238
B. L. ASTAUROV
any exception and, consequently, their triploidy. By heat activation of
unfertilized ova extracted from 532 3 n females, many parthenogenetic
3 n -f- daughters were produced in 52.9% of batches. Hatching was 6.4%
per batch, attaining 63.0% in the best individual batch (Astaurov and
Vereiskaya, 1960). Some of the parthenofemales were expected to be
mixoploids, 3 n + 6 n. After reduction of 6 n oocytes (4 n mon + 2 n
mandarina), if homologous pairing is preferential, 3 n pronuclei (2 n
mori + 1 n mandarina) ought to be formed. When fertilized with mandarina haploid spermatozoa, amphidiploid (2 n mori -\-2n mandarina)
progeny of both sexes is expected. Thus, when 1049 crosses of hybrid
parthenofemales 3 n and 3 n/6 n X
hatched larvae (17.9) and the total percentage of hatching (0.36) are
close to those observed in matings of parthenofemales 3 n mori X cf 2 n
mori yielding autotetraploids. Sex ratio in 235 fifth-instar caterpillars
was 1.00 $ :1.05 cf. Cytogenetic analysis proved their tetraploidy and
presumably amphiploidy. Phenotypically they resemble diploid mori X
mandarina F x hybrids (Vereiskaya and Astaurov, 1962). When 103 males
were repeatedly mated, 51 of them (49.5%) actually proved to be partially fertile. Hatching percentage fluctuated from 0.0 to 35.8% per batch;
the average was 1.1%. Poor fertility indicates the probable segmental
allotetraploidy versus amphidiploidy. Up to 1966, eight successive generations of allotetraploids were reared; 1°, primary "amphidiploids"
(A x ); 2°, "amphidiploids" A 2 and Ri-hybrids from backcrosses 9 4 n
mori X cT 4 n "amphi"; 3°, progeny of $ 4 n Ri X cf 4 n R x and of $ 4 n
mori X cf 4 n Ri crosses (Astaurov and Vereiskaya, 1963b) ; 4°, further
generations obtained by intercrossing of different $ 4 n X cf 4 n.
Presently (the winter of 1966-1967) several batches of the ninth
generation of bisexual tetraploid strain are hibernating. The total sum
of allotetraploids obtained in the silkworm amounts now to some 5500
individuals. It is worth mentioning that, in intercrosses of 4 n moths, the
sex ratio is fairly normal, approaching 1 $ : 1 cf · This depends on the
fact that the W chromosome in B. mori has a strong female-determining
ability ; the ZZZW individuals were females which, when mated to ZZZZ
males, produced typical 1:1 sex segregation.
The situation in the silkworm with respect to sex determination greatly
resembles that found in tetraploids of the plant Melandrium album
where the sex determining mechanism $ XXXX X cf XXXY also works
rather regularly despite polyploidy. Thus, Müller's well-known thesis
(Müller, 1925), that polyploidy in bisexual animals is rarely or never
seen because it is quite inconsistent with the chromosomal sex-determining mechanism, is invalid at least in some cases.
B. L. ASTAUROV
any exception and, consequently, their triploidy. By heat activation of
unfertilized ova extracted from 532 3 n females, many parthenogenetic
3 n -f- daughters were produced in 52.9% of batches. Hatching was 6.4%
per batch, attaining 63.0% in the best individual batch (Astaurov and
Vereiskaya, 1960). Some of the parthenofemales were expected to be
mixoploids, 3 n + 6 n. After reduction of 6 n oocytes (4 n mon + 2 n
mandarina), if homologous pairing is preferential, 3 n pronuclei (2 n
mori + 1 n mandarina) ought to be formed. When fertilized with mandarina haploid spermatozoa, amphidiploid (2 n mori -\-2n mandarina)
progeny of both sexes is expected. Thus, when 1049 crosses of hybrid
parthenofemales 3 n and 3 n/6 n X
hatched larvae (17.9) and the total percentage of hatching (0.36) are
close to those observed in matings of parthenofemales 3 n mori X cf 2 n
mori yielding autotetraploids. Sex ratio in 235 fifth-instar caterpillars
was 1.00 $ :1.05 cf. Cytogenetic analysis proved their tetraploidy and
presumably amphiploidy. Phenotypically they resemble diploid mori X
mandarina F x hybrids (Vereiskaya and Astaurov, 1962). When 103 males
were repeatedly mated, 51 of them (49.5%) actually proved to be partially fertile. Hatching percentage fluctuated from 0.0 to 35.8% per batch;
the average was 1.1%. Poor fertility indicates the probable segmental
allotetraploidy versus amphidiploidy. Up to 1966, eight successive generations of allotetraploids were reared; 1°, primary "amphidiploids"
(A x ); 2°, "amphidiploids" A 2 and Ri-hybrids from backcrosses 9 4 n
mori X cT 4 n "amphi"; 3°, progeny of $ 4 n Ri X cf 4 n R x and of $ 4 n
mori X cf 4 n Ri crosses (Astaurov and Vereiskaya, 1963b) ; 4°, further
generations obtained by intercrossing of different $ 4 n X cf 4 n.
Presently (the winter of 1966-1967) several batches of the ninth
generation of bisexual tetraploid strain are hibernating. The total sum
of allotetraploids obtained in the silkworm amounts now to some 5500
individuals. It is worth mentioning that, in intercrosses of 4 n moths, the
sex ratio is fairly normal, approaching 1 $ : 1 cf · This depends on the
fact that the W chromosome in B. mori has a strong female-determining
ability ; the ZZZW individuals were females which, when mated to ZZZZ
males, produced typical 1:1 sex segregation.
The situation in the silkworm with respect to sex determination greatly
resembles that found in tetraploids of the plant Melandrium album
where the sex determining mechanism $ XXXX X cf XXXY also works
rather regularly despite polyploidy. Thus, Müller's well-known thesis
(Müller, 1925), that polyploidy in bisexual animals is rarely or never
seen because it is quite inconsistent with the chromosomal sex-determining mechanism, is invalid at least in some cases.