CYTOGENETIC MECHANISMS IN SILKWORMS
209
parthenogenesis in B. mon failed and even leading authorities in silkworm biology were sometimes forced to the false conclusion that such
parthenogenesis is altogether impossible (Verson, 1872, 1890).
It seems possible in principle to raise a hatched parthenogenetic larva
up to the imago stage, which allows one to speak, although a little
arbitrarily, of parthenogenetic larvae as instances of complete parthenogenesis. Strictly speaking, however, only the development of unfertilized eggs up to the imago stage should be denoted as complete parthenogenesis. On the average, the viability of caterpillars in cases of
spontaneous parthenogenesis is very poor, considerable elimination always
taking place, particularly during the first instar and at the critical phase
of the first larval ecdysis. For instance, out of the total of 466 abovementioned parthenogenetic larvae obtained by Lecaillon (1916, 1917c),
Kawaguchi (1934a), and Astaurov (1940), only 54 (25 parthenogenetic moths (8 X 10"
6
) were raised. Thus, on the basis of a
very common phenomenon of rudimentary and usually abortive spontaneous parthenogenesis, extremely rare cases of accidental complete
parthenogenesis occur. Approximately one unfertilized egg out of about
100,000 has a chance to develop into the adult insect by this rare mode
of reproduction.
b. Cytogenetic Aspect. In all better-studied cases, spontaneous parthenogenesis in B. mori is deuterotokous, i.e., individuals of both sexes are
present (Lecaillon, 1918; Kawaguchi, 1934a; Astaurov, 1940), as is the
case in accidental parthenogenesis of many other Lepidoptera.
In all genetically controlled cases where the female parent was
heterozygous in regard to some mutational markers, the parthenogenetic
sons were proved to be diploid and homozygous for all characters under
control, approximately half of them displaying the dominant and the
other half the recessive allele (Astaurov, 1940; Kawaguchi, 1934a). Since
the female sex in B. mori is heterozygous ( $ = WZ), homogamety of
parthenogenetic sons (ZZ) is in perfect accordance with the rule just
mentioned. However, WW individuals and embryos with such a genetic
constitution, although they must arise, were not discovered, and were
evidently inviable. Contrary to this, parthenogenetic daughters of heterozygous mothers with very few and still unexplained exceptions retain
the heterozygous constitution of their mother for both autosomal alleles
and for hétérochromosomes (ZW). There are also cases of triploidy,
tetraploidy, and mixoploidy (haplo-diploid embryos, ditetraploid embryos and adults) which deserve special attention (cf. below, Sato,
1931).
The sex ratio among the spontaneous parthenogenetic progeny greatly
depends on the stage of sex discrimination. Recessive lethals, semilethals,
and depressors, occur in B. mori very frequently (Efroimson, 1932,
209
parthenogenesis in B. mon failed and even leading authorities in silkworm biology were sometimes forced to the false conclusion that such
parthenogenesis is altogether impossible (Verson, 1872, 1890).
It seems possible in principle to raise a hatched parthenogenetic larva
up to the imago stage, which allows one to speak, although a little
arbitrarily, of parthenogenetic larvae as instances of complete parthenogenesis. Strictly speaking, however, only the development of unfertilized eggs up to the imago stage should be denoted as complete parthenogenesis. On the average, the viability of caterpillars in cases of
spontaneous parthenogenesis is very poor, considerable elimination always
taking place, particularly during the first instar and at the critical phase
of the first larval ecdysis. For instance, out of the total of 466 abovementioned parthenogenetic larvae obtained by Lecaillon (1916, 1917c),
Kawaguchi (1934a), and Astaurov (1940), only 54 (25 parthenogenetic moths (8 X 10"
6
) were raised. Thus, on the basis of a
very common phenomenon of rudimentary and usually abortive spontaneous parthenogenesis, extremely rare cases of accidental complete
parthenogenesis occur. Approximately one unfertilized egg out of about
100,000 has a chance to develop into the adult insect by this rare mode
of reproduction.
b. Cytogenetic Aspect. In all better-studied cases, spontaneous parthenogenesis in B. mori is deuterotokous, i.e., individuals of both sexes are
present (Lecaillon, 1918; Kawaguchi, 1934a; Astaurov, 1940), as is the
case in accidental parthenogenesis of many other Lepidoptera.
In all genetically controlled cases where the female parent was
heterozygous in regard to some mutational markers, the parthenogenetic
sons were proved to be diploid and homozygous for all characters under
control, approximately half of them displaying the dominant and the
other half the recessive allele (Astaurov, 1940; Kawaguchi, 1934a). Since
the female sex in B. mori is heterozygous ( $ = WZ), homogamety of
parthenogenetic sons (ZZ) is in perfect accordance with the rule just
mentioned. However, WW individuals and embryos with such a genetic
constitution, although they must arise, were not discovered, and were
evidently inviable. Contrary to this, parthenogenetic daughters of heterozygous mothers with very few and still unexplained exceptions retain
the heterozygous constitution of their mother for both autosomal alleles
and for hétérochromosomes (ZW). There are also cases of triploidy,
tetraploidy, and mixoploidy (haplo-diploid embryos, ditetraploid embryos and adults) which deserve special attention (cf. below, Sato,
1931).
The sex ratio among the spontaneous parthenogenetic progeny greatly
depends on the stage of sex discrimination. Recessive lethals, semilethals,
and depressors, occur in B. mori very frequently (Efroimson, 1932,