Evolutionary transformations of chorioallantoic placental characters in rodentia with special reference to hystricognath species

Abstract

The aim of this paper is to reconstruct the evolution of chorioallantoic placental characters in Rodentia. The analysis is based on pre-existing hypotheses of rodent relationships and the tracing of character evolution. Data on 64 rodent species of 49 genera are derived from the literature. New results refer to the hystricognath species Petromus typicus A. Smith, 1831 and Octodon degus (Molina, 1782). This comprehensive analysis confirms that the stem species pattern of Rodentia is characterised by a haemochorial placenta which is divided horizontally. Inside the placental labyrinth, fetal vessels and their trophoblastic external border build up a network through which the maternal blood flows. The trophoblastic tissue is one-layered, syncytial and possess a considerable surface extension. Within Rodentia, evolutionary transformations occurred on the macroscopic as well as the fine structural level. The results suggest that the stem species of Hystricognathi underwent transformations only on the macroscopic level, i.e., forming a ring-shaped arrangement of placental regions with centrally situated maternal arteries and the acquisition of a subplacenta. By contrast, in Muridae the chorioallantoic placenta shows derived features only in regard to the fine structure of the labyrinth, i.e. the interhaemal membrane is modified in composition, and the fetal capillary endothelium is fenestrated. Geomyoidea underwent transformations on both levels. Macroscopically, their placenta is modified into a hillock shape. Microscopically, the interhaemal membrane is formed by the cytotrophoblast. In addition to the mentioned transformations, some aspects of other fetal membrane differentiation in Rodentia are briefly discussed.

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

“…4C). Within the latter clade, however, precocial newborns are part of the stem pattern of Ferungulata and the same transformation occurs in Cavia (and other hystricognath rodents; Mess, 2003) and in primates. Precocial young are part of the stem species pattern of Xenarthra as indicated by their single representative Bradypus as well as of the stem species pattern of Afrotheria, but with secondary transformation to altricial newborns in Tenrecoidea.…”

“…MacClade is a widely accepted tool for analyzing phylogenetic hypotheses and reconstructing and displaying character evolution on a phylogenetic tree. Character optimization is an approach previously used to trace character evolution in fetal membranes (Freyer et al, 2003;Mess, 2003;Asher, 2005;Vogel, 2005). Moreover, the parsimony-based method of character analysis in MacClade easily allows handling of missing data.…”

“…Even for Rodentia, where the pattern is not resolved, we know about several taxa with superficial implantation, indicating an independent achievement of more invasive blastocyst attachment (e.g. Luckett, '85;Mossman, '87;Mess, 2003).…”

“…However, this is not the case in Afrotheria or Bradypus. Precociality requires a longer gestation and usually occurs in relatively large animals such as Cetartiodactyla, Hystricognathi (Mess, 2003) and higher primates (Martin, 2003), where it is apomorphic.…”

“…Transformations within Glires include reversion to syncytiotrophoblast in Cavia and other hystricognaths, which also have precocial newborns (Mess, 2003). In addition Cavia, as a typical hystricognath rodent, has primary interstitial implantation; there is no true yolk sac placentation and the allantoic sac is rudimentary.…”