Molecular genetic markers provide no evidence for reproductive isolation among retreat building phenotypes of the net-spinning caddisfly Macrostemum carolina

Larvae of the stream-dwelling, filter-feeding caddisfly Macrostemum carolin a construct silken catchnets within protective retreats. In the Savannah Ri ver, M. carolina individuals make three different retreats, each with a dis tinct water entrance hole: (i) at the end of a silken tube; (ii) with a app roximate to 180 degrees silken backstop; and (iii) flush with the top of th e retreat. To resolve whether these different retreats represent alternativ e behavioural phenotypes within a single panmictic population or fixed phen otypes within three genetically distinct populations or species, we compare d the allele frequencies at three polymorphic nuclear loci (allozyme electr ophoresis for Gpi, Mpi and Pgm) and the mitochondrial DNA (mtDNA) haplotype frequencies among individuals displaying the three retreat morphs. We also calculated pairwise exact tests of population differentiation using the al lozyme and mtDNA allele frequencies. No significant genetic differentiation was detected among caddisflies exhibiting the different retreat morphs. Th erefore, these morphs apparently represent a single panmictic population in the Savannah River. Consequently, additional study is required to assess w hether this retreat polymorphism is a phenotypically plastic trait under co nditional control, or is mediated by alternative alleles at a Mendelian gen e or genes (or a combination of the two).