Mealybug Chromosome Cycle as a Paradigm of Epigenetics

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.

Segmental pycnotic reversion of facultative heterochromatin during the second male meiosis in tettigonioid species (Orthoptera)

The X chromosome invariably exhibits isopycnosis during the most condensed stages of male meiosis after conventional staining procedures in all studied species of Tettigonioidea (Orthoptera). However, differential negative staining of the X chromosome can be achieved during these stages by a silver impregnation method after extraction of chromosomal proteins with 2 × SSC. This differential response to silver may be indicative of a distinctive chromatin organization in the X chromosome with respect to that of the autosomes. Following 2 × SSC – silver impregnation treatment, pycnotic reversion of part of the X chromosome can be observed from the metaphase II stage on in tettigonioids. Pycnotic reversion is initiated from the telomeric region of the X chromosome in the subfamily Ephyppigerinae, while in the subfamily Pycnogastrinae pycnotic reversion occurs at the centromeric region of this chromosome. These two modes of segmental pycnotic reversion may indicate the onset of a reversion of the facultative heterochromatinized state of the X chromosome (euchromatinization) during male meiosis in tettigonioids. An initiation center may be postulated for this process that is located either at the end distal to or proximal to the centromere of the X chromosome. This hypothesis is supported by the observed pycnotic behaviour of neo-X chromosomes in two different neo-XY systems, one involving centric and the other tandem fusion. Facultative heterochromatinization never involves the autosomally derived segments (XR) of the neo-X chromosomes. Occasional failures in the heterochromatinization process of part of the X chromosome, as well as of the neo-X chromosome, in two species suggests that both facultative heterochromatinization and its reversion are two processes that act segmentally. Key words: male meiosis, X chromosome, facultative heterochromatin, segmental euchromatinization.