120
Μ. J. D. WHITE
Outside groups like the higher Díptera, in which chiasmata do not
occur in the male sex, heterozygosity for paracentric inversions may lead
to the formation of a certain number of "lethal" sperms with broken
chromosomes if chiasmata are formed in the mutually inverted
region.
However, in the midge Chironomus
tentans, Beermann (1956) has
shown that heterozygosity for a paracentric inversion does not reduce
the fertility of the male because spermatid nuclei connected by dicentric
"bridges" give rise to non-functional double sperms. It is quite uncertain how far this is true of other animal species with chiasmata in the
males, but the same situation seems to occur in another midge, Dicranomyia
(Wolf, 1941). In species lacking this "double sperm"
mechanism we might expect that there would be a powerful selective
action against the establishment of paracentric inversions except where,
for one reason or another, chiasmata are not formed between the mutually inverted sections. The difficulty here is that the chief means for
detecting such inversions in organisms other than the Díptera (where
the salivary gland technique is used) depends on the formation of such
chiasmata. We must consequently expect that some paracentric inversions which do manage to establish themselves in groups without
polytene chromosomes and with chiasmata in both sexes may be ones
that are extremely difficult to detect cytologically. Thus we really have
no satisfactory means of determining whether paracentric inversions are
rare or frequent, or even whether they are present at all in most groups
of animals.
The position with regard to pericentric inversions is somewhat different. Provided they are not too short, there is no difficulty in detecting
them by cytological means, although in some instances it may be difficult to distinguish a centromere shift (3-break rearrangement) from a
pericentric inversion. In both sexes crossing-over in pericentric inversions will give rise to gametes carrying deficiencies and duplications. It
is accordingly understandable that pericentric inversions should be
very rare in Orosophila populations, although instances are known in
several species (Miller, 1939; Carson and Stalker, 1947; Levitan, 1951).
Among the grasshoppers pericentric rearrangements which are probably in most or all cases inversions (the possibility of centromere shifts
cannot be excluded at present) have been found in the heterozygous
state in populations of sixteen species of the American group Trimerotropi (see White, 1954 for a general review), in an Indian species of
Orthacris (Rao, 1934), and in seven or eight species of Australian grasshoppers belonging to both the families Acrididae and Eumastacidae
(White, unpublished data). In all these instances their maintenance
in the population is possible because chiasma formation in the mutually
Μ. J. D. WHITE
Outside groups like the higher Díptera, in which chiasmata do not
occur in the male sex, heterozygosity for paracentric inversions may lead
to the formation of a certain number of "lethal" sperms with broken
chromosomes if chiasmata are formed in the mutually inverted
region.
However, in the midge Chironomus
tentans, Beermann (1956) has
shown that heterozygosity for a paracentric inversion does not reduce
the fertility of the male because spermatid nuclei connected by dicentric
"bridges" give rise to non-functional double sperms. It is quite uncertain how far this is true of other animal species with chiasmata in the
males, but the same situation seems to occur in another midge, Dicranomyia
(Wolf, 1941). In species lacking this "double sperm"
mechanism we might expect that there would be a powerful selective
action against the establishment of paracentric inversions except where,
for one reason or another, chiasmata are not formed between the mutually inverted sections. The difficulty here is that the chief means for
detecting such inversions in organisms other than the Díptera (where
the salivary gland technique is used) depends on the formation of such
chiasmata. We must consequently expect that some paracentric inversions which do manage to establish themselves in groups without
polytene chromosomes and with chiasmata in both sexes may be ones
that are extremely difficult to detect cytologically. Thus we really have
no satisfactory means of determining whether paracentric inversions are
rare or frequent, or even whether they are present at all in most groups
of animals.
The position with regard to pericentric inversions is somewhat different. Provided they are not too short, there is no difficulty in detecting
them by cytological means, although in some instances it may be difficult to distinguish a centromere shift (3-break rearrangement) from a
pericentric inversion. In both sexes crossing-over in pericentric inversions will give rise to gametes carrying deficiencies and duplications. It
is accordingly understandable that pericentric inversions should be
very rare in Orosophila populations, although instances are known in
several species (Miller, 1939; Carson and Stalker, 1947; Levitan, 1951).
Among the grasshoppers pericentric rearrangements which are probably in most or all cases inversions (the possibility of centromere shifts
cannot be excluded at present) have been found in the heterozygous
state in populations of sixteen species of the American group Trimerotropi (see White, 1954 for a general review), in an Indian species of
Orthacris (Rao, 1934), and in seven or eight species of Australian grasshoppers belonging to both the families Acrididae and Eumastacidae
(White, unpublished data). In all these instances their maintenance
in the population is possible because chiasma formation in the mutually