Genetic structure of leaf-beetles populations: microgeographic and sexual differentiation in <i>Oreina cacaliae</i> and <i>O. speciosissima</i>

We used <i>F</i>-statistics to quantify the population structure of two sympatric species of leaf beetles, <i>Oreina cacaliae</i> and <i>O. speciosissima</i> (Chrysomelinae, Coleoptera), which share the same microhabitat since they feed on the same herbaceous host plants. We measured genetic differentiation at six allozyme loci 1) among populations separated by relatively small distances (40 to 250 kilometers), 2) within each population, and 3) between sexes within populations. For both species, the populations studied are not panmictic. For each population, heterozygosities are relatively high, but the observed heterozygosities are generally lower than the expected values. Overall, within-population differentiation is high and similar for both species (<i>F</i>_is=0.326 for <i>O. cacaliae</i> and 0.332 for <i>O. speciosissima</i>). Additionally, populations of both species are highly differentiated (<i>F</i>_st=0.234 versus 0.051 for <i>O. speciosissima</i>). For <i>O. cacaliae</i>, <i>F</i>_is and <i>F</i>_st are greater among females than among males, while for <i>O. speciosissima</i>, <i>F</i>_st is sustantially greater among the males while <i>F</i>_is is slightly greater among males. Differences in gene frequency among the sexes were statistically tested using a modified <i>F</i> st with sex as the defining category, and the sexes differed significantly with the exception of one population in <i>O. cacaliae</i>. Possible explanations for this difference are discussed.