An analysis of histone modifications in relation to sex-specific chromatin organization in the mealybug Maconellicoccus hirsutus.

The male-specific heterochromatization of the paternal genome, observed in coccids, is an example of both genomic imprinting and differential regulation of homologous chromosomes. We observed a highly nuclease-resistant chromatin (NRC) organization of a part of the paternal genome in males of Maconellicoccus hirsutus as reported earlier in Planococcus lilacinus. The nuclease resistance of NRC is correlated with nuclear matrix association and is lost when NRC is dissociated from the matrix. We carried out a comparative analysis of epigenetic modifications of histones in matrix associated chromatin of male and female mealybugs by ELISA. We detected H3K27me3, H4K20me3, H3K9me2 and H3K4me3 in both males and females and observed significant enrichment of H3K27me3 in the nuclear matrix of males compared to that of females. To further examine the presence of NRC in the germ line, nuclei were sorted based on chromatin compaction. The analysis of sorted nuclei indicates the presence of NRC in nuclei with different DNA content including the haploid nuclei from males. We discuss these results in the light of the presence of NRC exclusively in male nuclei and the retention of the maternal genome in sperm nuclei of mealybugs.

Genomic imprinting in the mealybugs.

The coccid insects (Hemiptera; Sternorrhyncha; Aphidiformes; Coccoidea; Pseudococcidae) are well suited to study not only the mechanisms of genomic imprinting but also facultative heterochromatization, a phenomenon well exemplified by inactivation of the X chromosome in female mammals. Coccids show sex-specific heterochromatization of an entire set of chromosomes and transcriptional silencing of all the paternally contributed chromosomes in males. Thus, genomic imprinting and the resultant differential regulation operate on 50% of the genome in contrast to the single X chromosome in female mammals. A significant insight into the phenomenon of genomic imprinting has come from very elegant cytological analysis of the coccid system. Recently, efforts have been made to dissect out at the molecular level the phenomenon of genomic imprinting in these insects. The present review summarizes both of these aspects. In light of the accruing experimental evidence for chromatin-based differences in the maternal and paternal genomes, it appears that the mealybug system may provide evidence for stable maintenance of chromatin code not only through mitosis but also through meiosis.

Sex-specific organisation of middle repetitive DNA sequences in the mealybug Planococcus lilacinus.

Differential organisation of homologous chromosomes is related to both sex determination and genomic imprinting in coccid insects, the mealybugs. We report here the identification of two middle repetitive sequences that are differentially organised between the two sexes and also within the same diploid nucleus. These two sequences form a part of the male-specific nuclease-resistant chromatin (NRC) fraction of a mealybug Planococcus lilacinus. To understand the phenomenon of differential organisation we have analysed the components of NRC by cloning the DNA sequences present, deciphering their primary sequence, nucleosomal organisation, genomic distri-bution and cytological localisation. Our observations suggest that the middle repetitive sequences within NRC are functionally significant and we discuss their probable involvement in male-specific chromatin organisation.

Triplet repeat polymorphism & fragile X syndrome in the Indian context.

Mental retardation due to fragile X syndrome is one of the genetic disorders caused by triplet repeat expansion. CGG repeat involved in this disease is known to exhibit polymorphism even among normal individuals. Here we describe the development of suitable probes for detection of polymorphism in CGG repeat at FMR1 locus as well as the diagnosis of fragile X syndrome. Using these methods polymorphism at the FMR1 locus has been examined in 161 individuals. Ninety eight patients with unclassified mental retardation were examined, of whom 7 were found to have the expanded (CGG) allele at the FMR1 locus. The hybridization pattern for two patients has been presented as representative data.

Chromosomal fragility and human genetic disorders.

The first report of X-linked mental retardation correlated with the presence of marker chromosome came in 1940. It was in 1990 that the molecular basis of fragile X syndrome was deciphered. This elucidation marked the discovery of a novel process of mutation designated as dynamic mutations, resulting in the expansion of a triplet repeat sequence within the human genome. Subsequently several human genetic disorders involving triplet repeat expansion have been discovered. Almost all the disorders are known to affect the nervous system and/or the brain. This review presents an overview of fragile sites in the genome and the molecular genetics of fragile X syndrome.

Detection of a CpA methylase in an insect system: characterization and substrate specificity.

A cytosine-specific DNA methyltransferase (EC 2.1.1.37) has been purified to near homogeneity from a mealybug (Planococcus lilacinus). The enzyme can methylate cytosine residues in CpG sequences as well as CpA sequences. The apparent molecular weight of the enzyme was estimated as 135,000 daltons by FPLC. The enzyme exhibits a processive mode of action and a salt dependence similar to mammalian methylases. Mealybug methylase exhibits a preference for denatured DNA substrates.

Studies on 1-methyl adenine transfer RNA methyltransferase of Mycobacterium smegmatis.

The presence of 1-methyl adenine in transfer RNA is a feature that Mycobacterium smegmatis shares with only a few other prokaryotes. The enzyme 1-methyl adenine tRNA methyl transferase from this source has been purified and the preliminary results show the presence of two activity peaks with different substrate specificity.

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

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.