EVOLUTIONARY DEVELOPMENT

The birth of a thymus

In a broad-ranging study, Thomas Boehm of

the Max Planck Institute of Immunobiology

in Freiburg, Germany, and his colleagues

attempt to reconstruct the evolutionary

history of the thymus, the immune-system

organ where T cells accumulate and mature.

The authors looked at the expansion and

diversification of genes implicated in T-cell

and thymus development in 13 species

spanning the chordate phylogenetic tree.

They paid special attention to the jawed

vertebrates, which have a thymus, and jawless

fishes such as lampreys, which do not. The

authors conclude that the latter have many

but not all of the genes required to develop a

thymus. The study begins to show how the

duplication and cooption of genetic pathways

leads to the development of a complex organ.