CHROMOSOMAL EVOLUTION AND SPECIATION
133
Evolutionary polyploidy does not seem theoretically impossible in
bisexual animals whose method of sex determination depends on the
presence or absence of a Y (as in the silkworm moth and the axolotl)
and it may be that eventually some credible instances will be discovered,
probably in a group without cytologically recognizable sex chromosomes. In fact, the scale insect Gossyparia spuria studied by Schräder
(1929) may well be a tetraploid, although the original author made no
such claim. But dogmatic statements that the existence of evolutionary
polyploidy in bisexual animals is "well-established" or that it is "widespread" in such forms are simply irresponsible. There is, of course, no
doubt that polyploidy is widespread in parthenogenetic animals such as
certain weevils, oligochetes, etc., and that it has also been proved to
exist in some, but by no means all, of the groups composed of hermaphroditic species (Aeppli, 1952). Many of the parthenogenetic weevils
and earthworms are alloploid members of "agamic complexes" and must
owe their origin to occasional crossing between males of persisting bisexual races and females of parthenogenetic biotypes. Presumably many
of these alloploid strains owe their success to a form of heterosis. It is
likely that parthenogenesis would be much less widespread in some of
these groups if all strains were diploid or autoploid.
V. FUSIONS OF SEX CHROMOSOMES
We have already referred to fusions involving the sex chromosomes.
The effect of these is to lead to the inclusion of previously autosomal material in what may be called neo-X's and neo-Y's. Depending on whether
the heterogametic sex was originally XO or XY and on whether the
various chromosomes involved were acrocentric or metacentric, we may
have the following types of transformations (where X' and Y' represent
"neo" sex chromosomes, i.e., ones containing some material of recent
autosomal origin or consisting entirely of such material):
(1) XO
> XT'
(2) XO
> ΧΊΧ' 2 Υ'
(3)
XY
> XiXVT
(4)
XY
> X'YiY'o
These various transpositions of chromosome arms are shown diagrammatically in Fig. 2.
Evolutionary changes of type (1) have taken place repeatedly in the
grasshoppers and some allied groups through centric fusion between
an acrocentric X chromosome and an acrocentric autosome. Helwig
(1942) stated that cases were known in fourteen genera of Acrididae,
and several more instances (the North American Oedaleonotus
enigma
and the Australian Stenocatantops
angustifrons and Tolgadia spp.) may
now be added to that total. Similar transformations are known to have
133
Evolutionary polyploidy does not seem theoretically impossible in
bisexual animals whose method of sex determination depends on the
presence or absence of a Y (as in the silkworm moth and the axolotl)
and it may be that eventually some credible instances will be discovered,
probably in a group without cytologically recognizable sex chromosomes. In fact, the scale insect Gossyparia spuria studied by Schräder
(1929) may well be a tetraploid, although the original author made no
such claim. But dogmatic statements that the existence of evolutionary
polyploidy in bisexual animals is "well-established" or that it is "widespread" in such forms are simply irresponsible. There is, of course, no
doubt that polyploidy is widespread in parthenogenetic animals such as
certain weevils, oligochetes, etc., and that it has also been proved to
exist in some, but by no means all, of the groups composed of hermaphroditic species (Aeppli, 1952). Many of the parthenogenetic weevils
and earthworms are alloploid members of "agamic complexes" and must
owe their origin to occasional crossing between males of persisting bisexual races and females of parthenogenetic biotypes. Presumably many
of these alloploid strains owe their success to a form of heterosis. It is
likely that parthenogenesis would be much less widespread in some of
these groups if all strains were diploid or autoploid.
V. FUSIONS OF SEX CHROMOSOMES
We have already referred to fusions involving the sex chromosomes.
The effect of these is to lead to the inclusion of previously autosomal material in what may be called neo-X's and neo-Y's. Depending on whether
the heterogametic sex was originally XO or XY and on whether the
various chromosomes involved were acrocentric or metacentric, we may
have the following types of transformations (where X' and Y' represent
"neo" sex chromosomes, i.e., ones containing some material of recent
autosomal origin or consisting entirely of such material):
(1) XO
> XT'
(2) XO
> ΧΊΧ' 2 Υ'
(3)
XY
> XiXVT
(4)
XY
> X'YiY'o
These various transpositions of chromosome arms are shown diagrammatically in Fig. 2.
Evolutionary changes of type (1) have taken place repeatedly in the
grasshoppers and some allied groups through centric fusion between
an acrocentric X chromosome and an acrocentric autosome. Helwig
(1942) stated that cases were known in fourteen genera of Acrididae,
and several more instances (the North American Oedaleonotus
enigma
and the Australian Stenocatantops
angustifrons and Tolgadia spp.) may
now be added to that total. Similar transformations are known to have