122
Μ. J. D. WHITE
have become established, so that 2n has been reduced from 23 to 12 in
the male (1 fusion is between the X and an autosome). In a few species
of Cryptosacci a reduction in chromosome number has occurred, but
since there are no metacentrics there is no evidence of centric fusions.
Some of these may be instances in which tandem fusions occurred, although the alternative of centric fusion followed by pericentric inversion seems more probable. However we interpret these cases, it seems
clear that all changes in chromosome number which have occurred in
the Acrididae have been decreases rather than increases.
The concept of an "upper limit" to the chromosome number in a
group seems to be rather generally, but not universally, applicable in
groups with monocentric chromosomes. In the genus Orosophila, where
the chromosome numbers of several hundred species are known, there
seems to be an upper limit of η = 6, there being only two species with
η = 7 (Patterson and Stone, 1952; Clayton and Ward, 1954). Much of
Matthey's work on the cytology of various groups of reptiles may be
interpreted as evidence for an upper limit.
Such upper limits in particular groups could be due to several kinds
of causes. They could indicate that the cells or their spindles were incapable of accommodating an extra pair of chromosomes without ill
effects. They could mean that the population genetics of the groups in
question precluded, for some reason, an increased amount of recombination. Or they could simply be an indication that mechanisms of successful fragmentation are absent from these groups. Neither the first
nor the second hypothesis seems plausible, and it seems likely that the
third one explains most of the instances of upper limits discussed above.
In the two Orosophila species with η = 7 the "extra" pairs of chromosomes seem in both cases to be minute heterochromatic elements.
Natural populations which are polymorphic in respect of centric
fusions are not common. They have, however, been recorded in a number of species of grasshoppers and crickets (references in White, 1954)
and mantids (Wahrman, 1954). An analogous case was recorded in the
lizard Gerrhonotus
by Matthey (1931, 1933); and a most remarkable
polymorphism of this kind exists in certain populations of the mollusc
Purpura lapillus (Staiger, 1954). In some of these cases, such as the
grasshoppers Hesperotettix
viridis
(McClung, 1917), Ύ rimer oír opis
sparsa (White 1951), and Circotettix
undulatus (Evans, 1954), the
polymorphism is apparently restricted to a small area which is probably
in each instance a transition zone between areas inhabited by races
monomorphic for the fusion or its absence. On the other hand, in the
mantid Ameles heldreichi
(Wahrman, 1954) numerous populations in
Palestine and Turkey were all polymorphic in respect of the fusion. In
Μ. J. D. WHITE
have become established, so that 2n has been reduced from 23 to 12 in
the male (1 fusion is between the X and an autosome). In a few species
of Cryptosacci a reduction in chromosome number has occurred, but
since there are no metacentrics there is no evidence of centric fusions.
Some of these may be instances in which tandem fusions occurred, although the alternative of centric fusion followed by pericentric inversion seems more probable. However we interpret these cases, it seems
clear that all changes in chromosome number which have occurred in
the Acrididae have been decreases rather than increases.
The concept of an "upper limit" to the chromosome number in a
group seems to be rather generally, but not universally, applicable in
groups with monocentric chromosomes. In the genus Orosophila, where
the chromosome numbers of several hundred species are known, there
seems to be an upper limit of η = 6, there being only two species with
η = 7 (Patterson and Stone, 1952; Clayton and Ward, 1954). Much of
Matthey's work on the cytology of various groups of reptiles may be
interpreted as evidence for an upper limit.
Such upper limits in particular groups could be due to several kinds
of causes. They could indicate that the cells or their spindles were incapable of accommodating an extra pair of chromosomes without ill
effects. They could mean that the population genetics of the groups in
question precluded, for some reason, an increased amount of recombination. Or they could simply be an indication that mechanisms of successful fragmentation are absent from these groups. Neither the first
nor the second hypothesis seems plausible, and it seems likely that the
third one explains most of the instances of upper limits discussed above.
In the two Orosophila species with η = 7 the "extra" pairs of chromosomes seem in both cases to be minute heterochromatic elements.
Natural populations which are polymorphic in respect of centric
fusions are not common. They have, however, been recorded in a number of species of grasshoppers and crickets (references in White, 1954)
and mantids (Wahrman, 1954). An analogous case was recorded in the
lizard Gerrhonotus
by Matthey (1931, 1933); and a most remarkable
polymorphism of this kind exists in certain populations of the mollusc
Purpura lapillus (Staiger, 1954). In some of these cases, such as the
grasshoppers Hesperotettix
viridis
(McClung, 1917), Ύ rimer oír opis
sparsa (White 1951), and Circotettix
undulatus (Evans, 1954), the
polymorphism is apparently restricted to a small area which is probably
in each instance a transition zone between areas inhabited by races
monomorphic for the fusion or its absence. On the other hand, in the
mantid Ameles heldreichi
(Wahrman, 1954) numerous populations in
Palestine and Turkey were all polymorphic in respect of the fusion. In