Specificity of crowding response that induces sexuality in the rotifer Brachionus

Crowding induced the production of mictic (male-producing) females in Brachionus calyciflorus from two North American strains and an Australian strain. The specificity of this response to crowding was tested by culturing single individuals of a North American clone in three treatment conditions: a small volume (high density), a large volume (low density), and a large volume with a high density of an Australian clone. The results were consistent and clear in six experiments using different combinations of clones. Crowding low-density individuals of the North American strains with the Australian strain failed to induce them to produce mictic females. The mictic-female response in this treatment was similar to that in the low-density treatment, and both of these responses were significantly lower than that in the high-density treatment. Since the mictic-female response to crowding in Brachionus is mediated by a chemical produced by the rotifers themselves, the chemical inducers produced by the Australian and North American strains must be different. Taxonomically specific responses to crowding should increase fitness by assuring that sexual reproduction in the heterogonic life cycle coincides with a high population density of individuals able to mate with one another and, thus, when the production of fertilized resting eggs can be maximized. This would be especially important in plankton communities with diverse rotifer assemblages and multiple congeneric species. Otherwise, a low-density population of a species could be induced to initiate bisexual reproduction by populations of other species, curtailing its potential for population growth via female parthenogenesis and limiting its production of resting eggs in the future. Changes in zooplankton demographics and reproductive mode may occur at high population densities, and they can be induced by chemicals produced by the population itself. Crowding in laboratory populations of cladocerans and rotifers can affect feeding, growth, age at first reproduction, and fecundity. In many of these cases, the positive or negative effects of crowding are attributable to unidentified chemicals produced by the population (Seitz 1984; Helgen 1987; Matveev 1993; Goser and Ratte 1994; Burns 1995, 2000; Kirk 1998; Yoshinaga et al. 1999; Mitchell and Carvalho 2002). In addition, crowding provides a stimulus for the initiation of bisexual reproduction in both Daphnia (Stross and Hill 1965) and Brachionus (Gilbert 1963a, 2002; Hino and Hirano 1976; Snell and Boyer 1988). The basis for these crowding effects also is unidentified chemicals produced by the population (Hino and Hirano 1976; Hobæk and Larsson 1990; Kleiven et al. 1992; Carmona et al. 1993; Stelzer and Snell 2003). While considerable attention has been given to demonstrating the chemical nature of crowding effects, less has been devoted to determining the species specificity of crowding responses or crowding chemicals. The chemicals responsible for affecting growth and fecundity in Daphnia operate across species (Seitz 1984; Burns 1995, 2000), and water conditioned by Daphnia carinata inhibits feeding in conspecifics, Daphnia lumholtzi, Moina micrura, and Diaphanosoma unguiculatum (Matveev 1993). Three studies have tested the specificity of the crowding stimulus for sexual reproduction in Daphnia and Brachionus. Male production in Daphnia magna was higher in media