136
Μ. J. D. WHITE
not all these mechanisms have originated by centric fusions, they
clearly all involve the incorporation of autosomal material in the sex
chromosome mechanism.
Some multiple sex chromosome systems have resulted from processes
other than fusions between sex chromosomes and autosomes. Thus in
the Heteroptera (especially in the families Reduviidae, Gelastocoridae,
Nepidae, and Cimicidae) most of those that have been studied seem to
have arisen by various kinds of "fragmentations" or reduplications of
portions of the original X or Y (White, 1940; Schräder, 1947). In such
mechanisms the X's and Y's do not form chiasmata with one another in
the heterogametic sex, i.e., there are no pairing segments in the strict
sense. But in spite of this X's and Y's regularly pass to opposite poles
at meiosis.
In the ostracods (Crustacea), Dietz (1955) has recently described a
variety of extremely complicated multiple sex chromosome mechanisms.
Inclusion of autosomes in the sex system has undoubtedly taken place
in several species, but fragmentation or reduplication of portions of
Y chromosomes seems to have occurred as well in Cyclocypris
ovum.
The XiXoY system of the cricket Eneoptera surinamensis is unusual in
that, in spite of the fact that it has almost certainly arisen by incorporation of autosomes in the sex chromosome mechanism, the three elements
are not associated as a trivalent at diakinesis and first metaphase. But in
spite of this the X's and the Y regularly pass to opposite poles (Piza,
1946; Claus, 1954).
The adaptive significance of these X-autosome and Y-autosome fusions
(which are now known to have taken place in many other groups of
the animal kingdom besides those selected for mention above) is probably rather different from that of autosome-autosome fusions. And their
ultimate consequences are certainly different in kind (see below). The
following general interpretation, although not based on any direct evidence, seems a plausible explanation for the initial fixation of such
fusions in natural populations. We may imagine a population with an
autosomal genetic polymorphism such that heterozygotes aia 2 are at a
selective advantage over the homozygotes aia x and a 2 a 2 if they are of
the heterogametic
sex, hut not otherwise.
If the heterogametic sex is
the male we have the following six categories of individuals:
a 2 ai $
aia 2 $
a 2 a 2 $
a^i 9
aia 2 9
a 2 a 2 9
The individuals which are underlined doubly or singly are assumed to
be at a greater or less selective advantage.
In such circumstances a fusion which links either chromosome ai or a 2
Μ. J. D. WHITE
not all these mechanisms have originated by centric fusions, they
clearly all involve the incorporation of autosomal material in the sex
chromosome mechanism.
Some multiple sex chromosome systems have resulted from processes
other than fusions between sex chromosomes and autosomes. Thus in
the Heteroptera (especially in the families Reduviidae, Gelastocoridae,
Nepidae, and Cimicidae) most of those that have been studied seem to
have arisen by various kinds of "fragmentations" or reduplications of
portions of the original X or Y (White, 1940; Schräder, 1947). In such
mechanisms the X's and Y's do not form chiasmata with one another in
the heterogametic sex, i.e., there are no pairing segments in the strict
sense. But in spite of this X's and Y's regularly pass to opposite poles
at meiosis.
In the ostracods (Crustacea), Dietz (1955) has recently described a
variety of extremely complicated multiple sex chromosome mechanisms.
Inclusion of autosomes in the sex system has undoubtedly taken place
in several species, but fragmentation or reduplication of portions of
Y chromosomes seems to have occurred as well in Cyclocypris
ovum.
The XiXoY system of the cricket Eneoptera surinamensis is unusual in
that, in spite of the fact that it has almost certainly arisen by incorporation of autosomes in the sex chromosome mechanism, the three elements
are not associated as a trivalent at diakinesis and first metaphase. But in
spite of this the X's and the Y regularly pass to opposite poles (Piza,
1946; Claus, 1954).
The adaptive significance of these X-autosome and Y-autosome fusions
(which are now known to have taken place in many other groups of
the animal kingdom besides those selected for mention above) is probably rather different from that of autosome-autosome fusions. And their
ultimate consequences are certainly different in kind (see below). The
following general interpretation, although not based on any direct evidence, seems a plausible explanation for the initial fixation of such
fusions in natural populations. We may imagine a population with an
autosomal genetic polymorphism such that heterozygotes aia 2 are at a
selective advantage over the homozygotes aia x and a 2 a 2 if they are of
the heterogametic
sex, hut not otherwise.
If the heterogametic sex is
the male we have the following six categories of individuals:
a 2 ai $
aia 2 $
a 2 a 2 $
a^i 9
aia 2 9
a 2 a 2 9
The individuals which are underlined doubly or singly are assumed to
be at a greater or less selective advantage.
In such circumstances a fusion which links either chromosome ai or a 2