Mammary glands in male marsupials: I. Primordia in neonatal opossums Didelphis virginiana and Monodelphis domestica

An ovine model for investigation of the microenvironment of the male mammary gland

The specific biology of the male breast remains relatively unexplored in spite of the increasing global prevalence of male breast cancer. Delineation of the microenvironment of the male breast is restricted by the low availability of human samples and a lack of characterisation of appropriate animal models. Unlike the mouse, the male ovine gland persists postnatally. We suggest that the male ovine mammary gland constitutes a promising adjunctive model for the male breast. In this study, we evaluate the male ovine mammary gland microenvironment, comparing intact and neutered males. Assessment of the glandular histo-anatomy highlights the resemblance of the male gland to that of neonatal female sheep and confirms the presence of rudimentary terminal duct lobular units. Irrespective of neutered status, cell proliferation in epithelial and stromal compartments is similarly low in males, and cell proliferation in epithelial cells and in the intralobular stroma is significantly lower than in pubertal female sheep. Between 42% and 72% of the luminal mammary epithelial cells in the male gland express the androgen receptor and expression is significantly reduced by neutering. Luminal epithelial cells within the intact and neutered male gland also express oestrogen receptor alpha, but minimal progesterone receptor expression is observed. The distribution of leukocytes within the ducts and stroma is similar to the mammary gland of female sheep and females of other species. Both macrophages and T lymphocytes are intercalated in the epithelial bilayer and are more abundant in the intralobular stroma than the interlobular stroma, suggesting that they may have a protective immunological function within the vestigial glandular tissue of the male sheep. Mast cells are also observed within the stroma. These cells cluster near the glandular tissue and are frequently located adjacent to blood vessels. The abundance of mast cells is significantly higher in intact males compared to neutered males, suggesting that hormone signalling may impact mast cell recruitment. In this study, we demonstrate the utility of the male ovine mammary gland as a model for furthering our knowledge of postnatal male mammary biology.

Incomplete lineage sorting and phenotypic evolution in marsupials

Incomplete lineage sorting (ILS) makes ancestral genetic polymorphisms persist during rapid speciation events, inducing incongruences between gene trees and species trees. ILS has complicated phylogenetic inference in many lineages, including hominids. However, we lack empirical evidence that ILS leads to incongruent phenotypic variation. Here, we performed phylogenomic analyses to show that the South American monito del monte is the sister lineage of all Australian marsupials, although over 31% of its genome is closer to the Diprotodontia than to other Australian groups due to ILS during ancient radiation. Pervasive conflicting phylogenetic signals across the whole genome are consistent with some of the morphological variation among extant marsupials. We detected hundreds of genes that experienced stochastic fixation during ILS, encoding the same amino acids in non-sister species. Using functional experiments, we confirm how ILS may have directly contributed to hemiplasy in morphological traits that were established during rapid marsupial speciation ca. 60 mya.

Morphology and evolution of the oral shield in marsupial neonates including the newborn monito del monte (Dromiciops gliroides, Marsupialia Microbiotheria) pouch young

