ABSTRACT
ELENA GÓMEZ-DI ´AZ, MIREIA JORDA `, MIGUEL ANGEL PEINADO, and ANA RIVERO
WHAT IS EPIGENETICS?
Few areas in biology attract as much current attention and yet require as much presentation as the field of epigenetics. The term “epigenetics” was first used by Waddington to describe the process through which genotypes give rise to phenotypes during development [1]. Since then, there has been a burgeoning interest in the field of epigenetics that has been coupled with a diversification in the use of the term: epigenetics means different things to the different fields of biology, and even within a given field, different authors may use it in somewhat different contexts, generating a great deal of confusion in the process [2]. Broadly speaking, epigenetics refers to stimuli-triggered changes in gene expression due to processes that arise independent of changes in the underlying DNA sequence. Some of these processes have been elucidated and include DNA methylation [3], histone modifications and chromatin-remodeling proteins [4], and DNA silencing by noncoding RNAs (ncRNA) [5]. This general definition of “epigenetics” is, however, used in two broadly different contexts. For some authors, the term “epigenetics” includes all transient changes in gene expression that occur at the individual cell level, as well as those that are propagated during mitosis in multicellular organisms and remain stable at the time scale of an individual (Figure 1). For clarity, we refer to this as epigenetic plasticity (see [6]). A good example is the development of morphologically different castes of bees from genetically identical individuals through nutritionally triggered DNA methylation [7]. Yet for other authors, and most
notably for evolutionary biologists, the term epigenetics refers exclusively to epigenetic inheritance: the stimuli-triggered variation in gene expression that is heritable across generations. Here, the epigenetic changes are generated in the germ line in multicellular organisms (either directly or indirectly, see Jablonka and Raz [8]) or maintained clonally in single-cell organisms (Figure 1) [8,9]. A classic example of transgenerational epigenetic inheritance involves a change in flower symmetry from bilateral to radial in Linaria vulgaris, which relates to different levels of methylation of the geneLcyc [10]. In this review, we contend that both epigenetic plasticity and epigenetic inheritance are important in shaping host-pathogen interactions, and thus we use the term “epigenetics” to encompass both of these definitions.
