222
B. L. ASTAUROV
and stimulation to active life, come so close together. It may be suggested therefore that different examples of heat activation are due to
essentially similar physicochemical changes in the cytoplasm, linked
with early reversible stages of heat denaturation of proteins.
2. Cytogenetic Consequences of Heat Shock: Ameiotic Parthenogenesis
The results of complete heat parthenogenesis differ sharply from those
of spontaneous parthenogenesis. In the latter case, as we have seen,
both sexes, heterozygous females and homozygous males, are found.
Males are even in prevailing numbers, while all their characters exhibit
segregation and wide variability. Contrary to this, in all experiments on
artificial heat parthenogenesis females have been exclusively obtained,
since the thermal parthenogenesis is strictly thelytokous. In cases when
parthenogenetic caterpillars are produced by females heterozygous for
some autosomal hereditary factor, the parthenogenetic progeny possesses a dominant phenotype, exactly similar to that of their mothers.
Direct genetic analysis has shown that if we exclude the frequent
cases of polyploidy, which will be dealt with later on, we may consider
it proved that parthenogenetic females as a rule possess a diploid set of
chromosomes and are genotypically and phenotypically exactly like their
mothers.
This completely agrees with Dr. Frolova's findings that in heat activated eggs the reduction division is suppressed and only the equational
division remains, which is followed by the formation of a diploid cleavage
nucleus (Frolova, 1940a,b, 1948). Thus, thelytokous heat parthenogenesis
in B. mori belongs cytologically to the zygotic or ameiotic type (Fig.
2,E, 2-7).
According to Frolova (1940a, 1948) as we have seen (Section II), the
first maturation division in moths may normally occur only after a
prolonged preparation including reorientation of tetrads by their rotation
through 90° and lasting from the moment when the eggs are inseminated
and laid until the formation of the so-called elimination plate or elimination chromatin, peculiar to the first maturation division in oocytes
of Lepidoptera.
In the case of thermal artificial parthenogenesis, heat activation
disturbs the above normal course of the maturation division causing
stretching of the spindle and thereby apparently inhibiting the 90°
rotation of the tetrads and the formation of the elimination plate, which
are necessary for the completion of the reduction division. The equational split of the tetrad is retained in the plane of the equator perpendicularly to the axis of the spindle, and splitting of the tetrads occurs
along it. The diads migrating towards the poles are formed by two
B. L. ASTAUROV
and stimulation to active life, come so close together. It may be suggested therefore that different examples of heat activation are due to
essentially similar physicochemical changes in the cytoplasm, linked
with early reversible stages of heat denaturation of proteins.
2. Cytogenetic Consequences of Heat Shock: Ameiotic Parthenogenesis
The results of complete heat parthenogenesis differ sharply from those
of spontaneous parthenogenesis. In the latter case, as we have seen,
both sexes, heterozygous females and homozygous males, are found.
Males are even in prevailing numbers, while all their characters exhibit
segregation and wide variability. Contrary to this, in all experiments on
artificial heat parthenogenesis females have been exclusively obtained,
since the thermal parthenogenesis is strictly thelytokous. In cases when
parthenogenetic caterpillars are produced by females heterozygous for
some autosomal hereditary factor, the parthenogenetic progeny possesses a dominant phenotype, exactly similar to that of their mothers.
Direct genetic analysis has shown that if we exclude the frequent
cases of polyploidy, which will be dealt with later on, we may consider
it proved that parthenogenetic females as a rule possess a diploid set of
chromosomes and are genotypically and phenotypically exactly like their
mothers.
This completely agrees with Dr. Frolova's findings that in heat activated eggs the reduction division is suppressed and only the equational
division remains, which is followed by the formation of a diploid cleavage
nucleus (Frolova, 1940a,b, 1948). Thus, thelytokous heat parthenogenesis
in B. mori belongs cytologically to the zygotic or ameiotic type (Fig.
2,E, 2-7).
According to Frolova (1940a, 1948) as we have seen (Section II), the
first maturation division in moths may normally occur only after a
prolonged preparation including reorientation of tetrads by their rotation
through 90° and lasting from the moment when the eggs are inseminated
and laid until the formation of the so-called elimination plate or elimination chromatin, peculiar to the first maturation division in oocytes
of Lepidoptera.
In the case of thermal artificial parthenogenesis, heat activation
disturbs the above normal course of the maturation division causing
stretching of the spindle and thereby apparently inhibiting the 90°
rotation of the tetrads and the formation of the elimination plate, which
are necessary for the completion of the reduction division. The equational split of the tetrad is retained in the plane of the equator perpendicularly to the axis of the spindle, and splitting of the tetrads occurs
along it. The diads migrating towards the poles are formed by two