**8. Conclusions**

Gene duplication is a common feature of eucaryotic genomes although the degree varies among gene families and loci. It is estimated that ~5% of the human genome is comprised of duplicated genes (Lewin, 2004). Among these are genes of the immunoglobulin supergene family (IGSF). This polygeny is widespread in loci important to the immune system as well as

Immunoglobulin Polygeny: An Evolutionary Perspective 133

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encoding proteins with only indirect roles in immunity. The -barrel encoded by genes of the IGSF has obviously been a successful structural motif which can explain its conservation during evolution and diversification into many variants. While IGSF polygeny is widespread, the degree of polygeny is especially pronounced among those that encode the variable heavy and light chain genes of antibodies and the T cell receptor (TCR). Early estimates suggested there were as many as 1000 variable heavy (VH) genes in mice and hundreds in humans. While subsequently studies, including genome projects, have lowered the number of VH genes to ~100-150 in these species, this is still a very large number of homologous genes to occupy a single locus. Similar duplication is seen among genes encoding the variable light chain genes, i.e. V and V. However, there are large variations in the numbers and features of duplicated VH and VL genes among mammals and other vertebrates.

This article surveys the duplicated Ig genes in a number of species and uses examples indicating that Ig polygeny resulted from a combination of duplication and genomic gene conversion. Since understanding the evolutionary forces at work in this process requires some understanding of the role played by these duplicated genes in humoral immunity, we review the processes involved in the generation of the antibody repertoire such as Ig gene segment recombination, junctional diversity, somatic hypermutation (SHM) and somatic gene conversion (SGC). We review these processes in various vertebrates but focus on data obtained using the neonatal and newborn piglet model to suggest that evolutionary improvements in somatic processes have reduced the need for the Ig polygeny that evolved among lower vertebrates. We also describe the more recent duplication of the Cgenes of mammals that indicates the process was similar. C genes encode the subclasses of mammalian IgG, the "flagship mammalian antibody" that is unique to this vertebrate class. Since this duplication event occurred more recently, we thought it could provide insight into the advantages conferred by gene duplication.

We propose that the extensive polygeny of VH, Vand Vgenes among vertebrates gave adaptive advantage to the earliest vertebrates for generating a diverse repertoire of antibody specificities much as the more recent evolutionary diversification of C genes resulted in IgG subclass antibodies with different effector functions. We suggest that the evolutionary appearance of mechanisms to somatically alter V-region genes reduced the importance of polygeny in V-region loci for certain mammals and birds. In higher mammals these mechanisms make it possible for a complete functional repertoire to be generated using just one or a few VH genes. This hypothesis can explain why so many of the V-region genes of higher mammals are seldom used, and why deletions of VH genes and C genes have no effect. We propose that these genes remain as evolutionary vestiges or redundant back-ups in the genome in a manner that parallels the retention of IgD in most mammals. An alternative hypothesis is that the extensive somatic recombination which characterizes the variable region loci (Section 4) creates instability that promotes duplication and genomic gene conversion. In any case, these hypotheses challenge the existing paradigm that random VH, DH and JH recombination among the many gene segments is necessary for survival (presented in immunology textbooks) by placing Ig polygeny into evolutionary perspective.

### **9. Acknowledgement**

The authors acknowledge the Molecular Cell Biology Program of the National Science Foundation (USA) and Biological Mimetics of Fredrick, Md for their support of the studies described.
