Length variation in 18S rRNA expansion segment 43/e4 of Daphnia obtusa: ancient or recurring polymorphism?

Expansion segments in ribosomal DNA (rDNA) can show length variation at the level of the individual, yet our understanding of the evolutionary forces shaping this variation is incomplete. Previous studies of expansion segment 43/e4 of the 18S rRNA gene in Daphnia obtusa have examined this variation in six individuals; however, it is not known if the variation documented at this locus is representative of variation across the species' geographic range. Furthermore, it is unclear whether length variants found in multiple individuals share common ancestry, or were generated de novo through recombination. We quantified expansion segment length variant frequencies in 134 individual D. obtusa from 33 populations at 15 sites across the species range in the US, and used a phylogeographic approach to determine whether recombination continues to add to the standing crop of variation at this locus. We identified seven length variants across the sampling range, which spans almost 3000 km. Based on the phylogeographic distribution of length variants in the expansion segment, we conclude that they are shared ancient polymorphisms that have persisted despite the operation of molecular mechanisms that cause the concerted evolution of multigene families such as rDNA.