Mermithid (nematoa: mermithidae) infections of black flies (Diptera: Simuliidae) : seasonal variation and developmental characteristics /

Mermithid nematodes (Nematoda: Mermithidae) parasitize larval, pupal and adult black flies (Diptera: Simuliidae), oftentimes resulting in partial or complete host feminization. This study was designed to characterize parasite-host seasonal variation and to estabUsh the developmental life stage at which feminization is initiated. Data indicate that the total adult population of black flies collected from Algonquin Provincial Park throughout the spring of 2004 was comprised of 31.8% female, 67.8% male and 0.4% intersex individuals. Of the total population, 0.6% was infected by mermithid nematodes (69.0% female, 3.5% male and 27.6% intersex). Seasonal infection trends established over a 12-month period revealed that black flies with different life histories host the same mermithid subfamilies, while black flies with similar life histories host mermithids from different subfamilies. If a simuliid species simultaneously hosts two mermithid species, these parasites are from different subfamilies. Molecular mermithid identification revealed two mermithid subfamilies, Me.somermithinae and Gastromermithinae, present in the simuliid hosts. Mermithid colour variation was not found to be a reliable species indicator. The developmental stage at which feminization is initiated was determined by examining gonad morphology and meiotic chromosomal condition. Results indicate that mermithid-infected black flies exhibit feminization prior to larval histoblast formation. Larvae can be morphologically male (testes present) or female (ovaries present), with morphological males exhibiting either male (achiasmate) or female (chiasmate) meiotic chromosomes; morphological females were only genetically female. Additionally, mermithid infection inhibits simuliid gonad development.