112
Μ. J. D. WHITE
tropis and Circotettix, where pericentric rearrangements are of common
occurrence in natural populations, some of these have reached fixation
and now constitute "species characters" (details in White, 1954), while
in many other grasshopper genera such rearrangements do not occur
either within species or as cytotaxonomic characters. And in groups
where the species may be distinguished by the extent and distribution
of heterochromatic segments in the chromosomes, populations occur in
which the extent of such segments is variable, so that cytologically
heterozygous individuals with "unequal bivalents" at meiosis occur. The
only serious discrepancy in the correlation between cytological differences within and between the species of a group (apart from the apparent but probably unreal one in the vertebrates referred to earlier) seems
to be in the case of forms with supernumerary chromosomes. It must
be admitted that there are groups, such as many grasshopper genera,
in which supernumerary chromosomes (largely, but not entirely heterochromatic ) are of common occurrence in wild populations, but in which
such elements seem never to have reached fixation as regular members
of the karyotype. This is rather a special case, however. The inheritance
of such chromosomes, due to various peculiarities of their behavior at
mitosis or meiosis, is inherently irregular in most cases. Thus the only
way in which the genetic material contained in them can be expected to
undergo fixation is by incorporation in chromosomes whose mitotic and
meiotic behavior is regular.
There is, in fact, a close parallelism between the occurrence of
supernumerary chromosomes as separate elements and supernumerary
regions which are attached to ordinary autosomes and which, in the
heterozygous state, give rise to "unequal bivalents" of the type previously
mentioned. Thus in a number of species of the grasshopper genera
Trimerotropis and Circotettix both unequal bivalents and supernumerary
chromosomes are found, either in the same or different local populations.
In the Australian grasshopper Cryptobothrus
chrysophorus
both types
of polymorphism occur, but do not usually coexist in the same locality
(White, unpublished data). On the other hand, species are known in
which only one of these kinds of polymorphism has been found. Thus
in another Australian grasshopper, Atractomorpha
crenaticeps
australis,
the majority of individuals at most localities carry supernumerary chromosomes, but in this form no unequal bivalents have been encountered.
In species which possess both supernumerary chromosomes and supernumerary regions it is probable that the two have been rather freely
interconvertible in the past evolutionary history of the group.
The adaptive character of most cytological polymorphisms depends, of
course, on the fact that by suppressing crossing-over between certain
Μ. J. D. WHITE
tropis and Circotettix, where pericentric rearrangements are of common
occurrence in natural populations, some of these have reached fixation
and now constitute "species characters" (details in White, 1954), while
in many other grasshopper genera such rearrangements do not occur
either within species or as cytotaxonomic characters. And in groups
where the species may be distinguished by the extent and distribution
of heterochromatic segments in the chromosomes, populations occur in
which the extent of such segments is variable, so that cytologically
heterozygous individuals with "unequal bivalents" at meiosis occur. The
only serious discrepancy in the correlation between cytological differences within and between the species of a group (apart from the apparent but probably unreal one in the vertebrates referred to earlier) seems
to be in the case of forms with supernumerary chromosomes. It must
be admitted that there are groups, such as many grasshopper genera,
in which supernumerary chromosomes (largely, but not entirely heterochromatic ) are of common occurrence in wild populations, but in which
such elements seem never to have reached fixation as regular members
of the karyotype. This is rather a special case, however. The inheritance
of such chromosomes, due to various peculiarities of their behavior at
mitosis or meiosis, is inherently irregular in most cases. Thus the only
way in which the genetic material contained in them can be expected to
undergo fixation is by incorporation in chromosomes whose mitotic and
meiotic behavior is regular.
There is, in fact, a close parallelism between the occurrence of
supernumerary chromosomes as separate elements and supernumerary
regions which are attached to ordinary autosomes and which, in the
heterozygous state, give rise to "unequal bivalents" of the type previously
mentioned. Thus in a number of species of the grasshopper genera
Trimerotropis and Circotettix both unequal bivalents and supernumerary
chromosomes are found, either in the same or different local populations.
In the Australian grasshopper Cryptobothrus
chrysophorus
both types
of polymorphism occur, but do not usually coexist in the same locality
(White, unpublished data). On the other hand, species are known in
which only one of these kinds of polymorphism has been found. Thus
in another Australian grasshopper, Atractomorpha
crenaticeps
australis,
the majority of individuals at most localities carry supernumerary chromosomes, but in this form no unequal bivalents have been encountered.
In species which possess both supernumerary chromosomes and supernumerary regions it is probable that the two have been rather freely
interconvertible in the past evolutionary history of the group.
The adaptive character of most cytological polymorphisms depends, of
course, on the fact that by suppressing crossing-over between certain