224
B. L. ASTAUROV
passed through the laboratories, it may be taken for granted that no
males at all are produced by this mode of reproduction.
Although such a complete absence of males could hardly result from
preferential mortality of male embryos, some attempts have been made
to ascertain by means of direct cytological analysis whether there is
such a possibility.
As early as 1945, Smith discovered that in the moth Archips fumiferana cell nuclei of the female silk gland are clearly marked by the
presence of heteropycnotic masses similar to sex chromatin, which are
never found in male nuclei. This makes sex discrimination possible even
as early as the first instar larvae. It was soon demonstrated by Frizzi
(1948) and later on by Dolidze (1958) that the method is fairly
applicable to silkworms.
In several independent tests (Chowdhury, 1954; Springhetti and
Chowdhury, 1956, 1957; Springhetti and Merli, 1956), almost 1500
parthenogenetic larvae of the first instar were cytologically examined
with respect to their sex, and at least 99.9% of them proved to be
females.
Thus, on the ground of different cytogenetic data there is hardly any
doubt now that all heat-activated unfertilized eggs possess the female
chromosome constitution from the very beginning of their development,
i.e., immediately after the termination of artificially induced maturation
division.
3. Selection of Parthenogenetic Clones
There exists an extremely wide variation of the capacity for heat
parthenogenesis among the individuals belonging to every population
of the silkworm so far investigated. The variability limits range from
complete incapacity to the ability to yield up to 90% of viable parthenogenetic larvae from heat-treated eggs. This individual susceptibility
to parthenogenesis is a strictly hereditary character, and owing to genotypic identity between the female and her parthenogenetic progeny, it
is found in the parthenogenetic daughters to the same degree as in
their mothers.
It is very easy therefore to select individual females and their parthenogenetic progeny, possessing a rather high capacity for heat parthenogenesis, and to obtain parthenogenetic clones readily propagating by
this artificial mode of reproduction throughout an unlimited number of
generations. Owing to their genotypical identity, females belonging to
the clone resemble not only the maternal type, but each other, just like
identical twins or specimens of one and the same clone of asexually
reproducing plants.
B. L. ASTAUROV
passed through the laboratories, it may be taken for granted that no
males at all are produced by this mode of reproduction.
Although such a complete absence of males could hardly result from
preferential mortality of male embryos, some attempts have been made
to ascertain by means of direct cytological analysis whether there is
such a possibility.
As early as 1945, Smith discovered that in the moth Archips fumiferana cell nuclei of the female silk gland are clearly marked by the
presence of heteropycnotic masses similar to sex chromatin, which are
never found in male nuclei. This makes sex discrimination possible even
as early as the first instar larvae. It was soon demonstrated by Frizzi
(1948) and later on by Dolidze (1958) that the method is fairly
applicable to silkworms.
In several independent tests (Chowdhury, 1954; Springhetti and
Chowdhury, 1956, 1957; Springhetti and Merli, 1956), almost 1500
parthenogenetic larvae of the first instar were cytologically examined
with respect to their sex, and at least 99.9% of them proved to be
females.
Thus, on the ground of different cytogenetic data there is hardly any
doubt now that all heat-activated unfertilized eggs possess the female
chromosome constitution from the very beginning of their development,
i.e., immediately after the termination of artificially induced maturation
division.
3. Selection of Parthenogenetic Clones
There exists an extremely wide variation of the capacity for heat
parthenogenesis among the individuals belonging to every population
of the silkworm so far investigated. The variability limits range from
complete incapacity to the ability to yield up to 90% of viable parthenogenetic larvae from heat-treated eggs. This individual susceptibility
to parthenogenesis is a strictly hereditary character, and owing to genotypic identity between the female and her parthenogenetic progeny, it
is found in the parthenogenetic daughters to the same degree as in
their mothers.
It is very easy therefore to select individual females and their parthenogenetic progeny, possessing a rather high capacity for heat parthenogenesis, and to obtain parthenogenetic clones readily propagating by
this artificial mode of reproduction throughout an unlimited number of
generations. Owing to their genotypical identity, females belonging to
the clone resemble not only the maternal type, but each other, just like
identical twins or specimens of one and the same clone of asexually
reproducing plants.