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C banding in polytene chromosomes of Chironomus riparius from a Santena polluted station of the river Po, Italy was described. A large variation in the appearance of C bands was established. The process involved the transformation of euchromatin into heterochromatin. Activation of all chromosomes at telomeres was found. A structural modification of telomeric heterochromatin may exist. The centromeric heterochromatin was rarely observed. A high percentage of an amplification (88%) in arm F, B3h, was recorded. These changes in the appearance of heterochromatin in the population studied could be considered as a type of polymorphic system, existing in the polluted population. For the first time in C. riparius two types of position effect variegation are reported. In arm E the euchromatin near to the centromere became inactive due to a heterozygous inversion and in arms D and F the opposite phenomenon was observed in which a heterozygous inversion partly suppressed the inactivation of heterochromatin.

Ophryotrocha diadema is a simultaneously hermaphroditic polychaete worm with a brief adolescent protandrous phase. In the mating system of this worm, pairs are formed preferentially between simultaneous hermaphrodites. Both partners of a pair regularly alternate sex roles, reciprocate egg exchange, and care for developing embryos. It was experimentally shown that neglected eggs have a 31% probability of dying, but eggs cared for by both parents, by a single parent, or even by an unrelated individual have a 95% probability of developing. In pairs experimentally prevented from caring for eggs, intruders succeeded in substituting one of the partners significantly more frequently than in pairs allowed to care for eggs. Thus, the main adaptive value of biparental care would be to guarantee a partner for egg reciprocation. Protandrous males are inefficient brooders, and biparental care becomes fully established only when individuals are able to reciprocate egg exchange, i.e., they have reached the simultaneously hermaphroditic stage., (© 1991 The Society for the Study of Evolution.)