124
Μ. J. D. WHITE
majority of the fusions that have become established seem to have been
between the larger elements of the chromosome set (White 1954;
Matthey, 1954a, p. 32). In the case of many fusions between smaller
chromosomes, interference across the centromere will probably be strong
enough to prevent the regular formation of a chiasma in both arms, so
that fusion heterozygotes would have their fertility seriously reduced.
These considerations may also explain why relatively few fusions and
"fragmentations" have established themselves in the Heteroptera and
Homoptera, with their poly centric chromosomes. It is well known that
in these groups there is seldom more than a single chiasma per bivalent,
and interference is presumably strong. Also, it may be that the polycentric condition is ill suited for ensuring regular meiotic segregation of
the fused and unfused elements to opposite poles even if interference
does permit the regular formation of a trivalent. Schräder (1947) reasoned that "fragmentations" should establish themselves more easily in
a group with diffuse or multiple centromeres and pointed to the high
frequency of multiple sex chromosome mechanisms in the Heteroptera
and Homoptera in support of this view. It is significant that in these
groups the regular segregation of the sex chromosomes in the male (apart
from a few special cases) does not depend on chiasma formation. Thus
the considerations discussed above possibly explain the difference between the evolutionary behavior of the autosomes and the sex chromosomes in these two orders of insects (in which multiple sex chromosome
systems are not the result of X-autosome and Y-autosome fusions as
they are in many other groups).
Tandem fusions in groups with monocentrie chromosomes, whether
between two acrocentrics or between an acrocentric and a metacentric,
will in most cases give rise to extremely awkward meiotic configurations when heterozygous; and it is difficult to see how most of these
could give regular segregation. No clear case of a natural polymorphism based on a tandem fusion seems to have been recorded, and it is
likely that most of the cytotaxonomic differences which look as if they
were due to tandem fusions have arisen by centric fusion followed by
pericentric inversion, as suggested by Patterson and Stone (1952).
In species with more than one pair of acrocentric chromosomes the
way is wide open for centric fusions to occur, and such fusions must
surely take place repeatedly in a species with several pairs of acrocentric
chromosomes, provided that the number of individuals comprising the
species is large enough. Whether fusions will survive for any considerable number of generations is another matter. They will be eliminated
from the population if they are not genetically adaptive or if the meiotic
configurations in the heterozygotes undergo frequent misorientation,
Μ. J. D. WHITE
majority of the fusions that have become established seem to have been
between the larger elements of the chromosome set (White 1954;
Matthey, 1954a, p. 32). In the case of many fusions between smaller
chromosomes, interference across the centromere will probably be strong
enough to prevent the regular formation of a chiasma in both arms, so
that fusion heterozygotes would have their fertility seriously reduced.
These considerations may also explain why relatively few fusions and
"fragmentations" have established themselves in the Heteroptera and
Homoptera, with their poly centric chromosomes. It is well known that
in these groups there is seldom more than a single chiasma per bivalent,
and interference is presumably strong. Also, it may be that the polycentric condition is ill suited for ensuring regular meiotic segregation of
the fused and unfused elements to opposite poles even if interference
does permit the regular formation of a trivalent. Schräder (1947) reasoned that "fragmentations" should establish themselves more easily in
a group with diffuse or multiple centromeres and pointed to the high
frequency of multiple sex chromosome mechanisms in the Heteroptera
and Homoptera in support of this view. It is significant that in these
groups the regular segregation of the sex chromosomes in the male (apart
from a few special cases) does not depend on chiasma formation. Thus
the considerations discussed above possibly explain the difference between the evolutionary behavior of the autosomes and the sex chromosomes in these two orders of insects (in which multiple sex chromosome
systems are not the result of X-autosome and Y-autosome fusions as
they are in many other groups).
Tandem fusions in groups with monocentrie chromosomes, whether
between two acrocentrics or between an acrocentric and a metacentric,
will in most cases give rise to extremely awkward meiotic configurations when heterozygous; and it is difficult to see how most of these
could give regular segregation. No clear case of a natural polymorphism based on a tandem fusion seems to have been recorded, and it is
likely that most of the cytotaxonomic differences which look as if they
were due to tandem fusions have arisen by centric fusion followed by
pericentric inversion, as suggested by Patterson and Stone (1952).
In species with more than one pair of acrocentric chromosomes the
way is wide open for centric fusions to occur, and such fusions must
surely take place repeatedly in a species with several pairs of acrocentric
chromosomes, provided that the number of individuals comprising the
species is large enough. Whether fusions will survive for any considerable number of generations is another matter. They will be eliminated
from the population if they are not genetically adaptive or if the meiotic
configurations in the heterozygotes undergo frequent misorientation,