Page 1488 - Veterinary Immunology, 10th Edition

VetBooks.ir Immunity in Monotremes and

Marsupials

The least evolved mammals, the egg-laying monotremes, such as
the duck-billed platypus (Ornithorhynchus anatinus) and the echidna
(Tachyglossus aculeatus), branched from the other mammals about
166 Mya. They have a spleen, thymus, and gut-associated lymphoid

tissues that are as well developed as those in marsupials and
eutherian mammals. However, instead of typical mammalian
lymph nodes, they have structures consisting of several lymphoid
nodules, each containing a germinal center suspended by its blood

vessels within the lumen of a lymphatic plexus. Thus each nodule is
bathed in lymph. There is usually just one germinal center per
nodule. Monotremes express the typical mammalian
immunoglobulins. They have eight heavy chain isotypes, including

two IgG isotypes, two divergent IgA isotypes, one IgD, IgE, IgM,
and a unique isotype called IgO arranged in order -µ–δ–ο–γ2–γ1–
α1–ε–α2-.
The ο gene contains four C domains, and a hinge, and appears
H
to be structurally intermediate between IgY and IgG. The δ gene
contains 10 C domains and thus resembles the δ gene in reptiles,
H
amphibians, and fish. All the major structural changes that gave
rise to the immunoglobulin classes of modern mammals evolved
before the separation of the monotremes from the marsupials and

placental mammals and probably soon after the split from reptile
lineages 300 Mya. Monotremes, like other mammals, produce
predominantly IgM in the primary immune response and IgG in
secondary immune responses.

Marsupials split from the placental mammals about 147 Mya. The
complete sequencing of the genome of the marsupial opossum
Monodelphis domestica has allowed its immune system genes (its
immunome) to be examined in detail. Thus it contains genes for all

the key immune gene families. There has been substantial
duplication or gene conversion involving leukocyte receptors, NK
complexes, immunoglobulins, type I interferons, and defensins. The
opossum, like the platypus and wallaby, produces a TCR chain

1488