous (egg-laying) Monotremata on the one hand as well as the viviparous Metatheria (marsupials) and Eutheria (placental mammals) on the other [73,74]. However, only Eutheria developed a remarkable diversity with more than 4000 extant species, whereas the two other groups contain only about 3 and 350 species, respectively [75].…”

“…Particularly in amniotes, viviparity and placentas evolved multiple times in parallel [93]. The evolution of additional extraembryonic membranes in amniotes, the amnion, allantoic, and chorion, working in concert with the vertebrate yolk sacs, seemed to be a suitable prerequisite to evolve a very high degree of reproductive plasticity including complex placental structures within many sauropsid lineages as well as in marsupial and placental mammals [7,11,74,93,94].…”

Evolutionary transformations of fetal membrane characters in Eutheria with special reference to Afrotheria

“…Various other accessory structures within the chorioallantoic placenta are concerned with absorption of maternal secretions and cell debris, socalled histiotrophic nutrition (Mossman, '87;Mess et al, 2003). Typical of these are areolae associated with uterine glands (character 8, state 1, e.g.…”

“…To function as a placenta, the trilaminar yolk sac must maintain contact with the uterine wall, in order to take up histiotrophe (Mossman, '87;Mess et al, 2003). All marsupials rely on yolk sac placentation.…”