The spermatozoon of arthropoda. XXVII. Uncommon axoneme patterns in different species of the cecidomyid flies

Propulsive nanomachines: the convergent evolution of archaella, flagella and cilia

Echoing the repeated convergent evolution of flight and vision in large eukaryotes, propulsive swimming motility has evolved independently in microbes in each of the three domains of life. Filamentous appendages – archaella in Archaea, flagella in Bacteria and cilia in Eukaryotes – wave, whip or rotate to propel microbes, overcoming diffusion and enabling colonization of new environments. The implementations of the three propulsive nanomachines are distinct, however: archaella and flagella rotate, while cilia beat or wave; flagella and cilia assemble at their tips, while archaella assemble at their base; archaella and cilia use ATP for motility, while flagella use ion-motive force. These underlying differences reflect the tinkering required to evolve a molecular machine, in which pre-existing machines in the appropriate contexts were iteratively co-opted for new functions and whose origins are reflected in their resultant mechanisms. Contemporary homologies suggest that archaella evolved from a non-rotary pilus, flagella from a non-rotary appendage or secretion system, and cilia from a passive sensory structure. Here, we review the structure, assembly, mechanism and homologies of the three distinct solutions as a foundation to better understand how propulsive nanomachines evolved three times independently and to highlight principles of molecular evolution.

Overview on spermatogenesis and sperm structure of Hexapoda

The main characteristics of the sperm structure of Hexapoda are reported in the review. Data are dealing with the process of spermatogenesis, including the aberrant models giving rise to a reduced number of sperm cells. The sperm heteromorphism and the giant sperm exceeding the usual sperm size for length and width are considered. The characteristics of several components of a typical insect sperm are described: the plasma membrane and its glycocalyx, the nucleus, the centriole region and the centriole adjunct, the accessory bodies, the mitochondrial derivatives and the flagellar axoneme. Finally, a detailed description of the main sperm features of each hexapodan group is given with emphasis on the flagellar components considered to have great importance in phylogenetic considerations. This study may be also useful to those requiring an introduction to hexapod reproduction.

Insect sperm motility

The flagellosperm of insects, although following a general ground plan, exhibit considerable variation in morphology and ultrastructure across taxa, consistent with a history of rapid and divergent evolution. Sperm competition, which occurs when sperm of two or more males compete for the fertilization of a female's ova, has been recognized as a significant driving force in the evolution of insect sperm structure. Despite a considerable volume of data on sperm morphology, little is known about the motility of insect sperm. Understanding insect sperm motility would help to refine models of sexual selection on insect sperm, and would throw light on the selective mechanisms that shape insect sperm structure and function. This review updates our present knowledge of the proximate and ultimate aspects of insect sperm motility.

Unusual Axonemes of Hexapod Spermatozoa

Hexapod spermatozoa exhibit a great variation in their axoneme structure. The 9+2 pattern organization is present in a few basal taxa and in some derived groups. In most hexapods, a crown of nine accessory microtubules surrounds the 9+2 array, giving rise to the so-called 9+9+2 pattern. This general organization, however, displays a number of modifications in several taxa. In this review, the main variations concerning the number and localization of the accessory tubules, microtubular doublets, central microtubules, dynein arms, and axonemal length are summarized. We discuss the phylogenetic significance of all this structural information as well as the current hypotheses relating the sperm size and sperm polymorphism with reproductive success of some hexapod species. Also described are the biochemical data and the motility patterns which are currently known on some peculiar aberrant axonemes, in light of the contribution these models may give to the comprehension of the general functioning of the conventional 9+2 axoneme. Finally, we summarize methodological developments for the study of axoneme ultrastructure and the new opportunities for the molecular analysis of hexapod axonemes.

