Hybridization and gene flow between two cryptic species in the Macaffertium modestum complex (Ephemeroptera: Heptageniidae)

Genetic data (“barcoding” and allozymes) suggest the currently accepted concept of Macaffertium modestum (Banks) in eastern North America includes several morphologically cryptic species. Two of these (hereafter “dark” and “light” based on coloration in the imago) are commonly found together in small to medium-sized streams throughout much of eastern North America. Although larval morphology, habitat, phenology and ovipositional behaviors appear identical, mating swarms are spatially distinct and nuclear genetic data indicates little or no recent gene flow between them. In the laboratory, imagos of both species readily copulated with either conspecifics or heterospecifics. Pure and hybrid eggs exhibited similar rates of fertilized hatch and larval survivorship. Imaginal coloration bred true in conspecific crosses while hybrids were intermediate. Both male and female progeny were produced by “dark” females mated with “light” males, but most males died during metamorphosis and none mated successfully. Progeny reared from reciprocal crosses were entirely female. Eggs from F1 hybrid females backcrossed with pure males hatched at rates similar to those observed in conspecific crosses. Larval rearing experiments are ongoing. These experiments suggest that barriers to gene flow between “light” and “dark” Macaffertium modestum are largely prezygotic and depend upon differences swarm site selection.