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We evaluated genetic variation in 40 populations (five species) of ephemerellid mayflies collected from streams in the Atlantic and Gulf drainages of eastern North America. The principal study species included Eurylophella funeralis (McDunnough), Eurylophella verisimilis (McDunnough), Ephemerella subvaria McDunnough, Ephemerella aurivillii (Bengtsson) and Ephemerella septentrionalis McDunnough. For the 40 populations, proportion of polymorphic loci averaged 22.3% (SEM = 2.0; range, 0–50%), with relatively low genie heterozygosity (ݲ ± SEM = 0.079 ± 0.006; range, 0–0.160). Nei's genetic distance ( D ) averaged 0.006 among conspecific mayfly populations and 1.434 among species. A genetic distance of 0.704 suggests that Eu. funeralis and Eu. verisimilis are distantly related congeners. Genetic distances among populations of the three principal Ephemerella species were even higher, with D values averaging >1.0. Genetic distance data for five additional Ephemerella species, Ephemerella dorothea Needham, Ephemerella excrucians Walsh, Ephemerella inconstans Traver, Ephemerella invaria (Walker), and Ephemerella rotunda Morgan, revealed that D values

The genus Eurylophella includes nine species from eastern North America, one from western North America, and three from Europe. We examined 18 polymorphic enzyme loci in 82 populations (>2,000 individuals) of Eurylophella from 40 localities in eastern North America, representing eight of the nine currently recognized eastern species. Our data suggest that Eurylophella is actually represented by at least 15 species in eastern North America, some of which cannot be resolved morphologically. Although intraspecific comparisons revealed significant geographic variation in allele frequencies, there were no fixed allelic differences between populations. In contrast, most interspecific comparisons were characterized by the lack of shared alleles at several loci. Thus, >99% of the individuals studied could be sorted to species by electromorph characteristics. The average Nei's genetic distance ( D ) was 0.02 (range = 0-0.12) between conspecific populations and 0.74 (range = 0.11-1.87) between species. A phenogram generated from D values shows five groups of species branching at D < 0.60: • E. verisimilis, E. verisimilis-A, E. verisimilis-B, E. verisimilis-C, E. bicolor, E. minimella, E. prudentalis, E. aestiva , and E. aestiva-A; • E. temporalis-A, E. temporalis-B , and E. temporalis-C; • E. sp. 1; • E. funeralis; and • E. lutulenta. Interspecific D values for Eurylophella were in the upper range of reported literature values for congeneric insect species, a fact that supports the recent elevation of Eurylophella from subgeneric to generic status. Expected heterozygosities ranged from 0.03 to 0.21; those of most species averaged 0.10-0.12.