Nonreciprocal gonadal dysgenesis in Chironomus thummi hybrids: Temperature sensitivity of female sterility

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Abstract
In nonreciprocal hybrids of Chironomus thummi an environmental factor has been detected which, along with genetic factors, determines gonadal dysgenesis. Female hybrids of the cross Ch' thummi thummi ♂ ♂ × Ch. thummi piger ♀ ♀ show various degrees of rudimentary developed ovaries and sterility. The extent of these abnormalities is dependent on the developmental temperature of the hybrids. At a temperature of 21°C approximately 90% of the females are completely sterile and at 16°C only 30%. The curative effect of a temperature of 16°C on sterility occurs, however, only in those hybrid females which hatch from a specific type of egg mass (class A). Females of another type of egg mass (class B) show nearly as many dysgenic ovaries as do those developed at 21°C. At a developmental temperature of 21°C no such differentiation between the A and B class of egg masses is possible. Ovarian dysgenesis and sterility is induced during atemperature‐sensitive period which extends from the beginning of embryonic development through the first two‐thirds of the first larva instar stage. The abnormalities observed must be due to a failure in the early development of the germ line and are probably initiated by an inhibition of primordial germ cell divisions.