Reply from Ephraim Fuchs

Immunoglobulin genes, reproductive isolation and vertebrate speciation

Reproductive isolation drives the formation of new species, and many genes contribute to this through Dobzhansky–Muller incompatibilities (DMIs). These incompatibilities occur when gene divergence affects loci encoding interacting products such as receptors and their ligands. We suggest here that the nature of vertebrate immunoglobulin (IG) genes must make them prone to DMIs. The genes of these complex loci form functional genes through the process of recombination, giving rise to a repertoire of heterodimeric receptors of incredible diversity. This repertoire, within individuals and within species, must defend against pathogens but must also avoid pathogenic self‐reactivity. We suggest that this avoidance of autoimmunity is only achieved through a coordination of evolution between heavy‐ and light‐chain genes, and between these genes and the rest of the genome. Without coordinated evolution, the hybrid offspring of two diverging populations will carry a heavy burden of DMIs, resulting in a loss of fitness. Critical incompatibilities could manifest as incompatibilities between a mother and her divergent offspring. During fetal development, biochemical differences between the parents of hybrid offspring could make their offspring a target of the maternal immune system. This hypothesis was conceived in the light of recent insights into the population genetics of IG genes. This has suggested that antibody genes are probably as susceptible to evolutionary forces as other parts of the genome. Further repertoire studies in human and nonhuman species should now help determine whether antibody genes have been part of the evolutionary forces that drive the development of species.

Moving beyond the immune self?

We are witnessing a significant challenge to immunology's basic tenet, the immune self. Such an 'entity' is increasingly regarded as polymorphous and ill defined as transplantation biology and autoimmunity have demonstrated phenomena that fail to allow faithful adherence to a strict dichotomy of self/nonself discrimination. Instead of searching for elusive criteria of 'self' and 'other', immune responses are increasingly studied as arising within complex contexts, which determine various degrees of reactivity or dormancy. When the character of the immune 'object' is determined by the context in which it appears, not its character as 'foreign' per se, self/nonself discrimination recedes as a governing principle. In such context-based models, 'ecologic' controls arise from the entire organism in which the immune system is fully integrated. In these systems, subject-object relationships become blurred. Viewed from this perspective, a new theoretical construction of the immune system, one originally proposed by Jerne, is contending with Burnet's theory of immune identity. Although it is too early to judge which theory will prove more capacious, it is already apparent that Jerne's formulation has had a decisive impact in shaping new models of immunity.