114
Μ. J. D. WHITE
and that chromosomes with two or more centromeres are usually unstable because from time to time the centromeres in the same daughter
chromosome pass to opposite poles of the spindle at anaphase. Thus
in such groups only monocentric
chromosomes can survive. In certain
other groups of animals, however, it is fairly certain that the situation
is radically different. Thus in the insects of the orders Homoptera and
Heteroptera centromeres cannot be seen by direct observation, and the
orientation and maneuvers of the chromosomes strongly suggest that no
localized centromere region exists, the chromosomes being connected
to the spindle along their whole length (Schräder, 1935, 1947). And in
such groups when a chromosome is broken by irradiation into several
pieces each fragment attaches itself to the spindle at subsequent mitoses (Hughes-Schrader and Ris, 1941; Ris, 1942). There has been
considerable discussion as to whether these chromosomes possess many
centromeres distributed along their length or a "diffuse centromere
"activity," and the question cannot be regarded as settled. It is possible
that intermediate conditions between "monocentric' and ' polycentric"
chromosomes exist, and it may be, that some homopteran and heteropteran chromosomes, although polycentric, have the centromeric
activity mainly concentrated in the middle region. In at least one species
of pentatomid bug Dutt (1955) has produced evidence that the chromosomes are monocentric. The meiotic mechanism of species with polycentric chromosomes differs in certain respects from that of other organisms (Hughes-Schrader, 1948; Helenius, 1952; Suomalainen, 1953).
In animal species with monocentric chromosomes the centromere almost always seems to lie in a heterochromatic region. It is not clear
why this should be so, but it is a fact of considerable importance for
the processes of chromosomal evolution, since structural rearrangements
involving loss or gain of a centromere (together with a minute region
on either side) can occur with less disturbance of genetic balance than
if the centromere were situated in a genetically active region.
In the majority of animal groups a practical difference exists between
chromosomes whose centromeres are situated very close to one end
(acrocentrics)
and ones which have the centromere near the middle of
their length (metacentrics).
Both types are in reality two-armed structures; but in the former one arm is very minute, probably in all cases
heterochromatic and hence almost negligible, genetically. Thus the real
difference between acrocentric and metacentric chromosomes is that the
former have only one genetically significant limb, whereas the latter have
two, which may be cytologically equal, subequal, or quite unequal in
length. Some species of animals have all their chromosomes of one type or
the other, while other species have a karyotype that includes some elements of each type. Lima-de-Faria (1956) has recently claimed that in the
Μ. J. D. WHITE
and that chromosomes with two or more centromeres are usually unstable because from time to time the centromeres in the same daughter
chromosome pass to opposite poles of the spindle at anaphase. Thus
in such groups only monocentric
chromosomes can survive. In certain
other groups of animals, however, it is fairly certain that the situation
is radically different. Thus in the insects of the orders Homoptera and
Heteroptera centromeres cannot be seen by direct observation, and the
orientation and maneuvers of the chromosomes strongly suggest that no
localized centromere region exists, the chromosomes being connected
to the spindle along their whole length (Schräder, 1935, 1947). And in
such groups when a chromosome is broken by irradiation into several
pieces each fragment attaches itself to the spindle at subsequent mitoses (Hughes-Schrader and Ris, 1941; Ris, 1942). There has been
considerable discussion as to whether these chromosomes possess many
centromeres distributed along their length or a "diffuse centromere
"activity," and the question cannot be regarded as settled. It is possible
that intermediate conditions between "monocentric' and ' polycentric"
chromosomes exist, and it may be, that some homopteran and heteropteran chromosomes, although polycentric, have the centromeric
activity mainly concentrated in the middle region. In at least one species
of pentatomid bug Dutt (1955) has produced evidence that the chromosomes are monocentric. The meiotic mechanism of species with polycentric chromosomes differs in certain respects from that of other organisms (Hughes-Schrader, 1948; Helenius, 1952; Suomalainen, 1953).
In animal species with monocentric chromosomes the centromere almost always seems to lie in a heterochromatic region. It is not clear
why this should be so, but it is a fact of considerable importance for
the processes of chromosomal evolution, since structural rearrangements
involving loss or gain of a centromere (together with a minute region
on either side) can occur with less disturbance of genetic balance than
if the centromere were situated in a genetically active region.
In the majority of animal groups a practical difference exists between
chromosomes whose centromeres are situated very close to one end
(acrocentrics)
and ones which have the centromere near the middle of
their length (metacentrics).
Both types are in reality two-armed structures; but in the former one arm is very minute, probably in all cases
heterochromatic and hence almost negligible, genetically. Thus the real
difference between acrocentric and metacentric chromosomes is that the
former have only one genetically significant limb, whereas the latter have
two, which may be cytologically equal, subequal, or quite unequal in
length. Some species of animals have all their chromosomes of one type or
the other, while other species have a karyotype that includes some elements of each type. Lima-de-Faria (1956) has recently claimed that in the