Mitochondrial (mt) DNA control region sequences were used to test the genetic and phylogeographic structure of walleye Stizostedion vitreum populations at different geographical scales: among spawning sites, lake basins, lakes, and putative glacial refugia in the Great Lakes region. Sequencing 199 walleye revealed nucleotide substitutions and tandemly repeated sequences that varied in copy number, as well as in sequence composition, in approximately 1200 bp of the mtDNA control region. Variable numbers of copies of an 11-bp tandem repeat showed no geographical patterning and were not used in further analyses. Substitutions in the other areas of the control region yielded 19 haplotypes, revealing phylogeographic structure and significant differences among glacial refugia, lakes, basins and some spawning sites. Differences among spawning populations were consistent with reduced gene flow, philopatry and possible natal homing. Analysis of spawning populations showed consistency of genotypic frequencies among years and between males and females, supporting philopatry in both sexes. The unglaciated plateau in southern Ohio, USA housed a very different haplotype that diverged prior to the Missouri, Mississippi and Atlantic glacial refugia types. Haplotypes from the three refugia colonized the Great Lakes after retreat of the Wisconsin glaciers, and their present distribution reflects the geography of their prior isolation and differential colonization. Populations that became associated with spawning localities appear to have diverged further due to philopatry, resulting in fine-scale phylogeographic structuring.

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Summary 1.Males and females often differ in their optimal mating rates, resulting potentially in conflicts over remating. In species with separate sexes, females typically have a lower optimal mating rate than males, and can regulate contacts with males accordingly. The realized mating rate may therefore be closer to the female's optimum. In simultaneous hermaphrodites, however, it has been suggested that the intraindividual optimization between 'male' and 'female' interests generates more 'male'-driven mating rates. 2. In order to assess the consequences of variation in mating rate on 'female' reproductive output, we exposed the simultaneously hermaphroditic sea slug Chelidonura sandrana to four mating rate regimes and recorded the effects on a variety of fitness components. 3. In focal 'females', we found (i) a slight but significant linear decrease in fecundity with mating rate, whereas (ii) maternal investment in egg capsule volume peaked at an intermediate mating rate. 4. Combining the observed fecundity cost with the apparent benefits of larger offspring size suggests that total female fitness is maximized at an intermediate mating rate. With the latter being close to the natural mating rate of C. sandrana in the field, our findings challenge the assumption of 'male'-driven mating systems in simultaneous hermaphrodites. 5. Our study provides experimental evidence for various mathematical models in which female fitness is maximized at intermediate mating rates.

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