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 preand postmating isolation is common in this species complex and, due to its cosmopolitan distribution, species often occur in sympatry. Chemical signals play a key role in intraand interspecies communication of aquatic animals. Crowding chemicals, for example, influence a variety of life history traits, such as feeding, growth, age at first reproduction, or the induction of bisexual reproduction, and are thus important determinants of population dynamics (Kirk 1998; Yoshinaga et al. 1999; Mitchell and Carvalho 2002). Previous studies provided mixed evidence with regard to the specificity of such signals. Some studies with zooplankton found that crowding chemicals operate across species (Hobak and Larsson 1990; Carmona et al. 1993), whereas others demonstrated species specificity (Gilbert 1963, 2003). Intuitively, one would expect specificity to evolve when heterospecific signals interfere with important life history processes, i.e., when responding to a heterospecific cue would reduce fitness by triggering an inappropriate physiological response. In this case, selection would favor divergence of crowding signals during speciation. Accordingly, distantly related species should be less responsive to heterospecific crowding signals than to those of closely related species. The objective of this study was to examine this prediction in an explicitly phylogenetic context for representatives of the Brachionus plicatilis species complex. Crowding chemicals that induce bisexual reproduction are known in cyclical parthenogens, such as cladocerans or rotifers. Cyclical parthenogens may reproduce for generations 1 Corresponding author ([email protected]).