Chiasmate meiosis in male black fly (Diptera: Simuliidae) larvae associated with mermithid infections (Nematoda: Mermithidae)

Mermithid nematodes (Nematoda: Mermithidae) parasitize larval, pupal, and adult black flies (Diptera: Simuliidae) and may result in the development of intersex adults, generally considered to be feminized males. Simuliid gonad morphology and meiotic chromosome condition were examined in both early-instar larvae and late-instar larvae (≈ pharate pupae) to determine the developmental stage at which mermithid-associated feminization was initiated. We found that feminization occurred in early-instar larvae prior to gill histoblast formation. Infected larvae were morphologically male (rounded testes present) or female (elongated ovaries present), with morphological males exhibiting either male (achiasmate) or female (chiasmate) meiotic chromosomes; morphological females only had chiasmate chromosomes. Mermithid infection can also inhibit simuliid gonad development completely.