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Absence of mammary development in male Dromiciops gliroides: another link to the Australian marsupial fauna

Abstract The South American microbiotherian marsupial Dromiciops gliroides is regarded as a sister taxon to the monophyletic Australasian marsupial clade. Mammary anlagen have been documented in male American marsupials but have not been observed in males of any Australian marsupials. The aim of this study was to determine if mammary anlagen were present in developing pouch young (PY) of Dromiciops. The developmental anatomy, particularly the presence of mammary anlagen, was investigated in 3 neonatal male (n  =  2) and female (n  =  1) Dromiciops PY (∼15–20 days postpartum). External and internal structures were examined to confirm PY gender and to provide basic information on development, particularly of the genitourinary system. In the female PY early pouch development was observed as well as 4 clearly defined abdominal mammary anlagen. No external teat or mammary anlagen development was seen in the 2 male PY and serial sections from the external genitalia to upper thorax failed to reveal any histological structures similar to mammary anlagen seen in the female. Absence of mammary anlagen in male Dromiciops PY provides further evidence that Dromiciops shares a closer relationship with the Australasian marsupial clade than with the American marsupial fauna.