Giant sperm cells with accessory macrotubules in a neuropteran insect

The flagellar axoneme of the atypical spermatozoa (paraspermatozoa) of Mantispa perla (Neuroptera, Planipennia) contains accessory microtubules or rather macrotubules that are 55 nm in diameter and that has a wall consisting of about 40 protofilaments. The sperm tail further contains two giant mitochondrial derivatives, which during spermiogenesis store an electron dense material. The mature spermatozoon has a flattened acrosome and a elliptical nucleus. These giant spermatozoa may furnish nutrients to the functional spermatozoa (euspermatozoa) when they reach the female genital tracts or/and they function in sperm competition filling the spermatheca.

Glutamylated and glycylated tubulin isoforms in the aberrant sperm axoneme of the gall-midge fly, Asphondylia ruebsaameni

The axonemal organization expressed in the sperm flagella of the cecidomyiid dipteran Asphondylia ruebsaameni is unconventional, being characterized by the presence of an exceedingly high number of microtubular doublets and by the absence of both the inner dynein arms and the central pair/radial spoke complex. Consequently, its motility, both in vivo and in vitro, is also peculiar. Using monoclonal antibodies directed against posttranslational modifications, we have analyzed the presence and distribution of glutamylated and glycylated tubulin isoforms in this aberrant axonemal structure, and compared them with those of a reference insect species (Apis mellifera), endowed with a conventional axoneme. Our results have shown that the unorthodox structure and motility of the Asphondylia axoneme are concomitant with: (1). a very low glutamylation extent in the alpha-tubulin subunit, (2). a high level of glutamylation in the beta-subunit, (3). an extremely low total extent of glycylation, with regard to both monoglycylated and polyglycylated sites, either in alpha- or in beta-tubulin, (4). the presence of a strong labeling of glutamylated tubulin isoforms at the proximal end of the axoneme, and (5). a uniform distribution of glutamylated as well as glycylated isoforms along the rest of the axoneme. Thus, our data indicate that tubulin molecular heterogeneity is much lower in the Asphondylia axoneme than in the conventional 9+2 axoneme with regard to both isoform content and isoform distribution along the axoneme.

How the sperm lost its tail: the evolution of aflagellate sperm

The typical sperm is comprised of a head, midpiece and flagellum. Around this theme there is an enormous diversity of form--giant sperm, multi-flagellate sperm and also sperm that lack flagella entirely. Explaining this diversity in sperm morphology is a challenging question that evolutionary biologists have only recently engaged in. Nonetheless, one of the selective forces identified as being an important factor in the evolution of sperm form is sperm competition, which occurs when the sperm of two or more males compete to fertilize a female's ova. In species with a truly monandrous mating system, the absence of sperm competition means that the selection pressure on males to produce motile sperm may be relaxed. Potentially aflagellate sperm are less costly to produce, both in terms of energy and time. Thus, selection may therefore favour the loss of the sperm flagellum and any other motile mechanisms in monandrous taxa. A review of the literature revealed that 36 taxonomic groups, from red algae to fish, were found independently to have evolved aflagellate sperm. I review what is known about the mating systems of each of these taxa and their nearest sister taxa. A sister-group analysis using this information provided weak evidence suggesting that the evolution of aflagellate sperm could be linked to the removal of selective pressures generated by sperm competition.

The best of all worlds or the best possible world? Developmental constraint in the evolution of beta-tubulin and the sperm tail axoneme

Through evolutionary history, some features of the phenotype show little variation. Stabilizing selection could produce this result, but the possibility also exists that a feature is conserved because it is developmentally constrained--only one or a few developmental mechanisms can produce that feature. We present experimental data documenting developmental constraint in the assembly of the motile sperm tail axoneme. The 9+2 microtubule architecture of the eukaryotic axoneme has been deeply conserved. We argue that the quality of motility supported by axonemes with this morphology explains their long conservation, rather than a developmental necessity for the 9+2 architecture. However, our functional tests in Drosophila spermatogenesis reveal considerable constraint in the coevolution of testis-specific beta-tubulin and the sperm tail axoneme. The evolution of testis beta-tubulins used in insect sperm tail axonemes is highly punctuated, indicating some pressure acting on their evolution. We provide a mechanistic explanation for their punctuated evolution by testing structure-function relationships between testis beta-tubulin and the motile axoneme in D. melanogaster. We discovered that a highly conserved sequence feature of beta-tubulins used in motile axonemes is needed to specify central pair formation. Second, our data suggest that cooperativity in the function of internal beta-tubulin amino acids is needed to support the long axonemes characteristic of Drosophila sperm tails. Thus, central pair formation constrains the evolution of the axoneme motif, and intramolecular cooperativity makes the evolution of the internal residues path dependent, which slows their evolution. Our results explain why a highly specialized beta-tubulin is needed to construct the Drosophila sperm tail axoneme. We conclude that these constraints have fixed testis-specific beta-tubulin identity in Drosophila.

