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Chromosome imprinting is the process by which one of two genetically homologous chromosomes is predetermined to function differently from the other at a subsequent stage in development. In the coccid insects, imprinting occurs in the egg, at the time of fertilisation; it probably occurs at the same time and site in mammals, and possibly also in Sciara.

THE mealy bug, P. citri, affords a unique opportunity for analysis of biochemical changes underlying chromosome behaviour. In the male embryos, the entire paternal set, that is, one half the total number of chromosomes, remains condensed in the non-dividing nuclei; this facultative behaviour is similar to that of typical heterochromatic elements such as sex chromosomes1,2. Experiments with tritiated thymidine show that, like heterochromatic elements in other organisms3,4, the paternal set in the males reproduces asynchronously from the maternal, or euchromatic, set5. As measured by dominant lethality2 and mutant genes6, the paternal set is genetically inert in the males, but some ‘residual’ influences can be demonstrated7,8.

THE chromosome constitution of marsupial, Potorous tridactylis, was described by Sharman and Barber1. The males have five pairs of autosomes and three sex chromosomes. The low number of chromosomes plus the fact that the same investigators reported somatic pairing made this species desirable for genetic studies in tissue cultures.

UNLIKE gene mutations the expression of which may be subject to considerable modification, chromosomal changes will remain the same whether they occur as sporadic aberrations or become incorporated in the karyotype of a species. If the complements of two closely related species differ, spontaneous chromosomal mutations paralleling the karyotypic differences should be demonstrable in the same group of organisms.