C Y T O G E N E T I C M E C H A N I S M S IN S I L K W O R M S
231
progeny closely approaches 4 $ : 1 cf (instead of the usual 1 $ : 1 cf )
varying in individual families according to the law of probability. In
families of mixoploid females (2n + 4w), the triploid progeny hatching
from large 4 n eggs display the above peculiar segregation patterns,
whereas the progeny hatching from normal diploid eggs show the common
Mendelian ratios characteristic of diploids. Owing to this, the total
segregation and sex ratio in the progeny of mixoploid females may
display any value intermediate between those inherent to pure triploids
and pure diploids.
Segregation of autosomal characters in progeny of 4 n females is precisely of the nature that would be expected theoretically on the basis of
randomly paired conjugation of homologous chromosomes and subsequent
formation of diploid gametes. It was found however, that when four
homologous chromosomes of the AAaa tetraploid are composed of A A
and aa pairs from different races, so that the chromosomes of one of
those pairs are strictly identical with regard to their hereditary content
and differ from the likewise identical chromosomes of the other pair,
random chromosome conjugation is to some extent obscured by partial
preferential conjugation of identical chromosomes. In such cases, segregation differs from the theoretically expected (5 A : 1 a) by a greater
prevalence of the dominant class (Astaurov, 1940).
The sex ratio (4 $ : 1 cf ) occurring among triploids so far has no
entirely satisfactory explanation. A number of possible explanations
have been attempted, however, though none of them can presently be
accepted without additional experiments (Astaurov, 1940; Hasimoto,
1933b; Kawaguchi, 1934b).
Three kinds of zygote may be expected in the triploid progeny from
the crosses $ 4 n (ZZWW, 4A) X d 2n {TIL, 2A) : (1) zygotes with a
purely male chromosome balance (3Z, 3A), where A denotes the haploid
autosome set; (2) superfemales (Z, WW, 3A), and (3) intersexes (ZZ,
W, 3A). In spite of this, no individuals with an apparent intersexual
phenotype are to be found in the triploid progeny unless a certain
degeneration of the female gonads is considered as intersexuality. The
possibility is not excluded that owing to the relatively slight sex dimorphism of B. mori moths compared, for instance, with that in the gypsy
moth, Lymantria dispar, it is very difficult to discern intersexuality in
triploids, which can be established only by accurate quantitative investigations of their morphology.
However, the main cause of this lack of morphological intersexuality
evidently lies in the strong female determining power of the W chromosome. Sex determination in the silkworm depends not on the quantitative balance between sex-determining genes of the Z (or X) chromosomes and autosomes as in Drosophila, but on the presence or absence
231
progeny closely approaches 4 $ : 1 cf (instead of the usual 1 $ : 1 cf )
varying in individual families according to the law of probability. In
families of mixoploid females (2n + 4w), the triploid progeny hatching
from large 4 n eggs display the above peculiar segregation patterns,
whereas the progeny hatching from normal diploid eggs show the common
Mendelian ratios characteristic of diploids. Owing to this, the total
segregation and sex ratio in the progeny of mixoploid females may
display any value intermediate between those inherent to pure triploids
and pure diploids.
Segregation of autosomal characters in progeny of 4 n females is precisely of the nature that would be expected theoretically on the basis of
randomly paired conjugation of homologous chromosomes and subsequent
formation of diploid gametes. It was found however, that when four
homologous chromosomes of the AAaa tetraploid are composed of A A
and aa pairs from different races, so that the chromosomes of one of
those pairs are strictly identical with regard to their hereditary content
and differ from the likewise identical chromosomes of the other pair,
random chromosome conjugation is to some extent obscured by partial
preferential conjugation of identical chromosomes. In such cases, segregation differs from the theoretically expected (5 A : 1 a) by a greater
prevalence of the dominant class (Astaurov, 1940).
The sex ratio (4 $ : 1 cf ) occurring among triploids so far has no
entirely satisfactory explanation. A number of possible explanations
have been attempted, however, though none of them can presently be
accepted without additional experiments (Astaurov, 1940; Hasimoto,
1933b; Kawaguchi, 1934b).
Three kinds of zygote may be expected in the triploid progeny from
the crosses $ 4 n (ZZWW, 4A) X d 2n {TIL, 2A) : (1) zygotes with a
purely male chromosome balance (3Z, 3A), where A denotes the haploid
autosome set; (2) superfemales (Z, WW, 3A), and (3) intersexes (ZZ,
W, 3A). In spite of this, no individuals with an apparent intersexual
phenotype are to be found in the triploid progeny unless a certain
degeneration of the female gonads is considered as intersexuality. The
possibility is not excluded that owing to the relatively slight sex dimorphism of B. mori moths compared, for instance, with that in the gypsy
moth, Lymantria dispar, it is very difficult to discern intersexuality in
triploids, which can be established only by accurate quantitative investigations of their morphology.
However, the main cause of this lack of morphological intersexuality
evidently lies in the strong female determining power of the W chromosome. Sex determination in the silkworm depends not on the quantitative balance between sex-determining genes of the Z (or X) chromosomes and autosomes as in Drosophila, but on the presence or absence