Nutritional reserves that are stored in egg yolk are crucial for the development of the embryo of nonmammalian oviparous vertebrates [1
]. In the extant egg-laying (oviparous) species that are closest to mammals—reptiles and birds—the composition of yolk is well known [1
]. It mainly consists of proteins, lipids, phosphorous, and calcium, most of which are either contained in or transported to the egg by vitellogenin (VTG), which is produced in the liver. Thus, yolk constitutes an essential resource in these species, because these nutrients cannot be provided from the exterior to the developing egg [3
In contrast, “placental” mammals (eutherians) are thought to have replaced the role of VTG through the establishment of a vascularized, chorioallantoic placenta, which builds a controlled interface between the developing embryo/fetus and its mother, together with subsequent milk feeding of the suckling after birth [4
In marsupials (metatherians), lactation is prolonged and more sophisticated than in eutherians [7
] (). Marsupials also have a placenta, originating from the yolk sac [9
], but the marsupial oocyte contains considerably more yolk than that of eutherians [10
], which is virtually devoid of it. The marsupial yolk reserve is assumed to be essential during the earliest development of the embryo, complementing the uptake of uterine secretions by the yolk sac, prior to shell coat rupture [12
]. However, the content of marsupial yolk is not well known [11
]. The presence of (transient) yolk-sac placentae [13
] and lecithotrophic (yolk-dependent) viviparity in lizards may provide a model for an early form of a still VTG-dependent marsupial. However, the increasing provision of nutrients through more advanced lactation and a placenta during marsupial evolution may have gradually reduced selective pressure to preserve large yolk reserves, which are exclusively designated to the developing embryo/fetus until birth.
Monotremes (prototherians) are the only extant oviparous mammalian species (). They possess mammary glands like marsupials and eutherians, but teats are absent and milk is supplied to the offspring by leakage onto the abdominal milk patch [4
]. Thus, the combination of a primitive mode of lactation—which is likely similar to that of the common mammalian ancestor [6
]—and oviparity in these species may give insights into the relationship between lactation and nutrient reserves in the oocyte, as lactation might have at least partially replaced oocyte resources. Indeed, the eggs (~2 cm in diameter) of the duck-billed platypus, one of three extant monotreme lineages, are very small in proportion to body size, when compared with, for example, bird and reptile eggs [4
]. Nevertheless, monotreme eggs still contain considerable quantities of yolk compared with those of marsupials and eutherians. However, the molecular composition of this yolk is not documented in detail [14
To understand the transition from yolk-dependent nourishment toward the alternative resources—lactation and placentation—available for the mammalian embryo, fetus, and new-born offspring, we set out to elucidate in detail the evolutionary fate of the genes coding for the fundamental egg yolk resource, VTG, in mammals.