Discussion

The GxE concept, meaning that genoytpe x environment interactions bring about the phenotype is widely used to describe adaptation phenomena. We propose to extend the initial notion of the GxE concept, and to replace G by “Inheritance system”. This system is composed of several elements: the genotype, the epigenotype, cytoplasmic components but also microorganisms. They interact as an inheritance system with the environment, leading to the development of a phenotype. The elements of this system can be defined using their molecular composition, for instance the DNA as genetic information carrier and then the bearers of epigenetic information such as the chromatin marking system. However, it is not the system itself that generates the phenotype but the developmental process that produces over time and in interaction with the environment a phenotypic trait. In each of these processes, genetic, epigenetic and holobiont diversity can change and result in ephemeral, fluctuating or stable, i.e. heritable phenotypic variations that are important for adaptation. To understand how parasitic worms can sometimes rapidly adapt to changing environments the parasitologist must ‘simply’ define (i) the units or elements of interactions i.e. the boundaries of these elements, and (ii) the types of interactions that interrelates them. When the GxE=>P concept was introduced originally, one of the immediate practical consequences was that breeding programs should be carried out in a range of different environments. Equally, the major consequence of our systems approach to inheritance is that if one wishes to understand the heritability of a trait, all elements of the inheritance system must be analysed comprehensively using a range of different genotypes, epigenotypes and holotypes of the Inheritance System. This is, however, almost never feasible. To cope with the caveat, one should remember that the elements of the Inheritance system are operationally defined and depends on the experimenter. We believe it is legitimate to exclude (operationally) some of the elements from the experiment as long as one does not exclude them from the conclusions and generalizations, e.g. the finding that genetic variants have a strong association with a phenotype does not exclude similar or even stronger epiallelic associations and vice-versa.