Male sterility in interspecific meadow katydid hybrids

Spermatogenesis was cytologically analyzed in males of the conocephaline ka tydids Orchelimum nigripes and O. pulchellum, their hybrids obtained in the laboratory, and males from intermediate populations belonging to a broad h ybrid zone. The results showed a very high similarity between the two speci es for the cytogenetic markers analyzed (C-heterochromatin content and loca tion of nucleolus organizer regions), but artificial hybrids appeared to be completely sterile due to a complete absence of meiosis and spermatozoa in their reduced testes. Orchelimum nigripes and O. pulchellum are abundant and widespread conocepha line katydids found throughout much of the eastern United States. They are distributed west and east, respectively, of the Appalachian Mountains. Neit her is known from the Appalachians themselves. South of the Appalachians, t hey form a very broad, and very old, hybrid zone, as evidenced by analysis of allozyme and morphological data. In addition, O. nigripes occurs east of the Appalachians along the Potomac River above Washington, D.C., where it has apparently been established for only 75 years or less and where it now forms a narrow hybrid zone with O. pulchellum (SHAPIRO 1998). There are no known differences between the calling songs of the two taxa, and in prelimi nary phonotaxis trials females showed no evidence of discriminating between the two (unpublished data). However, in laboratory mating trials using Pot omac katydids, females of O. nigripes showed a strong preference for conspe cific males when offered a choice between a male of each species (L. Shapir o, submitted), although in no-choice trials O. nigripes females mated freel y with O. pulchellum males and produced normally viable offspring. The present study had two objectives. First, we wished to look for cytologi cal markers that might be useful for the study of the Orchelimum hybrid zon es. Second, we wished to analyze spermatogenesis and possible sterility of males collected from populations within the hybrid zone and hybrid males pr oduced in the laboratory.