C Y T O G E N E T I C M E C H A N I S M S I N S I L K W O R M S
213
disharmony and to a fatal end (Fig. 2D, 4c-7c). If the restoration of
diploidy takes place in the course of the second maturation division or
shortly after maturation in the course of the first cleavage, all somatic
cells receive at least the diploid chromosome number. By this process
strictly homozygous ZZ, 27 AA individuals, i.e., diploid males, arise in
the case of Z, 27 A female pronucleus (Fig. 2D, 6b). The WW and 27 AA
lethal chromosome constitution must appear with exactly the same frequency (Fig. 2D, 6a).
Apparently, a quite analogous automictic cytogenetic mechanism is at
play in case of facultative spontaneous parthenogenesis in the turkey,
where parthenogenesis is arrhenotokous and only males arise (Kosin and
Sato, 1960; Olsen, 1960).
With respect to the origin of heterozygous B. mori females, it is easy
to imagine several almost equally plausible explanations quite consistent
with the cytological findings. However, it is not so easy to decide which
of the possible mechanisms actually works. Without any doubt there is
one or the other kind of "polar body fertilization" shortly after reductional division. In the case of prereduction, it must be a fusion of the
female pronucleus with the first polar body nucleus or that of the second
polar body nucleus with one of the dérivâtes of the first polar body
nucleus [Fig. 1C, the so-called "Richtungskopulationkern" of Goldschmidt and Katsuki (1928); cf. VI,C,1]. In the case of fusion of 3
haploid nuclei, the 3 n females encountered in Sato's experiments could
arise. Mixoploid, 2 n / 4 n females laying partly normal diploid, partly
large tetraploid eggs which were also found in Sato's experiments certainly result from somatic polyploidization which, according to all the
data and especially to those of Frolova (1935), is the most trivial event
in the course of parthenogenetic development.
Automictic restoration of diploidy in the course of haploid cleavage
leading to "maleness" and homozygosity is obviously a much more frequent cytogenetic mechanism in spontaneous parthenogenesis than "polar
body fertilization" leading to "femaleness" and heterozygosity. For instance, applying in one of my experiments the recessive marker chch
(chocolate instead of black coloration of first instar larvae) which could
be discriminated from ch/-\- and + / + (black) in late embryogenesis,
the sex ratio could be determined before hatching as 160 cf : 46 $ .
Taking into account that an equal number of homozygous WW embryos
should be eliminated during early development, the primary ratio
must be at least 320 automictic fusions:46 "polar body fertilizations" or
7:1. Moreover, many homozygotes would be eliminated owing to
lethal and depressor genes, so that the primary ratio must be far more
in favor of homozygosity. Consequently, the restoration of diploidy in
the course of the first cleavage must be many times more frequent in
213
disharmony and to a fatal end (Fig. 2D, 4c-7c). If the restoration of
diploidy takes place in the course of the second maturation division or
shortly after maturation in the course of the first cleavage, all somatic
cells receive at least the diploid chromosome number. By this process
strictly homozygous ZZ, 27 AA individuals, i.e., diploid males, arise in
the case of Z, 27 A female pronucleus (Fig. 2D, 6b). The WW and 27 AA
lethal chromosome constitution must appear with exactly the same frequency (Fig. 2D, 6a).
Apparently, a quite analogous automictic cytogenetic mechanism is at
play in case of facultative spontaneous parthenogenesis in the turkey,
where parthenogenesis is arrhenotokous and only males arise (Kosin and
Sato, 1960; Olsen, 1960).
With respect to the origin of heterozygous B. mori females, it is easy
to imagine several almost equally plausible explanations quite consistent
with the cytological findings. However, it is not so easy to decide which
of the possible mechanisms actually works. Without any doubt there is
one or the other kind of "polar body fertilization" shortly after reductional division. In the case of prereduction, it must be a fusion of the
female pronucleus with the first polar body nucleus or that of the second
polar body nucleus with one of the dérivâtes of the first polar body
nucleus [Fig. 1C, the so-called "Richtungskopulationkern" of Goldschmidt and Katsuki (1928); cf. VI,C,1]. In the case of fusion of 3
haploid nuclei, the 3 n females encountered in Sato's experiments could
arise. Mixoploid, 2 n / 4 n females laying partly normal diploid, partly
large tetraploid eggs which were also found in Sato's experiments certainly result from somatic polyploidization which, according to all the
data and especially to those of Frolova (1935), is the most trivial event
in the course of parthenogenetic development.
Automictic restoration of diploidy in the course of haploid cleavage
leading to "maleness" and homozygosity is obviously a much more frequent cytogenetic mechanism in spontaneous parthenogenesis than "polar
body fertilization" leading to "femaleness" and heterozygosity. For instance, applying in one of my experiments the recessive marker chch
(chocolate instead of black coloration of first instar larvae) which could
be discriminated from ch/-\- and + / + (black) in late embryogenesis,
the sex ratio could be determined before hatching as 160 cf : 46 $ .
Taking into account that an equal number of homozygous WW embryos
should be eliminated during early development, the primary ratio
must be at least 320 automictic fusions:46 "polar body fertilizations" or
7:1. Moreover, many homozygotes would be eliminated owing to
lethal and depressor genes, so that the primary ratio must be far more
in favor of homozygosity. Consequently, the restoration of diploidy in
the course of the first cleavage must be many times more frequent in