Newborn marsupials can be arranged into three grades of developmental complexity based on their external form, as well as based on their organ systems and their cytology. The dasyurids are considered the least developed marsupials at birth, while didelphids and peramelids are intermediate, and macropods are the most developed. Currently there is still little information on caenolestid and microbiotherid development at birth. Developmental stages can be graded as G1, G2 and G3, with G1 being the least developed at birth, and G3 the most developed. Marsupials are also characterized by having an extremely developed craniofacial region at birth compared with placentals. However, the facial region is also observed to vary in development between different marsupial groups at birth. The oral shield is a morphological structure observed in the oral region of the head during late embryological development, which will diminish shortly after birth. Morphological variation of the oral shield is observed and can be arranged by developmental complexity from greatly developed, reduced to vestigial. In its most developed state, the lips are fused, forming together with the rhinarium, a flattened ring around the buccal opening. In this study, we examine the external oral shield morphology in different species of newborn marsupials (dasyurids, peramelids, macropods and didelphids), including the newborn monito del monte young (Dromiciops gliroides - the sole survivor of the order Microbiotheria). The adaptive value of the oral shield structure is reviewed, and we discuss if this structure may be influenced by developmental stage of newborn, pouch cover, species relatedness, or other reproductive features. We observe that the oral shield structure is present in most species of Marsupialia and appears to be exclusively present in this infraclass. It has never been described in Monotremata or Eutherians. It is present in unrelated taxa (e.g. didelphids, dasyurids and microbiotherids). We observe that a well-developed oral shield may be related to ultra altricial development at birth, large litter size (more than two), and is present in most species that lack a pouch in reproductive adult females or have a less prominent or less developed pouch with some exceptions. We try to explore the evolution of the oral shield structure using existing databases and our own observations to reconstruct likely ancestral character states that can then be used to estimate the evolutionary origin of this structure and if it was present in early mammals. We find that a simple to develop oral shield structure (type 2-3) may have been present in marsupial ancestors as well as in early therians, even though this structure is not present in the extant monotremes. This in turn may suggest that early marsupials may have had a very simple pouch or lacked a pouch as seen in some living marsupials, such as some dasyurids, didelphids and caenolestids. The study's results also suggest that different morphological stages of the oral shield and hindlimb development may be influenced by species size and reproductive strategy, and possibly by yet unknown species-specific adaptations.

The role of androgens in development of the scrotum of the grey short-tailed Brazilian opossum ( Monodelphis domestica)

In eutherian mammals, sex differentiation is initiated by expression of the testis-determining gene on the Y chromosome. Subsequent phenotypic development of the reproductive tract and genitalia depends on the production of hormones by the differentiated testis. In marsupials the mechanisms of phenotypic development may vary from this pattern, as differentiation of the scrotal primordia has been shown to occur before that of the gonad. Thus, the development of the scrotum in the marsupial has been regarded as an androgen-independent process. We have sought to clarify the ontogeny of scrotal development and the appearance of androgen receptor immunoreactivity by examining Monodelphis domesticaembryos/pups from 1 day prior to birth until 2 days after birth. We have also used immunocytochemistry to determine the expression of the key steroidogenic enzyme 3beta-hydroxysteroid dehydrogenase as an indicator of when the developing gonad may be capable of synthesizing androgens. Expression of this enzyme was first detected in the gonads and adrenals of both sexes 1 day prior to birth and before the appearance of scrotal bulges. Androgen receptor immunoreactivity was detected in the scrotal anlagen of male opossum pups as early as 1 day following birth. This finding is significantly earlier than previous reports and coincides with the appearance 1 day after birth of distinct scrotal bulges. Androgen receptor immunoreactivity was also observed in the genital tubercles of male pups, but not female pups, 2 days after birth. These results suggest that androgens may play an important role in the development of the male genitalia at a much earlier stage than that indicated by previously published work and that scrotal development in this species may not be androgen-independent.

The marsupial male: a role model for sexual development

Sexual differentiation in male marsupials has many similarities with that of eutherians. Marsupials have an XX-XY sex determining mechanism, and have a homologue of the testis-determining SRY gene on their Y-chromosome. However, the development pattern of SRY gene expression is different from the mouse in that it is expressed for a much longer period. SRY is expressed in a range of non-gonadal tissues in male pouch young and adults which is similar to the human pattern, and raises questions as to its particular role(s) in sexual differentiation. Similarly Müllerian inhibiting substance (MIS) is produced in the developing testis over a longer period than in the mouse. Since ovaries cultured with MIS or transplanted into male recipient pouch young develop tubular structures, MIS may induce Sertoli cell formation. Testosterone is produced by the neonatal testis, and this stimulates Wolffian duct development to form the vas deferens and epididymis. Virilization of urogenital sinus is also androgen-dependent. However, virilization of the prostate and phallus occurs more than three weeks after the onset of testosterone production, suggesting that the timing of this may be regulated by delayed activation of the androgen receptor pathway. Unlike in eutherians, differentiation of the scrotum and mammary glands is not dependent on testicular hormones, but is independently regulated by an X-linked genetic mechanism. Clearly marsupials provide a unique perspective to help us clarify the mechanisms underlying sexual development in all mammals.