Evolution of mammals: a comparative study of the developing middle ear

In this project we will study the developmental processes underlying the transition in jaw articulation during marsupial development. In doing so, we will gain insights in to how changes in embryonic development lead to the production of novel features, such as those which define one group of animals from another. This work will help define and illuminate the developmental and evolutionary processes by which different species and groups of species arise, and how specifically mammals evolved.

There are three unique features common to all living mammals: the ability to lactate, a lower jaw consisting of a single bone, and the existence of three bones in the middle ear. During evolution, mammals evolved a new jaw articulation linking the upper and lower jaws, freeing up the old articulation to be incorporated into the ear.

Unlike the arrival of lactation, this remarkable transformation has been documented in the fossil record. Amazingly it can also be observed during the development of modern marsupials. When newborn, marsupials are significantly underdeveloped compared to newborn placental mammals. At this early stage of development the newborn lacks the typical mammal jaw articulation, and yet they need a functioning jaw to enable them to suckle. Consequently, newborn marsupials use the developing bones of the future middle ear to form their first jaw joint, which are the same bones used in the mature jaw joint in non-mammal four-limbed animals with back bones, including all birds, reptiles and amphibians. As the marsupial continues to develop outside the womb, the typical mammalian jaw joint forms, freeing up the middle ear bones to become incorporated into the hearing system, echoing the transition observed in the fossil record.

RPG - TUCKER A - IMG1

The new born opossum (Monodelphis) has a jaw articulation similar to that of the chick, however as it grows, the first jaw joint becomes part of the middle ear, and the opossum develops a typical mammal jaw joint, such as that seen in mice.

The anatomical development of the middle ear bones in marsupials has been studied in the grey-tailed opossum based on descriptions of the shape and size of the bones and surrounding tissues. No information about the genes influencing these structures is known.  Modern molecular biology provides new tools that shed light on old questions, such as the evolution of the mammalian middle ear bones. Work by ourselves and others has given insight into key genes regulating the formation of the jaw and middle ear joints and bones in the standard model animals of mice and chickens. We will use this information to compare formation of the middle ear bones and jaw joint in non-mammals (chicken and gecko), in a placental mammal (mouse) and in a marsupial (the grey tailed opossum). We will use gene expression analysis and advanced imaging techniquesavailable to us, such as optical positional tomography, which allows 3D visualisations of gene expression to be made, essential in complex structures such as those found the jaw and middle ear.

Dr Abigail Tucker
King's College London

Dr Tucker was awarded a Leverhulme Trust Research Project Grant in March 2013.