Structural and molecular characterization of dynein in a gall-midge insect having motile sperm with only the outer arm

The dipteran Monarthropalpus flavus possesses a peculiar sperm axoneme, characterized by multiple rows of microtubular doublets linked by the outer dynein arms only, lacking any equivalent of the central pair/radial spoke complex. The structure of these dynein molecules was studied by electron microscopy (EM). Using the quick-freeze, deep-etch method of EM, they were found to be similar to outer dynein arms described previously. Two globular "heads," each subdivided by a cleft, are clearly discernible. "Stalks" extend from proximal head to contact the B-tubule of the adjacent doublet. Unlike the situation in vertebrate sperm, the stalks sometimes branch into two thinner strands that contact the B-tubule at different sites. Treatment of demembranated sperm cells with ATP and vanadate induces conformational changes in the dynein outer arms. These are interpreted as the result of rotation of the dynein head with respect to what is observed in axonemes in rigor condition (after ATP depletion). SDS-PAGE indicates that the high-molecular-weight complement of this molecule comprises a single heavy chain. Specific dynein heavy chain-related DNA sequences corresponding to the catalytic-phosphate binding region were amplified by RT-PCR. Only one axonemal dynein sequence was identified among all amplified fragments. Southern blot analysis performed on genomic DNA using this sequence as a probe identified two hybridizing genes, only one of which is able to encode a functional product. Thus, genetic analysis indicates that this axonemal outer arm dynein is a homodymer of a single heavy chain subunit. In vivo, spermatozoa of this species are stored in a rolled configuration in female spermatheca, where they move rapidly with a wave-like motion. This movement could not be reproduced in vitro, except when spermatozoa were constrained in a bent configuration by some mechanical impediment. We propose that, in the absence of both the central pair/radial spoke complex and the inner arms, a curvature-dependent activation acts to trigger motility in these spermatozoa.

Structural and functional characteristics of the centrosome in gametogenesis and early embryogenesis of animals

We present a description of the wide spectrum of centrosome behavior during gametogenesis, early development, and cell differentiation. During meiosis and terminal differentiation of gametes there occurs a process of centrosome maturation which includes alterations in characteristics such as the number of centriolar cylinders and their structure if the basal body is formed and ability to function as MTOC, reduplicate, split, and serve as a polar organizer. Such centrosome properties require modifications of the molecular composition. Maturation of the centrosome in gametes may be compared to transformation of centrosome characteristics during terminal differentiation of other cells. After fertilization different properties of maternal and paternal centrosomes are supposed to combine, adding to each other in the fused (hybrid) centrosome of a zygote. Restoration of centrosome features typical in diploid somatic cells takes place in cells of a developing embryo in the course of early cell cycles.

The aflagellate spermatozoon of Diplozoon (Platyhelminthes: Monogenea: Polyopisthocotylea): a demonstrative case of relationship between sperm ultrastructure and biology of reproduction

Diplozoon is known to display an exceptional biology of reproduction: the hermaphroditic adults are permanently fused together and their genital ducts communicate. In contrast to all other polyopisthocotylean monogeneans in which the spermatozoa show an homogeneous biflagellate structure, the spermatozoon of Diplozoon is aflagellate. It is filiform, and composed of a cytoplasmic region and a nuclear region. The cytoplasmic region exhibits mitochondria, a well-developed smooth endoplasmic reticulum, and up to 450 longitudinal singlet microtubules. The microtubules show links between them; seen in cross section, they are arranged as rows or polygons. The spermatozoon nuclear region contains the nucleus surrounded by cortical longitudinal microtubules. The spermiogenesis shows no zone of differentiation, a typical structure found in all other parasitic Platyhelminthes. Diplozoon is the first case of aflagellate spermatozoon found in the parasitic Platyhelminthes. The atypical sperm structure is not linked with phylogeny, but is well correlated with the atypical biology of reproduction.

