12. G E N E T I C C Y T O L O G Y
433
of Drosophila reveals, besides the predominating diffused mesh-work,
some indistinct bands. The heterochromatic proximal segment of the
X chromosome contains about six such bands, and the entire Y chromosome about eight. The heterochromatic segment of chromosome II,
which in somatic metaphase comprises of about ^ the length of the
chromosome, is represented by only a single band in the salivary
chromosome. These observations suggest that a part of the heterochromatin, the ^-heterochromatin, may be similar to euchromatin
(formation of bands in salivary gland, ability to uncoil, and containing
genes), and the remaining part, the a-heterochromatin, is not (Heitz,
1934).
As studies on the morphology and behavior of chromosomes progressed, more and more types of chromatin were found. Application
of the term "heterochromatin" to various types of chromatin somewhat enhanced the complexity of the problem. In treating plant roottips with low temperature (0°C), Darlington and La Cour (1940)
found that many small, intercalated segments of chromosomes, ordinarily identical with other parts, failed to show condensation in metaphase. There were distinct gaps. The "nucleic acid starvation", as
these authors called it, helped to differentiate two types of chromatin.
The segments that showed "starvation" were regarded as heterochromatin. Thus the definition of heterochromatin was out of necessity
changed from the original rather specific category (condensed throughout the cell cycle) to any chromatin that behaves differently from
euchromatin. In the case of Darlington and La Cour's study, the
heterochromatin not only does not maintain a condensed, positively
heteropyknotic state, but becomes negatively heteropyknotic. Therefore, heterochromatin may be either positively or negatively heteropyknotic. Whether the negatively heteropyknotic regions correspond
to the heterochromatin that forms the chromocenter is not known.
T o complicate the situation further, there is the peculiar behavior of
the X chromosome in the spermatogonia and spermatocytes of grasshoppers. In short-horned grasshoppers (Acrididae), males are of the X O
genotype. In the spermatogonial cells (mitosis), the X chromosome can
be identified relatively easily by its negative heteropyknosis. It is thin
and relatively faintly stained. During meiotic prophase, it again is an
outstanding element, this time positively heteropyknotic. At meiotic
metaphase, its behavior is more or less similar to that of other chromosomes and is considered isopyknotic. Thus the same chromosome may
be negatively heteropyknotic, positively heteropyknotic, or showing no
heteropyknosis, depending upon the stage of development. Then is
this chromosome made of heterochromatin in its original sense? As we
have learned, the original heterochromatin described by Heitz and his
433
of Drosophila reveals, besides the predominating diffused mesh-work,
some indistinct bands. The heterochromatic proximal segment of the
X chromosome contains about six such bands, and the entire Y chromosome about eight. The heterochromatic segment of chromosome II,
which in somatic metaphase comprises of about ^ the length of the
chromosome, is represented by only a single band in the salivary
chromosome. These observations suggest that a part of the heterochromatin, the ^-heterochromatin, may be similar to euchromatin
(formation of bands in salivary gland, ability to uncoil, and containing
genes), and the remaining part, the a-heterochromatin, is not (Heitz,
1934).
As studies on the morphology and behavior of chromosomes progressed, more and more types of chromatin were found. Application
of the term "heterochromatin" to various types of chromatin somewhat enhanced the complexity of the problem. In treating plant roottips with low temperature (0°C), Darlington and La Cour (1940)
found that many small, intercalated segments of chromosomes, ordinarily identical with other parts, failed to show condensation in metaphase. There were distinct gaps. The "nucleic acid starvation", as
these authors called it, helped to differentiate two types of chromatin.
The segments that showed "starvation" were regarded as heterochromatin. Thus the definition of heterochromatin was out of necessity
changed from the original rather specific category (condensed throughout the cell cycle) to any chromatin that behaves differently from
euchromatin. In the case of Darlington and La Cour's study, the
heterochromatin not only does not maintain a condensed, positively
heteropyknotic state, but becomes negatively heteropyknotic. Therefore, heterochromatin may be either positively or negatively heteropyknotic. Whether the negatively heteropyknotic regions correspond
to the heterochromatin that forms the chromocenter is not known.
T o complicate the situation further, there is the peculiar behavior of
the X chromosome in the spermatogonia and spermatocytes of grasshoppers. In short-horned grasshoppers (Acrididae), males are of the X O
genotype. In the spermatogonial cells (mitosis), the X chromosome can
be identified relatively easily by its negative heteropyknosis. It is thin
and relatively faintly stained. During meiotic prophase, it again is an
outstanding element, this time positively heteropyknotic. At meiotic
metaphase, its behavior is more or less similar to that of other chromosomes and is considered isopyknotic. Thus the same chromosome may
be negatively heteropyknotic, positively heteropyknotic, or showing no
heteropyknosis, depending upon the stage of development. Then is
this chromosome made of heterochromatin in its original sense? As we
have learned, the original heterochromatin described by Heitz and his