A male-specific nuclease-resistant chromatin fraction in the mealybug Planococcus lilacinus

Abstract

In mealybugs, chromatin condensation is related to both genomic imprinting and sex determination. The paternal chromosomal complement is condensed and genetically inactive in sons but not in daughters. During a study of chromatin organization in Planococcus lilacinus, digestion with micrococcal nuclease showed that 3% to 5% of the male genome is resistant to the enzyme. This Nuclease Resistant Chromatin (NRC) apparently has a nucleosomal organization. Southern hybridization of genomic DNA suggests that NRC sequences are present in both sexes and occur throughout the genome. Cloned NRC DNA is A+T-rich with stretches of adenines similar to those present in mouse alpha-satellite sequences. NRC DNA also contains sequence motifs that are typically associated with the nuclear matrix. Salt-fractionation experiments showed that NRC sequences are matrix associated. These observations are discussed in relation to the unusual cytological features of mealybug chromosomes, including the possible existence of multiple centres of inactivation.

Conserved characteristics of heterochromatin-forming DNA at the 15q11-q13 imprinting center

“…(mealybugs) (53), heterochromatin formation of entire chromosomes is both gamete of origin-dependent and sex-determining. In mealybugs, the DNA sequences forming heterochromatin have been found to bind to nuclear matrices (54). The recent demonstration of genomic imprinting in Drosophila (55) is of particular interest, because the mechanism appears to involve gamete-determined position effect mediated by adjacent heterochromatic DNA.…”

Heterochromatin and the molecular mechanisms of ‘parent-of-origin’ effects in animals

“…Attempts to identify the 'imprint' that enables discrimination at blastoderm have focussed on the epigenetic signature of paternal chromatin. This has included analysis chromatin structure (Khosla et al 1996(Khosla et al , 1999, histone modification (Bongiorni et al 2009) and DNA methylation (Bongiorni et al 1999;Buglia et al 1999;Mohan and Chandra 2005). Approximately 10% of sperm chromatin is nuclease resistant (Khosla et al 1996).…”

“…This has included analysis chromatin structure (Khosla et al 1996(Khosla et al , 1999, histone modification (Bongiorni et al 2009) and DNA methylation (Bongiorni et al 1999;Buglia et al 1999;Mohan and Chandra 2005). Approximately 10% of sperm chromatin is nuclease resistant (Khosla et al 1996). The sperm and the pro-nucleus it forms in the zygote are enriched in trimethylated lysine 9 of histone H3 (H3K9me3; Bongiorni et al 2009).…”

“…There was no need to posit any imprint carried on the sperm that might direct heterochromatinization. Nevertheless, the identification of epigenetic modifications or lack thereof in paternal chromatin (Khosla et al 1996(Khosla et al , 1999Buglia et al 1999;Mohan and Chandra 2005;Bongiorni et al 1999Bongiorni et al , 2009) is noteworthy, albeit their relationship to an imprint that causes heterochromatinization at blastoderm has yet to be proven. Putting the case for a paternal epigenetic imprint at its highest, depending on the conditioning of the ooplasm by the mother, an imprint on the sperm may be registered at blastoderm leading to heterochromatinization and male development, ignored, or even erased, leaving the chromosomes euchromatic thereby instructing female development ( Figure 1A).…”

Genomic imprinting in the mealybugs

“…This inference, along with the previous observation that the heterochromatic set of chromosomes degenerates after spermiogenesis, would mean that the maternally derived euchromatic chromosomes of the father become heterochromatic in the sons (Brown and Nur, 1964). Thus implying that the signals for heterochromatization in cleavage stages which is present on the paternally contributed genome and absent on the maternally inherited genome of the males, are acquired by the maternally derived euchromatic chromosomes during spermatogenesis (Khosla et al, 1996).…”