Epstein, H. 1991 . Chromosomal proteins from a mealybug, Planococcus citri. Ph.D. Thesis. Cambridge, UK

Notes: (Ph.D. thesis, Cambridge.) Genomic imprinting is the name given to the process which permits some genes to behave differently according to parental origin. It is the term used to explain the failure of parthenogenesis in all mammals, the selective inactivation of certain chromosomes from the male parent in female mammals and in some insect species, and the parental origin effects seen in the inheritance of some genetic diseases in humans. Mealybugs are interesting because they seem to use genomic imprinting for sex determination. In embryos which are to become male, the entire paternal genome set becomes heterochromatic and inactive at the blastoderm stage, while the maternal set remains active. Somehow, the embryo can tell which chromosomes came from which parent at this stage. How this occurs is completely unknown. There are apparently no sex limited chromosomes in mealybugs with this mechanism of sex determination. HP1 is a protein from Drosphila melanogaster which binds preferentially to heterochromatin on polytene chromosomes and is also a modifier of position effect variegation (PEV). I thought it would be interesting to see if mealybug HP1 homologs could tell the maternal and paternal chromosomes apart. This thesis describes the isolation and sequencing of two HP1 homologues from the mealybug, Planococcus citri. The protein product of one of these genes has been expressed as a fusion protein in E. coli and polyclonal rat antibodies have been raised against it. Immunohistochemistry to mealybug squash preparations has been carried out. The antibodies react very strongly with some male nuclei, but are not specific to the heterochromatic chromosomes. The mealybug fusion protein was found to be a dimer in solution, and according to their sequences, both mealybug proteins contain an internal peptide repeat. The isolation of a DNA repeat sequence which is specific for the supernumerary B chromosomes of another mealybug species, Pseudococcus affinis, is also described.