Sperm axoneme of two rows of doublets reversely oriented in the gall-midge Lestremia (Diptera, Cecidomyiidae)

The spermatozoon of Lestremia lacks an acrosome and has a giant centriole that gives origin to a giant axoneme with about 150 doublets. The axonemal doublets, disposed in two opposite rows oriented antiparallel, have A doublets with two dynein arms and a B tubule filled with dense proteinaceous material. Mitochondria fuse in two derivatives and show cristae and a longitudinal crystallized axis. The probable origin of the giant axoneme is hypothesized and the more prolonged motility of Lestremia sperm in comparison with that of other gall midges is related to the presence of a more precise axonemal organization. The spermatological results agree with the systematic position of Lestremiinae at the base of the evolutionary trend of the family Cecidomyiidae.

Characteristics of the sperm structure in heteroptera (Hemiptera, Insecta)

Spermatozoa from eight heteropterans, each representing a different family, have been examined by electron microscopy in order to determine whether there exist characters typical for this insect group. Two such characters were found, namely bridges from the mitochondrial derivatives to the axonemal microtubules nos. 1 and 5, and two or three, rather than one, crystalline bodies within the mitochondrial derivatives. It is suggested that these characters are synapomorphic traits. The heteropteran spermatozoa lack accessory bodies typical of spermatozoa from many related groups of insects. The acrosome of the aquatic or semi-aquatic heteropterans (the infraorders Nepomorpha and Gerromorpha) has a peculiar inner structure consisting of tightly packed tubules. On the common theme of the heteropteran sperm structure, there were many variations, and the spermatozoa of each species examined can be recognized.

Biophysics of flagellar motility

One feature characterizing the transition from prokaryote to eukaryote is the ‘sudden’ appearance of centrioles and their highly structured products, the typical eukaryotic flagella and cilia. These mechanochemical systems appear as fully developed machines, containing some 200 diffierent proteins (Lucket al.1978) arranged in a remarkably complex organization which has undergone little modification since the advent of the first eukaryotic cells. It is now well established (see, for example, Satir, 1974) that ciliary and flagellar motility is based on a sliding filament mechanism that superficially resembles the far more extensively studied sliding filament system of striated skeletal muscle.The flagellar system, however, appears to be much more complex than the muscle system, because it does not ‘merely’ shorten and generate force, but develops propagating waves and exerts its effects via hydrodynamic interactions with a viscous medium.

The spermatozoon of arthropoda. XXX. The multiflagellate spermatozoon in the termite Mastotermes darwiniensis

In this paper the spermatozoon of the termite Mastotermes darwiniensis is described. It is the first example of a multiflagellate sperm cell in animals. The sperm consists of a conical head and 100 flagella. Other remarkable features of this sperm cell are the absence of an acrosome, the presence of centrioles containing doublet microtubules instead of triplets, and the presence of axonemes devoid of central tubules and with doublets bearing only one arm. The flagella are feebly motile.

Fine structure of the spermatozoon of an onychophoran, Peripatopsis

The spermatozoon of the Onychophoran Peripatopsis moseleyi is described. This cell is characterized by the general filiform shape, the absence of an acrosome, the presence of mitochondrial derivatives, of an annulus and of nine accessory tubules and a manchette of microtubules around the axoneme. All of these characters are typical of highly evolved sperm models, like those of insects and mammals, and suggest a long evolutionary history. Only the position of the mitochondria, inserted between nucelus and axoneme, is reminiscent of annelid features.