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
229
considered complete tetraploids. The frequency of primary polyploidization is very high; for instance, of 262 primarily parthenogenetic females
which have been studied in this respect, 72 females, or 27.5%, proved to
be tetraploid or partially tetraploid, while the average percentage of
tetraploid ova among those laid by all primary tetraploid parthenogenetic
females was 5-6%. During the first maturation division 112 chromosomes (28 X 4) of the unreduced 4 n oocyte form 56 ordinary tetrads.
Thus, every four homologous chromosomes of a tetraploid female conjugate as two pairs and form two simple tetrads and not one double
tetrad.
C. Parthenogenetic Reproduction of Tetraploids and the
Origin of 4 n Clones
Unfertilized tetraploid eggs may be stimulated to parthenogenetic
development. For this purpose the same method of heat activation may
be applied as that used for diploid eggs, although with much less success.
In the case of successful heat activation, tetraploid eggs give rise to
tetraploid females, which are capable of further parthenogenetic or bisexual propagation. Autotetraploid clones propagating by means of
repeated thermoactivation could be obtained in this way (Fig. 2E,
6a-8a, F,l-6).
The preservation of tetraploidy through parthenogenetic propagation
proceeds by means of a cytogenetic mechanism, which is apparently
similar to that ensuring the preservation of diploidy after artificial heat
parthenogenesis in diploid eggs, i.e., the reduction division of tetraploid
eggs is suppressed and only the equational division is retained. Neither
complete nor partial reversion from tetraploid to diploid structure ever
takes place in the parthenogenetic development of tetraploid heat-treated
ovarian eggs (see Section III,A,2).
D. Mating of 4 n Females and Bisexual Production of
Triploid Females and Males
Crossing of tetraploid females with diploid males results in triploid
offspring of both sexes. The triploid females and males are sterile both
when crossed with one another or with normal diploids (Fig. 5). Triploid
females crossed with diploid males lay egg batches containing eggs of
various size and shape. About 25% of the eggs oviposited by triploid
females fail to develop and have the appearance of unfertilized ova ; the
rest develop at least to the stage of normally pigmented serosa and then
perish.
During spermatogenesis of triploid males, every three homologous
chromosomes either conjugate together forming a trivalent, or only two
of them conjugate and one behaves as an independent element. Owing
229
considered complete tetraploids. The frequency of primary polyploidization is very high; for instance, of 262 primarily parthenogenetic females
which have been studied in this respect, 72 females, or 27.5%, proved to
be tetraploid or partially tetraploid, while the average percentage of
tetraploid ova among those laid by all primary tetraploid parthenogenetic
females was 5-6%. During the first maturation division 112 chromosomes (28 X 4) of the unreduced 4 n oocyte form 56 ordinary tetrads.
Thus, every four homologous chromosomes of a tetraploid female conjugate as two pairs and form two simple tetrads and not one double
tetrad.
C. Parthenogenetic Reproduction of Tetraploids and the
Origin of 4 n Clones
Unfertilized tetraploid eggs may be stimulated to parthenogenetic
development. For this purpose the same method of heat activation may
be applied as that used for diploid eggs, although with much less success.
In the case of successful heat activation, tetraploid eggs give rise to
tetraploid females, which are capable of further parthenogenetic or bisexual propagation. Autotetraploid clones propagating by means of
repeated thermoactivation could be obtained in this way (Fig. 2E,
6a-8a, F,l-6).
The preservation of tetraploidy through parthenogenetic propagation
proceeds by means of a cytogenetic mechanism, which is apparently
similar to that ensuring the preservation of diploidy after artificial heat
parthenogenesis in diploid eggs, i.e., the reduction division of tetraploid
eggs is suppressed and only the equational division is retained. Neither
complete nor partial reversion from tetraploid to diploid structure ever
takes place in the parthenogenetic development of tetraploid heat-treated
ovarian eggs (see Section III,A,2).
D. Mating of 4 n Females and Bisexual Production of
Triploid Females and Males
Crossing of tetraploid females with diploid males results in triploid
offspring of both sexes. The triploid females and males are sterile both
when crossed with one another or with normal diploids (Fig. 5). Triploid
females crossed with diploid males lay egg batches containing eggs of
various size and shape. About 25% of the eggs oviposited by triploid
females fail to develop and have the appearance of unfertilized ova ; the
rest develop at least to the stage of normally pigmented serosa and then
perish.
During spermatogenesis of triploid males, every three homologous
chromosomes either conjugate together forming a trivalent, or only two
of them conjugate and one behaves as an independent element. Owing