Molecular differentiation of two sibling species of the black fly Simulium vittatum (Diptera: Simullidae) based on random amplified polymorphic DNA

Larvae of the black fly morphospecies Simulium vittatum front Colorado, Montana, Nebraska, and New Hampshire were cytologically identified as either the IS-7 or the IIIL-1 cytospecies. DNA was PCR amplified from cytotyped larvae using eight 10-mer primers, labeled with 33 P, and run on polyacrylamide gels. The entire data set of 96 amplicons produced incomplete separation of the two cytospecies when subjected to neighbor-joining and maximum parsimony analyses. However, when analyzed within geographical regions, separate species status was supported. Bootstrap support for distinctness of the two cytospecies was best in Colorado where they were collected in true sympatry. The IS-7 cytospecies was more polymorphic in the western states, where it differed most from IIIL-1, which was most variable in the eastern states. The frequencies of the 17 most common amplicons in the two cytospecies were inversely correlated. A model of speciation derived from the molecular evidence suggests that IS-7 evolved in the west and spread eastward, whereas IIIL-1 later originated in the east and spread westward.