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SUMMARY
In cross-breeding experiments, males and females of Gammarus pulex (L.) and G. fossarum Koch can form mixed couples in both possible combinations. Copulation takes place shortly after moulting of the female. The sperm of the male stimulates oviposition in the female of both G. pulex and G. fossarum. Cleavage of some eggs starts just after oviposition.
Nevertheless the two species are intersterile, the isolating mechanism consisting of : (1) the preference of the males to mate with conspecific females, (2) better linkage within conspecific precopulated pairs, and (3) after fertilisation, hybrid eggs do not develop beyond the early morula stages.
The plumosity of the setae an the exopod of the 3rd uropod, a character on which the presence of intermediates (in nature) between G. pulex and G. fossarum was presumed, proved to be typical for very old and large males of G. fossarum.
