Heterochromatin of Drosophila Melanogaster

We examined the behavior of an autosomal recessive maternal-effect mutation, abnormal-oocyte (abo), that is located in the euchromatin of the left arm of chromosome 2. When homozygous in females, abo results in a marked reduction in the probability that an egg produced by a mutant mother will develop into an adult. However, this probability is increased if the fertilizing sperm delivers to the egg either a normal allele of the maternal-effect gene or a specific type of heterochromatin (called ABO) that is located in small regions of the X and Y chromosome constitutive heterochromatin as well as in some autosomal heterochromatin. These regions, moreover, all react to Hoechst 33258 fluorescent dye identically and specifically. The amelioration of the maternal effect produced by this heterochromatin differs temporally from that caused by the normal allele of the euchromatic gene: the heterochromatin reduces only precellular blastoderm mortality, whereas the normal allele of the euchromatic gene reduces only postblastoderm mortality. Thus, although the genome of the preblastoderm Drosophila embryo is apparently mostly silent, the ABO-containing heterochromatin functions at this early time. Finally, preliminary data indicate that abo is but one member of a cluster of linked genes, each of which interacts with its own normal allele and with a different, locus-specific, heterochromatic factor. From these observations, it appears that Drosophila heterochromatin contains developmentally important genetic elements, and that a functional concomitant of heterochromatic location is gene action at a developmental stage during which the activity of the euchromatic genome is as yet undetectable. Some general implications of these inferences are considered. N a series of previous reports (SANDLER 1970, 1975; PARRY and SANDLER I 1974), evidence was provided for the existence of an autosomal recessive hypomorphic maternal-effect mutation called abnormal-oocyte ( d o ) that is located in the euchromatin of chromosome 2 some four map units proximal to the dominant marker J and contained within region 32A-E on the standard salivary map. [Information about J and all other gene symbols undefined here is found in LINDSLEY and GRELL ( 1 968).] The abo mutation has no obvious morphological effects in either sex, and no effect of any kind has been detected in males. However, when homozygous in females, abo results in the production of defective eggs whose probability of developing into an adult is reduced to less than onehalf that of eggs from heterozygous sister females. This probability, however, is 'Permanent address: Dipartimento di Genetica e Biologia Molecolare, Citti Universitaria 00185, Roma, Italy. Genetics 109: 701-724 April, 1985. 702 S. PIMPINELLI E T AL.