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locations in which the breaks are not adjacent to the centromeres, i.e.,
rings or chains of four chromosomes will be formed, and a certain degree
of sterility will usually result.
Centric fusions result from two proximal breaks. There may be another type of fusion in which the break in one chromosome is proximal
while that in the other is distal, i.e., adjacent to the telomere. We may
call such rearrangements tandem fusions. They may take place between
two acrocentrics or between an acrocentric and a metacentric (if the
break is proximal in the former, distal in the latter), but not between two
metacentrics. The process of tandem fusion, like centric fusion, involves the loss of minute segments.
In the case of polycentric chromosomes, such as those of the Homoptera and Heteroptera, there is probably no distinction between proximal
and distal ends. Thus there is no real difference between "centric" and
"tandem" fusions in such a group. All fusions will simply result from
subterminal breaks in two chromosomes, the minute end sections being
lost as in other types of fusions.
Centric and tandem fusions lead to a decrease in chromosome number. Apart from losses of whole chromosomes from the karyotype
(which will almost always be lethal or sublethal), this is probably the
only way whereby evolutionary decreases in chromosome number can
come about.
It is more difficult to understand how chromosome numbers can be
increased in evolution, except for polyploidy and reduplication of whole
chromosomes (the latter will probably upset the genie balance so seriously as to cause near-lethality in most instances) . Simple fragmentation
(due to single breaks) will merely give rise to two pieces lacking telomeres. And in the case of monocentric chromosomes one of these fragments will also lack a centromere. It is generally agreed that the acentric
fragment will be unable to persist, but if Muller's conclusions as to the
inviability of chromosomes lacking one telomere can be extended from
Drosophila to other animals both fragments will be nonviable. Thus the
process of simple fragmentation, which has been invoked by almost all
writers on chromosomal evolution to account for increases in chromosome number, must be abandoned as an explanation for such increases,
even in species with multiple or diffuse centromeres where it seemed
plausible to a number of cytologists in the past.
It follows from the above discussion that as far as monocentric chromosomes are concerned, any "fragmentation," in order to be viable and to
lead to an increase in chromosome number, requires a "donor" chromosome to provide a centromere and two telomeres. In the case of chromosomes with multiple or diffuse centromeres only the telomeres will be
Μ. J. D. WHITE
locations in which the breaks are not adjacent to the centromeres, i.e.,
rings or chains of four chromosomes will be formed, and a certain degree
of sterility will usually result.
Centric fusions result from two proximal breaks. There may be another type of fusion in which the break in one chromosome is proximal
while that in the other is distal, i.e., adjacent to the telomere. We may
call such rearrangements tandem fusions. They may take place between
two acrocentrics or between an acrocentric and a metacentric (if the
break is proximal in the former, distal in the latter), but not between two
metacentrics. The process of tandem fusion, like centric fusion, involves the loss of minute segments.
In the case of polycentric chromosomes, such as those of the Homoptera and Heteroptera, there is probably no distinction between proximal
and distal ends. Thus there is no real difference between "centric" and
"tandem" fusions in such a group. All fusions will simply result from
subterminal breaks in two chromosomes, the minute end sections being
lost as in other types of fusions.
Centric and tandem fusions lead to a decrease in chromosome number. Apart from losses of whole chromosomes from the karyotype
(which will almost always be lethal or sublethal), this is probably the
only way whereby evolutionary decreases in chromosome number can
come about.
It is more difficult to understand how chromosome numbers can be
increased in evolution, except for polyploidy and reduplication of whole
chromosomes (the latter will probably upset the genie balance so seriously as to cause near-lethality in most instances) . Simple fragmentation
(due to single breaks) will merely give rise to two pieces lacking telomeres. And in the case of monocentric chromosomes one of these fragments will also lack a centromere. It is generally agreed that the acentric
fragment will be unable to persist, but if Muller's conclusions as to the
inviability of chromosomes lacking one telomere can be extended from
Drosophila to other animals both fragments will be nonviable. Thus the
process of simple fragmentation, which has been invoked by almost all
writers on chromosomal evolution to account for increases in chromosome number, must be abandoned as an explanation for such increases,
even in species with multiple or diffuse centromeres where it seemed
plausible to a number of cytologists in the past.
It follows from the above discussion that as far as monocentric chromosomes are concerned, any "fragmentation," in order to be viable and to
lead to an increase in chromosome number, requires a "donor" chromosome to provide a centromere and two telomeres. In the case of chromosomes with multiple or diffuse centromeres only the telomeres will be