Recently, epigenetics has had an ever-growing impact on research not only for its intrinsic interest but also because it has been implied in biological phenomena, such as tumor emergence and progression. The first epigenetic phenomenon to be described in the early 1960s was chromosome imprinting in some insect species (sciaridae and coccoideae). Here, we discuss recent experimental results to dissect the phenomenon of imprinted facultative heterochromatinization in Lecanoid coccids (mealybugs). In these insect species, the entire paternally derived haploid chromosome set becomes heterochromatic during embryogenesis in males. We describe the role of known epigenetic marks, such as DNA methylation and histone modifications, in this phenomenon. We then discuss the models proposed to explain the noncanonical chromosome cycle of these species. 1. Epigenetics The first appearance of the term epigenetics can be ascribed to Conrad Waddington, who stated in 1942 that “epigenetics is the branch of biology which studies the causal interactions between genes and their products, which bring the phenotype into being” [1]. In the modern view, epigenetics encompasses all those hereditary (genetic) phenomena not depending on the DNA sequence itself but on some functionally relevant molecular signatures which are imposed over the sequence (“epi” in Greek means “over”). All the systems involved in gene expression regulation are based on interactions between proteins and DNA. Some mechanisms inhibit or activate the expression of a single gene, acting on the promoter region, and thus reflect the structural organization of the gene itself (gene regulation). However, the epi-genetic systems can regulate phenotypic expression regardless of the gene sequence and are transmitted from one cell generation to the next one or from the parents to their progeny. These systems modulate the functional behavior of chromosomal regions, entire chromosomes, or even whole sets of chromosomes [2]. According to Denise Barlow “epigenetics has always been all the weird and wonderful things that cannot be explained by genetics.” Epigenetic phenomena occur in all the kingdoms from yeast to metazoans and plants. Some are limited to just one or few species. For example, RIP (rearrangement induced premeiotically) [3] and MIP (methylation induced premeiotically) [4] were reported in fungi, where they seem to protect the genome from transposable elements. The term paramutation, on the other hand, was coined to describe a heritable change in gene expression of an allele imposed by the presence of
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