Chromosome or Gene Imprinting by Methylation in Insects

Imprinting is a reversible, differential marking of genes or chromosomes that is determined by the sex of the parent from whom the genetic material is inherited. One method of imprinting DNAin some organisms involves methylation. Imprinted genes or chromosomes behave differently.

Methylation of cytosines at the carbon 5 position of CpG dinucleotides is common in many prokaryotes and eukaryotes (Colot and Rossignol 1999, Ng and Bird 1999). In prokaryotes, methylation is apparently part of a defense system against invading DNA parasites. However, in eukaryotes methylation can be associated with several functions, including inhibiting transcription initiation, arresting transcript elongation, serving as a signal for imprinting, and suppressing homologous recombination (Colot and Rossignol 1999). Usually, methylated DNA is inactive or expressed at a very low level. In mammals, DNA methylation is common, and mammalian DNA contains about 1 to 2% of the cytosine as 5-methylcytosine. In mammals, DNA methylation regulates chromatin structure, gene repression, parental imprinting, andX-chromosomeinactivation in females. However, DNA methylation is not always the cause of gene inactivity.

The role of methylation in insects is controversial. In some insects, such as scale insects or mealybugs, differential marking of paternally transmitted chromosomes leads to the inactivation or elimination of the paternal chromosomes (White 1973). For example, citrus mealybug males and females develop from fertilized eggs, and there are no sex chromosomes. However, in embryos destined to be males, one haploid set of chromosomes becomes heterochromatic and remains so in most of the tissues. Thus, males are functionally haploid because one chromosomal set, invariably the chromosomes inherited from the father, is inactivated. Bongiorni et al. (1999) found that the chromosomes derived from the citrus mealybug fathers actually are hypomethylated (containing a lower amount of methylation of the DNA) than are the chromosomes from the females. As a result, the mechanism by which imprinting and heterochromatinization occurs is unknown in these mealybugs, but methylation of the DNA is not involved (Buglia et al. 1999).

The amount of DNA methylation observed in insects varies. DNA methylation has been found in homopterans, including Megoura viciae, Planococcus lilacius, Pseudo-coccus calceolariae, P. obscurus, and Myzus persicae; orthopterans, including Locusta migratoria, Eyprepocnemis ploras, Pyrgomorpha conica, Gryllotalpa fossor, and Baetica ustalata; and the lepidopteran Bombyx mori (Manicardi et al. 1994, Regev et al. 1998). Differential expression of alleles due to imprinting has been observed in Drosophila melanogaster (Golic et al. 1998, Regev et al. 1998), but the subject of DNA methylation in D. melanogaster has been controversial (Gowher et al. 2000, Lyko et al. 2000). Gowher et al. (2000) estimated that D. melanogaster DNA has approximately 50 times less methylcytosine than do mammals, which explains why it is difficult to detect methylation in Drosophila. However, using a sensitive detection method, 5-methylcytosine was found in all stages of D. melanogaster development (Gowher et al. 2000). DNA methylation appears to be lacking in some other Diptera (D. virilis, Sciara coprophila, Musca domestica, Sarcophaga bullata) or present at only low levels (Culex bitaeniorhynchus, Chironomus plumosus, Anopheles maculipennis, Aedes albopictus) (Regev et al. 1998, Tweedie et al. 1999).

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