Zygotic doubling is an additional way in which polyploidy can arise. This mode, which is less common in a natural setting than either of the two other main ways of producing polyploids (i.e., somatic multiplication and the union unreduced gametes), involves a doubling of the chromosome number in the zygote (hence the name). All reported cases of chromosome doubling in the zygote seem to involve artificial processes (e.g., exposure to chemicals, heat and cold shock, and hydrostatic pressure). While similar conditions are no doubt found at times in a state of nature, this mode of polyploidization is poorly known in a natural setting and requires further investigation to assess its significance.
For example, maize zygotes exposed 24 hours after pollination to 40˚C produced 1.8 percent tetraploid and 0.8 percent octoploid seedlings. Dorsey (1936) obtained similar results by heat shocking rye (Secale), wheat (Triticum), and rye-wheat hybrids. Polyploidy has also been induced by this means in many other plants, as well as in fungi, insects, amphibians, and fish by various combinations of heat shock, cold shock, chemical treatment, radiation, centrifugal force, and hydrostatic pressure.
Astaurov (1936, 1957, 1967a, 1967b) used an artificial process of this sort to create a parthenogenetic silkworm. He subjected unfertilized domestic silkworm (Bombyx mori) eggs to a temperature of 46˚C for 18 minutes and then treated them with hydrochloric acid. Individuals developing from such eggs are almost exclusively parthenogenetic tetraploid females. Hybridization is often used in conjunction with physical inducers of polyploidy such as those just mentioned. For example, Astaurov went on to produce a sexual derivative by (1) crossing his new tetraploid with wild silkworm males (B. mandarina); (2) fertilizing triploid eggs from the resulting hybrid with haploid sperm from B. mori to produce a fertile sexual tetraploid. Since this new form was reproductively isolated from its parents, Astaurov claimed it should be treated as a species and named it B. allotetraploidus.
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