Conceptus Coats of Marsupials and Monotremes.

Mammals evolved from oviparous reptiles that laid eggs in a dry, terrestrial environment, thus requiring large amounts of yolk to support development and tough, outer coats to protect them. Eutherian mammals such as humans and mice exhibit an "extreme" form of viviparity in which yolk and conceptus coats have become largely redundant. However, the "other" mammals-monotremes and marsupials-have retained and modified some features of reptilian development that provide valuable insights into the evolution of viviparity in mammals. Most striking of these are the conceptus coats, which include the zona pellucida, the mucoid coat, and the shell coat. We discuss current knowledge of these coats in monotremes and marsupials, their possible roles, and recently identified components such as the zona pellucida protein ZPAX, conceptus coat mucin (CCM), and nephronectin (NPNT).

Quantitative analysis of somatosensory cortex development in metatherians and monotremes, with comparison to the laboratory rat.

Metatherians and monotremes are born in an immature state, followed by prolonged nurturing by maternal lactation. Quantitative analysis of isocortical sections held in the collections at the Museum für Naturkunde, Berlin was used to compare the pace of somatosensory cortex development relative to body size and pallial thickness between metatherian groups, monotremes, and the laboratory rat. Analysis indicated that the pace of pallial growth in the monotremes is much lower than that in the metatherians or laboratory rat, with an estimated 8.6-fold increase in parietal cortex thickness between 10 and 100 mm body length, compared to a 10- to 20-fold increase among the metatherians and the rat. It was found that aggregation of cortical plate neurons occurs at similar embryo size in the mammals studied (around 8-14 mm body length) and a similar pallial thickness (around 200 µm), but that proliferative zone involution occurs at a much higher body size and pallial thickness in the monotremes compared to the metatherians and the laboratory rat. The observations suggest that cortical development in the monotremes is slower and subject to different regulatory signals to the therians studied. The slow pace may be related to either generally slower metabolism in monotremes or less efficient nutrient supply to the offspring due to the lack of teats.

Vestibular development in marsupials and monotremes.

The young of marsupials and monotremes are all born in an immature state, followed by prolonged nurturing by maternal lactation in either a pouch or nest. Nevertheless, the level of locomotor ability required for newborn marsupials and monotremes to reach the safety of the pouch or nest varies considerably: some are transferred to the pouch or nest in an egg (monotremes); others are transferred passively by gravity (e.g. dasyurid marsupials); some have only a horizontal wriggle to make (e.g. peramelid and didelphid marsupials); and others must climb vertically for a long distance to reach the maternal pouch (e.g. diprotodontid marsupials). In the present study, archived sections of the inner ear and hindbrain held in the Bolk, Hill and Hubrecht collections at the Museum für Naturkunde, Berlin, were used to test the relationship between structural maturity of the vestibular apparatus and the locomotor challenges that face the young of these different mammalian groups. A system for staging different levels of structural maturity of the vestibular apparatus was applied to the embryos, pouch young and hatchlings, and correlated with somatic size as indicated by greatest body length. Dasyurids are born at the most immature state, with the vestibular apparatus at little more than the otocyst stage. Peramelids are born with the vestibular apparatus at a more mature state (fully developed semicircular ducts and a ductus reuniens forming between the cochlear duct and saccule, but no semicircular canals). Diprotodontids and monotremes are born with the vestibular apparatus at the most mature state for the non-eutherians (semicircular canals formed, maculae present, but vestibular nuclei in the brainstem not yet differentiated). Monotremes and marsupials reach the later stages of vestibular apparatus development at mean body lengths that lie within the range of those found for laboratory rodents (mouse and rat) reaching the same vestibular stage.

Reproductive biology in egg-laying mammals.

The duck-billed platypus and short-beaked echidna are iconic species in Australia. Their morphology and physiology have puzzled scientists all over the world for more than 200 years. Recent genetic studies, particularly the platypus whole-genome sequencing project, have revealed the molecular basis of some of the extraordinary characteristics of monotremes. This and other works demonstrate the great value of research on our most distantly related mammalian relatives for comparative genomics and developmental biology. In this review we focus on the reproductive biology of monotremes and discuss works that unravel genes involved in lactation, testicular descent, gamete biology and fertilization, and early development. In addition we discuss works on the evolution of the complex sex chromosome system in platypus and echidna, which has also significant impact on our general understanding of mammalian sex chromosomes and sex determination.

Monotreme and marsupial reproduction.

Marsupials were regarded as curiosities by their early European discoverers, animals to be wondered at. Monotremes were even more surprising; the platypus was such an amalgam of characters that it was thought to be a hoax. They were recognized very early as mammals that could make a major contribution to our understanding of reproductive processes, and work on marsupials at the turn of the century was much in evidence. It is, however, only in the past two decades, and especially in the past few years that marsupial research has regained this position. There is no doubt that future research will strengthen this contribution, but we are faced with serious conservation questions that must be solved if we are to maintain these wonderful animals as a resource for future generations.

Monotreme development with particular reference to the extraembryonic membranes.

This paper considers many of the salient features of monotreme development, particularly morphogenesis of the extraembryonic membranes. The uterine endometrium of both monotremes and marsupials exhibits a progesterone driven luteal phase where accelerated utilization of endometrial nutrients is evidenced by a rapid post-primitive streak expansion in the dimensions of the extraembryonic membranes. Monotremes share with marsupials, birds, and reptiles an unspecialized vertebrate mode of genesis of the embryonic disc on the peripheral surface of the yolk-sac. The fused vascularized respiratory chorioallantois is estimated to have a functional life of not more than the terminal 4 to 5 days of the monotreme incubation period. This time interval is of a slightly greater order of magnitude than that found in marsupials with a fused chorioallantois, so far described, but in the context of the proportional elapse of post-primitive streak organogenesis would fall within the marsupial grade. The dominant extraembryonic membrane for nutritive and respiratory function in both monotremes and marsupials is the yolk-sac. This contribution shows that monotremes and marsupials share a much larger suite of developmental anatomical features than previously reported. The evolutionary biologist is confronted with the challenge of how to assign an appropriate weighting to these features as marsupials are considered by many researchers to be allied phylogenetically more closely with eutherian mammals than with monotremes.