In mealybugs, chromatin condensation is related to both genomic imprin
ting and sex determination. The paternal chromosomal complement is con
densed and genetically inactive in sons but not in daughters. During a
study of chromatin organization in Planococcus lilacinus, digestion w
ith micrococcal nuclease showed that 3% to 5% of the male genome is re
sistant to the enzyme. This Nuclease Resistant Chromatin (NRC) apparen
tly has a nucleosomal organization. Southern hybridization of genomic
DNA suggests that NRC sequences are present in both sexes and occur th
roughout the genome. Cloned NRC DNA is A+T-rich with stretches of aden
ines similar to those present in mouse alpha-satellite sequences. NRC
DNA also contains sequence motifs that are typically associated with t
he nuclear matrix. Salt-fractionation experiments showed that NRC sequ
ences are matrix associated. These observations are discussed in relat
ion to the unusual cytological features of mealybug chromosomes, inclu
ding the possible existence of multiple centres of inactivation.