Cold Spring Harb Symp Quant Biol 42 Pt 2: 1137-46 (1978)[78237819]
Drosophila melanogaster is an ideal organism for the study of the
structure and function of heterochromatin because it has only four
chromosomes and 25% of its genome is heterochromatic. All of the
heterochromatin is constitutive and much of it is located in the sex
chromosomes, which have been well characterized genetically and
cytogenetically (Cooper 1959; Ashburner and Novitski 1976). The study
of Drosophila heterochromatin is also attractive at the molecular
level since the bulk of the DNA of these regions can be isolated as
several discrete satellite DNAs in CsCI equilibrium gradients
(Peacock et al. 1974; Endow et al. 1975; Brutlag at al. 1977a). These
unusual DNAS consist of short nucleotide sequences (5878 base pain
[bp] repeated in tandem arrays over 1,000,000 bp long
(Goldring at 83.1975; Brutlag et al., 1977a).
The biological roles of satellite DNA and heterochromatin have long
been controversial. The molecular properties of satellite DNA make it
unlikely that it is involved in transcription or the normal
regulation of genetic expression. The misconception that satellite
DNA has no essential function is based on the lack of genes in
heterochromatin and on the viability of individuals with large
heterochromatic deletions. However, individual Drosophila males
carrying deletions of heterochromatin have abnormal meiosis,
defective spermatogenesis, and a markedly reduced fertility
(Gershenson 1983; Sandler and Braver 1958; Peacock et al. 1975).
These germ-line aberrations are directly correlated with the failure
of deficient X chromosomes to pair properly during meiosis. Recent
evidence shows that females carrying heterochromatic deletions also
have a defective meiotic mechanism resulting in a reduced level of
recombination (Yamamoto and Miklos 1977). These results indicate a
strong selective pressure against chromosomes carrying
heterochromatic deletions. They also argue strongly for a role of
heterochromatin in the germ line rather than in somatic cells.
Heterochromatin, therefore, may be dispensable for the survival of a
cell or an individual, but it is essential for the survival of a
chromosome in the germ line. A germ-line function is also consistent
with the large variations of heterochromatin and satellite DNA
between closely related species (Hennig and Walker 1970; Sutton and
McCallum 1972; Gall at al. 1974). Indeed, the necessity for proper
meiotic pairing of heterochromatin for successful gametogenesis or
recombination suggests that variations in satellite DNA could lead to
speciation.
To begin an analysis of these meiotic functions at the molecular
level, we have studied the organization of the satellite DNA
sequences that compose the bulk of the heterochromatin of D.
melanogaster. We describe here the properties of two simple-sequence
satellites (1.705 and 1.672 g/cm3) and of a complex satellite (1.688
g/cm3) which has been located primarily in the regions of the X and Y
chromosomes essential for proper meiotic pairing (Gershenson 1940;
Peacock et al., this volume).
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