CHROMOSOMAL EVOLUTION AND SPECIATION
137
with the X or the Y must lead to a situation in which selection will
operate in favor of individuals with the fusion and will promote the
development of a population in which males homozygous for the a
chromosome and females heterozygous for it will soon cease to exist.
Sex-limited heterosis was studied by da Cunha (1953), but only in cases
where it was associated with inversions in the X chromosome.
The establishment and fixation of a fusion between an autosome and
a sex chromosome represent the first step in the origin of a new sex
chromosome mechanism. The genetic material symbolized by ai and a 2
will, after fusion, constitute a new "pairing segment" in which chiasmata
will, as a rule, be formed at meiosis (but not in Orosophila spp., where
chiasma formation has been suppressed in the heterogametic sex). The
heterozygosity of this segment may initially be confined to a single locus
or may involve several loci or even an inverted region. But the cytological evidence clearly indicates that in many instances the subsequent
evolution of such mechanisms has involved an increase in the structural
and presumably also the genie heterozygosity of the a x and a 2 regions,
with gradual restriction of pairing and chiasma formation to relatively
minute distal segments. This process usually involves the "heterochromatinization" of the material in the neo-Y and sometimes also that in the
neo-X; and it may also include deletion, duplication, or both, of small
sections. These processes have been carried to considerable lengths in
the ΧχΧ 2 Υ mantids, in the grasshopper Mermiria intertexta
(King,
1950), and in the XiX 2 Y species of Moraba previously mentioned, in
all of which the distal pairing regions have become reduced to very
minute segments. On the other hand, in the grasshopper
Hypochlora
alba (King, 1950), and the tettigoniid lorkietta
picta (White, unpublished) the pairing segments are extensive, and two chiasmata may frequently be formed in them.
Patterson and Stone (1952) have listed 12 different species groups
in the genus Orosophila in which X-autosome fusions have taken place
(it is not certain that as many as 12 separate X-autosome fusions occurred, since some of the groups may have a common origin). With the
exception of D. americana
(a member of the virilis group), with an
XYiY 2 system in which Y 2 is the unaltered fourth chromosome that is
entirely homologous to one limb of the neo-X, these are all XY species.
Thus in 11 species groups we must assume that one member of an autosomal pair has become fused to the X while the other became fused to
the Y and subsequently became heterochromatic or was lost by successive deletions. The two fusions would not necessarily be simultaneous
and either could have taken place first. D. americana shows us an evolutionary stage in which an X-autosome fusion has taken place but in
137
with the X or the Y must lead to a situation in which selection will
operate in favor of individuals with the fusion and will promote the
development of a population in which males homozygous for the a
chromosome and females heterozygous for it will soon cease to exist.
Sex-limited heterosis was studied by da Cunha (1953), but only in cases
where it was associated with inversions in the X chromosome.
The establishment and fixation of a fusion between an autosome and
a sex chromosome represent the first step in the origin of a new sex
chromosome mechanism. The genetic material symbolized by ai and a 2
will, after fusion, constitute a new "pairing segment" in which chiasmata
will, as a rule, be formed at meiosis (but not in Orosophila spp., where
chiasma formation has been suppressed in the heterogametic sex). The
heterozygosity of this segment may initially be confined to a single locus
or may involve several loci or even an inverted region. But the cytological evidence clearly indicates that in many instances the subsequent
evolution of such mechanisms has involved an increase in the structural
and presumably also the genie heterozygosity of the a x and a 2 regions,
with gradual restriction of pairing and chiasma formation to relatively
minute distal segments. This process usually involves the "heterochromatinization" of the material in the neo-Y and sometimes also that in the
neo-X; and it may also include deletion, duplication, or both, of small
sections. These processes have been carried to considerable lengths in
the ΧχΧ 2 Υ mantids, in the grasshopper Mermiria intertexta
(King,
1950), and in the XiX 2 Y species of Moraba previously mentioned, in
all of which the distal pairing regions have become reduced to very
minute segments. On the other hand, in the grasshopper
Hypochlora
alba (King, 1950), and the tettigoniid lorkietta
picta (White, unpublished) the pairing segments are extensive, and two chiasmata may frequently be formed in them.
Patterson and Stone (1952) have listed 12 different species groups
in the genus Orosophila in which X-autosome fusions have taken place
(it is not certain that as many as 12 separate X-autosome fusions occurred, since some of the groups may have a common origin). With the
exception of D. americana
(a member of the virilis group), with an
XYiY 2 system in which Y 2 is the unaltered fourth chromosome that is
entirely homologous to one limb of the neo-X, these are all XY species.
Thus in 11 species groups we must assume that one member of an autosomal pair has become fused to the X while the other became fused to
the Y and subsequently became heterochromatic or was lost by successive deletions. The two fusions would not necessarily be simultaneous
and either could have taken place first. D. americana shows us an evolutionary stage in which an X-autosome fusion has taken place but in