Spermatozoa and phylogeny of Curculionoidea (Coleoptera)

Abstract

In the examined families of Curculionoidea (Coleoptera), the sperm, although characteristic of typical pterygote insects, shows a few peculiarities that suggest Curculionoidea to be a homogeneous group. The curculionid sperm, in fact, always follows a similar structural design, without any variation. For example, it has 2 mitochondrial derivatives of different sizes, the larger of which is almost completely filled with a crystalline protein, the other being more moderately crystallized and almost completely occupied by cristae, and 2 accessory bodies of different sizes that are made up of a crystalline portion, crescent-shaped in section, and a “puff”-like expansion that is of different consistency, shape, and symmetry in various cases. The different extensions of the accessory bodies seem, therefore, to compensate for the high degree of asymmetry due to the largely different sizes of the 2 mitochondrial derivatives. The examined families and subfamilies can be arranged in 2 groups: Rhynchitidae appear drastically isolated, because they have a peculiar “9 + 9 + 0” axoneme, and show, moreover, a limited degree of asymmetry in the tail organelles. The remaining families and subfamilies are more closely related to one another by the presence of a “9 + 9 + 2” classical axoneme and by the same degree of asymmetry in the tail, typical of curculionid sperm. Among them, Apionidae are distinguished for the space containing the extraacrosomal layer, which may be hollow or absent, a twice-stepped nucleus-tail connection, and a thick glycocalyx at the end of the tail. The Curculionidae conserve primitive characters, such as the 3-layered acrosomal complex and “9 + 9 + 2” axoneme, but also present a high degree of differentiation in the shape of the asymmetrical tail organelles. There appear to be 3 clusters: the first cluster includes Brachyderinae, Leptopiinae, Gymnetrinae, Cryptorhynchinae, Rhynchophorinae. The second cluster includes Scolytidae, Cleoninae, Hylobiinae. The third cluster is more numerous and heterogeneous and shows 3 subgroups. The first of these includes only Otiorhynchinae. The sum of their characters shows that they have most of the common features of primitive curculionids; however the differences between a genus and another are so large that they could be assigned to different subfamilies. The second subgroup includes Hyperinae, Pissodinae, Magdalinae, Ceutorhynchinae and Cossoninae, and the third group includes Cioninae, Anthonominae, and Barimae. It is difficult to arrange these subfamilies (frequently recognizable for a different combination of the same recurrent characters) in a phylogenetic tree. However, we observed signs of primitiveness in Brachyderinae (small crescents) and their cluster; advanced ones in the third and fourth clusters all evolved with different patterns of the puff-like expansion of one of the accessory bodies, the latter being the most peculiar character of the superfamily. A tentative reconstruction is given. The functional significance of the variations seems to be that Rhynchitidae seem to be evolving towards immotility (their spermatozoon, in fact, is able to produce only a series of vibrations, not the progressive series of waves as in all other species studied), while all the other families and subfamilies show no signs of alterations in axonemal pattern and motility. The main evolutionary pathways observed in them are towards compensating for an exaggerated lengthening and a greater degree of asymmetry in the tail organelles: one of them, the major mitochondrial derivative, acts as a rigid axis, while the axoneme produces undulations in the opposite portion of the axonemal section.