Abstract
Phyciodes “tharos” in the northeastern United States is actually composed of two entities, “Type A” and “Type B,” which differ in phenotypic appearance, geographic range, ecology, flight dates, voltinism, and development rate. Laboratory crosses between populations of Type A from southwestern Pennsylvania and Type B from Pennsylvania, New York State, and Vermont showed a syndrome of hybrid breakdown manifested by reduction in F1 hybrid fertility and F1 and backcross embryonic viability, and abnormal adult sex ratios, development times and eclosion patterns. In addition there was significant hybrid incompatibility involving reductions in embryonic viability and abnormal sex ratios in crosses between the Type B populations from New York and Vermont. This incompatibility seems to be due to parental differences in comparatively few specialized and highly critical genetic systems responsible for growth and development. Type A (typical P. tharos ) and Type B (eastern P. pascoensis ) are full species which maintain reproductive isolation in a narrow band of sympatry.
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