The Evolution of Sperm Axoneme Structure and the Dynein Heavy Chain Complement in Cecidomid Insects S. Ciolfi, C. Mencarelli, and R. Dallai* Department of Life Sciences, University of Siena, Siena, Italy Received 25 September 2015; Revised 10 February 2016; Accepted 1 March 2016 Monitoring Editor: Ritsu Kamiya The 9 1 2 axoneme of cilia and flagella is specialized machinery aimed at the production of efficient, finely tuned motility, and it has been evolutionarily conserved from protists to mammals. However, the sperm cells of several insects express unconventional axonemes, which represent unique models for studying the structural– functional relationships underlying axonemal function and evolution. Cecidomids comprise a group of dipter- ans characterized by an overall tendency to deviate from the standard axonemal pattern. In particular, the subfamily Cecidomyiinae shows a series of progressive modifications of the sperm axoneme. We previously analyzed the unusual sperm axonemes of Asphondylia ruebsaameni (Asphondyliidi) and Monarthropalpus buxi (Cecidomyiidi), which are characterized by the absence of any structure related to the control of motil- ity (that is, the central pair complex, radial spokes and inner dynein arms); however, these sperm are motile, and motility is driven by the outer dynein arms only. This simplification of the motility machinery is accom- panied by a parallel reduction in the dynein isoform complement. Here, we complete our survey of the axo- nemal organization and the parallel evolution of sperm dynein complement in cecidomids with the characteri- zation of both the sperm ultrastructure and the dynein genes in Dryomyia lichtensteini, a representative of Lasiopteridi, the cecidomid taxon with aberrant and immotile sperm cells. On the basis of the whole set of our data, we discuss the potential molecular mecha- nism(s) underlying the progressive modification of axo- neme in cecidomids, leading first to a reduction of dynein genes and eventually to the complete loss of motility. V C 2016 Wiley Periodicals, Inc. Key Words: sperm axoneme; dynein; insect; cecidomid Introduction T he 9 1 2 axoneme of cilia and flagella is a specialized and strictly integrated supramolecular protein machin- ery that has been achieved during eukaryotic evolution to provide cells with an efficient and finely tuned motility. As a consequence, both the architecture of this organelle and the basic molecular mechanisms underlying motility gener- ation and control have been generally conserved from uni- cellular protists to mammals. Notwithstanding, unconventional axonemal organiza- tions have been observed in several species scattered among evolutionarily distant taxa [e.g., Schr  evel and Besse, 1975; Prensier et al., 1980; Gibbons et al., 1983; Perkins, 1991; Justine, 1998; Dallai et al., 2006, 2014; B^ a et al., 2007; Mencarelli et al., 2008; Bru nansk  a et al., 2014; Diagne et al., 2015; Kacem et al., 2015]. These peculiar axonemes are generally endowed with a reduced or even absent motil- ity which appears, however, to be fully compatible with spe- cies viability. In particular, insects provide an unusually high number of unconventional axonemal models expressed in the sperm cells of different species [Dallai, 2014]. Such variability in sperm architecture can reasonably be related to the wide- spread radiation and the high number of species in this ani- mal group that—due to a short generation lifespan and a consequent high speciation rate, as well as to their capabil- ity of tolerating a variety of environments [Mayhew, 2007]—is the largest and most diversified among animal taxa. Starting from the basic 9 1 2 model that is still expressed in the basal collembolans, insects have achieved the 9 1 9 1 2 model, so-called for the presence of an important apomorphic feature, that is, a crown of nine accessory microtubules that surrounds the central axoneme. Such a feature is also present in Diplura, which thus can be consid- ered the sister group of insects sensu strictu [Dallai et al., 2011; Misof et al., 2014]. In several insect taxa, however, this general organization displays a number of modifica- tions, and several aberrant and even bizarre axonemes have *Address correspondence to: R. Dallai, Department of Life Sciences, University of Siena, Siena, Italy. E-mail: dallai@unisi.it Published online 3 March 2016 in Wiley Online Library (wileyonlinelibrary.com). RESEARCH ARTICLE Cytoskeleton, April 2016 73:209–218 (doi: 10.1002/cm.21291) V C 2016 Wiley Periodicals, Inc. 209 