CYTOGENETIC MECHANISMS IN SILKWORMS
249
polocyte nuclei. When cleavage begins, the derivatives of the two resulting cleavage nuclei with different genetic constitutions populate the
opposite (right and left) sides of the ovum.
It can be taken for granted that two different diploid cleavage nuclei
really participate in the formation of bilateral mosaics and gynandromorphs. There exists, however, no such clarity with regard to the mode
of origin of the extra nucleus.
According to the Goldschmidt-Katsuki hypothesis, the additional
female pronucleus is formed from the second polar body nucleus, i.e.,
both female pronuclei arise as sister nuclei of the second maturation
division. It is clear, however, that the observed appearance of bilateral
mosaics and gynandromorphs in progeny of ? momo, ZW, Aa X cf ZZ,
aa, is possible via double fertilization only when both female pronuclei
have a different genetic constitution (A and a or Z and W). Therefore
the Goldschmidt-Katsuki supposition that the extra cleavage nucleus is
formed by means of fertilization of the second polar body nucleus holds
true only if maturation is always or at least often postreductional. Contrary to this, as we have seen (Section II) there are now ample grounds
for believing that, in the B. mori females, the second maturation division
is equational. If so, the additional cleavage nucleus in momo females
must have another origin: (1) it can arise by means of fertilization of
the first polocyte or of one of its derivatives or, (2) it corresponds to the
so-called "Richtungscopulationskern" of German authors, i.e., to the
diploid product of fusion taking place between two polar body nuclei,
one of them being derivative of the first, the other corresponding to the
second polar body. Both possibilities seemed to be conceivable when
Goldschmidt and Katsuki investigated this intricate case.
The original hereditary mosaic strain is still maintained in Japan and
there exists hope that all possible explanations can be checked.
2. Nonhereditary Cases of Mosaicism
Of several other examples of mosaicism caused by disturbances taking
place, at the time of maturation and fertilization, frequent cases of
androzygotic mosaics deserve mention.
When inducing dispermic androgenesis by heat shocking of freshly
fertilized eggs laid in crosses of 9 A A χ tf aa (Section VI,B) besides
the normal phenotypically dominant zygotic individuals Aa of both
sexes and the exceptional recessive androgenetic males aa, we always
find some admixture of rare specimens with the mosaic combination of
phenotypically dominant A and recessive parts in their body. Taking as
a recessive marker the mutational character w 2 (white two depigmentation of serosa membrane), we obtain besides "black" and "white" some
motley eggs with black and white patches in different combinations,
249
polocyte nuclei. When cleavage begins, the derivatives of the two resulting cleavage nuclei with different genetic constitutions populate the
opposite (right and left) sides of the ovum.
It can be taken for granted that two different diploid cleavage nuclei
really participate in the formation of bilateral mosaics and gynandromorphs. There exists, however, no such clarity with regard to the mode
of origin of the extra nucleus.
According to the Goldschmidt-Katsuki hypothesis, the additional
female pronucleus is formed from the second polar body nucleus, i.e.,
both female pronuclei arise as sister nuclei of the second maturation
division. It is clear, however, that the observed appearance of bilateral
mosaics and gynandromorphs in progeny of ? momo, ZW, Aa X cf ZZ,
aa, is possible via double fertilization only when both female pronuclei
have a different genetic constitution (A and a or Z and W). Therefore
the Goldschmidt-Katsuki supposition that the extra cleavage nucleus is
formed by means of fertilization of the second polar body nucleus holds
true only if maturation is always or at least often postreductional. Contrary to this, as we have seen (Section II) there are now ample grounds
for believing that, in the B. mori females, the second maturation division
is equational. If so, the additional cleavage nucleus in momo females
must have another origin: (1) it can arise by means of fertilization of
the first polocyte or of one of its derivatives or, (2) it corresponds to the
so-called "Richtungscopulationskern" of German authors, i.e., to the
diploid product of fusion taking place between two polar body nuclei,
one of them being derivative of the first, the other corresponding to the
second polar body. Both possibilities seemed to be conceivable when
Goldschmidt and Katsuki investigated this intricate case.
The original hereditary mosaic strain is still maintained in Japan and
there exists hope that all possible explanations can be checked.
2. Nonhereditary Cases of Mosaicism
Of several other examples of mosaicism caused by disturbances taking
place, at the time of maturation and fertilization, frequent cases of
androzygotic mosaics deserve mention.
When inducing dispermic androgenesis by heat shocking of freshly
fertilized eggs laid in crosses of 9 A A χ tf aa (Section VI,B) besides
the normal phenotypically dominant zygotic individuals Aa of both
sexes and the exceptional recessive androgenetic males aa, we always
find some admixture of rare specimens with the mosaic combination of
phenotypically dominant A and recessive parts in their body. Taking as
a recessive marker the mutational character w 2 (white two depigmentation of serosa membrane), we obtain besides "black" and "white" some
motley eggs with black and white patches in different combinations,