CYTOGENETIC MECHANISMS IN SILKWORMS
225
IV. Parthenogenesis and Hybridization
A. Ability to Undergo Artificial Parthenogenesis in Pure Races
and Their Fi Hybrids
In the domesticated silkworm, the average capacity for heat parthenogenesis varies markedly from race to race but especially important
is that it is most pronounced in F ± interracial hybrids. Almost all of
our clones, yielding as a rule 50-60% of parthenogenetic larvae on the
average, are therefore of hybrid origin. They retain constant heterozygosity and clearly manifest greatly increased fecundity, vigor, and other
signs of heterosis appropriate to the silkworm interracial ¥ x hybrids.
B. Heat Parthenogenesis in Interspecific Reciprocal Hybrids
Bombyx mori L X Bombyx mandarina Moore
During the last few years some studies of artificial heat parthenogenesis in the wild silkworm species B. mandarina Moore and in reciprocal
interspecific hybrids between domesticated B. mori and wild mandarina
have also been performed (Astaurov, 1960, 1962).
The laboratory stock of B. mandarina, which has been used as a
source for living material, takes its origin from the South-Ussurie
population. It has the same haploid number of chromosomes (28) as
B. mori (Astaurov et al., 1959).
It was found that heat parthenogenesis could be readily induced in
pure mandarina species and its hybrids by the same method as in
B. mori. Parthenogenetic maturation is ameiotic, and only females arise.
It is important to emphasize the extremely pronounced difference
existing between both pure paternal species and their reciprocal hybrids
with respect to the capacity for parthenogenetic development. As a rule
the tendency to parthenogenesis in both pure species is comparatively
small.
In cases of primary activation of the diapausing B. mori eggs, as a
rule only some 20% of treated-egg batches yield hatched larvae and the
average percentage of parthenogenetic hatching in a random sample of
batches is usually of the order of 3-5%. Only in one case of primary
activation did the average percentage of hatching reach 18%. Still poorer
is the result of heat activation in the wild B. mandarina; the number
of diapause batches yielding larvae was 10-15%, and the average number
of hatched larvae was 0.5%.
A quite different picture is offered by heat parthenogenesis in eggs
taken from interspecific Y x hybrids. Heat treatment at 46°C for 15, 16,
and 18 min gives the best, and practically similar, results, but there are
225
IV. Parthenogenesis and Hybridization
A. Ability to Undergo Artificial Parthenogenesis in Pure Races
and Their Fi Hybrids
In the domesticated silkworm, the average capacity for heat parthenogenesis varies markedly from race to race but especially important
is that it is most pronounced in F ± interracial hybrids. Almost all of
our clones, yielding as a rule 50-60% of parthenogenetic larvae on the
average, are therefore of hybrid origin. They retain constant heterozygosity and clearly manifest greatly increased fecundity, vigor, and other
signs of heterosis appropriate to the silkworm interracial ¥ x hybrids.
B. Heat Parthenogenesis in Interspecific Reciprocal Hybrids
Bombyx mori L X Bombyx mandarina Moore
During the last few years some studies of artificial heat parthenogenesis in the wild silkworm species B. mandarina Moore and in reciprocal
interspecific hybrids between domesticated B. mori and wild mandarina
have also been performed (Astaurov, 1960, 1962).
The laboratory stock of B. mandarina, which has been used as a
source for living material, takes its origin from the South-Ussurie
population. It has the same haploid number of chromosomes (28) as
B. mori (Astaurov et al., 1959).
It was found that heat parthenogenesis could be readily induced in
pure mandarina species and its hybrids by the same method as in
B. mori. Parthenogenetic maturation is ameiotic, and only females arise.
It is important to emphasize the extremely pronounced difference
existing between both pure paternal species and their reciprocal hybrids
with respect to the capacity for parthenogenetic development. As a rule
the tendency to parthenogenesis in both pure species is comparatively
small.
In cases of primary activation of the diapausing B. mori eggs, as a
rule only some 20% of treated-egg batches yield hatched larvae and the
average percentage of parthenogenetic hatching in a random sample of
batches is usually of the order of 3-5%. Only in one case of primary
activation did the average percentage of hatching reach 18%. Still poorer
is the result of heat activation in the wild B. mandarina; the number
of diapause batches yielding larvae was 10-15%, and the average number
of hatched larvae was 0.5%.
A quite different picture is offered by heat parthenogenesis in eggs
taken from interspecific Y x hybrids. Heat treatment at 46°C for 15, 16,
and 18 min gives the best, and practically similar, results, but there are