The fat-tailed dunnart, Sminthopsis crassicaudata, emerges as a powerful comparative animal model for evolutionary developmental biology. Image credit: Alan Couch (CC BY 2.0)
Marsupials are a distinctive group of mammals best known for their defining trait: a pouch. Unlike monotremes, which lay eggs, and placental mammals, in which young develop fully in the womb, marsupials give birth to highly premature offspring. These young complete most of their development within the mother’s pouch.
Because of this strategy, marsupial newborns possess traits critical for immediate survival outside the womb. These include well-developed forelimbs for climbing into the pouch and a mature oral region for attaching to the teat and suckling.
Evolution has shaped the development of the head to match the diverse environments vertebrates inhabit. Marsupials diverged from placental mammals over 160 million years ago, and their requirement for early limb and oral development makes them a powerful system for investigating the genetic mechanisms underlying development.
The fat-tailed dunnart (Sminthopsis crassicaudata), often called a marsupial mouse, is emerging as an important model organism. With its short gestation period and extremely undeveloped state at birth, the dunnart provides an excellent comparison to the laboratory mouse – a well-established placental model – for studying evolutionary and developmental differences.
Cook et al. investigated the genes and regulatory elements driving early orofacial development in the dunnart, comparing their findings with existing craniofacial expression and epigenomic data in mice.
Their results showed that although the genes involved in craniofacial development are highly conserved between the two species, the regulatory elements controlling those genes differ markedly. In dunnarts, regulatory elements linked to skin, nervous system, and muscle development were highly active, whereas in mice they were inactive or only weakly expressed.
An important feature of marsupial development is the postnatal retention of the periderm (a transient outer cell layer in embryonic skin). This may support gas exchange in prematurely born young. In addition, elevated activity of genes regulating sensory and muscle development suggests early maturation of mechanosensory and olfactory systems, which are essential for the newborn’s journey to the pouch. Notably, pharyngeal and facial muscles develop before the skeletal system, contrasting with the developmental sequence in placental mammals.
In summary, marsupials exhibit a unique reproductive strategy, in which young are born in an extremely underdeveloped state and survival depends on accelerated development of specific traits after birth. By comparing gene regulation in marsupials and placentals, researchers gain insights into how evolution shaped developmental pathways. Expanding genomic resources for the dunnart, such as genome editing tools and transgenic models, will further enhance its role as a powerful comparative system for evolutionary developmental biology.
