Phylogenetic branching of Trichoptera and Lepidoptera: an ultrastructural analysis on comparative spermatology

The divergence during the evolution of the sister orders of Trichoptera (caddisflies) and Lepidoptera (moths and butterflies) from their common ancestors is still subject to disagreement. This is partially due to lack of clarity concerning the phylogenetic position of Zeugloptera which is considered to be either a very primitive suborder of Lepidoptera or an order of its own, more archaic than both Trichoptera and Lepidoptera. The ultrastructural analysis on comparative spermatology reported here shows that dichotomous spermatogenesis producing concomitant, normal, anucleate (apyrene) and nucleate (eupyrene) spermatozoa: (1) is generalized in Lepidoptera as it occurs, in addition to higher Lepidoptera, also in the primitive Hepialoidea, (2) is present in Zeugloptera, and (3) is absent from Trichoptera which produce only nucleate spermatozoa. It is concluded that dichotomous spermatogenesis is an evolutionary novelty of Lepidoptera (including Zeugloptera) that was established at the origin of the order, after the divergence of Trichoptera and Lepidoptera.