Sponges are the most primitive multicellular organisms
alive today. Like modern sponges, the first animals must have possessed
collagen and immunoglobulin proteins. Collagen is the major extracellular
protein of all animals. Higher animals possess hundreds of immunoglobulin
genes which perform a variety of functions ranging from fighting microbes
to guiding embryonic development.
Collagen is the major extracellular protein of all animals and is even
found in the simplest animals. A number of invertebrates have collagen
fibrils very similar to the type of collagen found in vertebrates; collagen
fibrils are even known from cnidarians and sponges. Sponge collagen is
homologus to that of vertebrates. Since sponges can contain both fibrillar
and non-fibrillar collagens, the amplification of this gene family had
begun in the early animals (Exposito, 1990). Although collagen was once
thought to exist only in animals, it has been found in fungi where it
composed the fimbriae which function in cell to cell communication. Animal
cells can interact with fungal collagen in a way similar to the manner
in which they interact with animal collagens (Celerin, 1996). Collagen
sequences may have functioned long before collagen was an extracellular
protein. Collagenous sequences are known from vertebrate proteins such
as acetylcholinesterase, C1q (a complement protein), pulmonary surfactant
apoprotein, several lectins, and type I macrophage scavenger receptor.
The bacteria Streptococcus pyogenes possesses a collagen-like sequence
in enzyme hyaluronidase (Stern, 1992).
One of the first problems that animals had to solve was how to be multicellular.
In a multicellular organism, cells must interact with each other. Once
primitive animal cells began to express the immunoglobulin domain on their
cell surfaces, other cells expressing the same (or slightly modified)
domains could interact with them. As vertebrate animals began to live
longer (and put off reproduction until later in life), the issue of distinguishing
between one's own cells and foreign cells became ever more important and
important families of immunoglobulins within the immunoglobulin superfamily
evolved such as MHC proteins, antibodies, and T-Cell receptors.
A very early duplication of ancestral immunoglobulin/fibronectin molecules
produced fibronectin and immunoglobulin families. The functional domains
of each of these molecules became incorporated into a variety of multi-domain
proteins which can have single or multiple fibronectin domains and single
or multiple immunoglobulin domains.
The immunoglobulin branch of the immunoglobulin/fibronectin superfamily
includes genes known in both vertebrates and invertebrates, including
the simplest invertebrates. An immunoglobulin-like domain is present in
the extracellular part of the receptor tyrosine kinase in some sponges
(Schacke, 1994). Two Ig-like molecules are known from fruit flies, another
from squid, and from sponges (the C2 set of Ig domains) (Schacke, 1994a).
Sponges can distinguish between self and non-self in that a sponge can
reject a graft made from another sponge. Wandering phagocytes combat microbes
in sponges and some flatworms, indicating that a primitive immune resistance
predated the evolution of circulatory systems. Invertebrates do not have
a lymphatic system or any aspects of acquired immunity, but certain parts
of their immune systems do display characteristics observed in vertebrates.
Although there is no complement system, there is a prophenoloxidase system
which involves a cascade, including enzymes which kill microbes and clot
blood. Although invertebrates do not possess antibodies, there are molecules
called lectins (found in bacteria, plants, and all animals) which can
cause foreign cells to clump by binding to sugar groups.
In tunicates, both macrophages and morula cells participate in the rejection
of foreign cells. The numbers of morula cells can increase four times
within the first 2 hours of the reaction. Morula cells are similar to
vertebrate lymphocytes in this feature and in some morphological characterisitics
Lectins are a group of cell surface proteins which, like the immunoglobulin
domain, have been utilized by many animals for immune reactions (Hofer,
2001). Sponges possess lectins homologous to those found in vertebrates
(Gamulin, 1994; Schacke, 1994a). In invertebrates, lectins bind to foreign
particles and facilitate phagocytosis (as do antibodies in vertebrates).
The human protein collectin (related to the lectins) coats foreign particles
before phagocytosis, just as antibodies do (OMIM). Mannose-binding lectin
(MBL) is produced by liver hepatocytes and functions in the blood. Variable
immunoglobulin domains are known in cell surface molecules of plants and
fungi in addition to a diversity of animals, including the most primitive
animals, the sponges (Muller, 2001; Muller, 2001a).