Specificity of the crowding response in the Brachionus plicatilis species complex

Crowding chemicals influence a wide variety of life history traits in zooplankton communities. In the rotifer Brachionus, sexual reproduction (mixis) is induced by a chemical signal produced by the rotifers that accumulates during population growth. The specificity of the reaction to the mixis induction signal could play a central role in maintaining reproductive barriers between closely related sympatric species. Using cross-induction assays between different species, we tested whether this signal has diversified in the Brachionus plicatilis species complex. We found that closely related, as well as more distant species in this complex, could induce mixis in each other. This suggests that there are no species barriers in sex induction and that the mixis signal did not diversify for several million years during the evolution of the Brachionus plicatilis complex. This apparent stasis is remarkable because pre- and postmating isolation is common in this species complex and, due to its cosmopolitan distribution, species often occur in sympatry.