1,721,189 research outputs found
A phylogenetic analysis of Tubificinae and Limnodriloidinae (Annelida, Clitellata, Tubificidae) using sperm and somatic characters
No full textThe spermatozoa and the sperm aggregates of 13 species belonging to four genera (
Smithsonidrilus,Limnodriloides, Thalassodrilides, Doliodrilus
) in the tubificid subfamily Limnodriloidinae
(Annelida, Oligochaeta) were studied and compared with the spermatozoal patterns already described in the subfamily Tubificinae. Two characters considered exclusive for the Tubificinae were found in the more spermatologically variable Limnodriloidinae: the production of two
kinds of spermatozoa, eusperm and parasperm, and the presence, in the spermathecae, of sperm aggregates formed by a combination of the two sperm types. A parsimony analysis was performed on the spermatozoal data of the species examined and compared with that based on the somatic characters of the same species: a critical revision of the already codified eusperm characters was carried out and the ultrastructure of parasperm was used as a new subset of spermatozoal characters. In a total evidence approach, a further parsimony analysis was run using a matrix combining both sets of characters. This analysis suggested that the double sperm
line and the sperm aggregates composed of both eusperm and parasperm may well be homologous in tubificines and limnodriloidines. It thus supported the previous notion that Tubificinae and
Limnodriloidinae are closely related and indicated that these subfamilies may be sister taxa
Spermiogenesis and seminal receptacles in Aeolosoma singulare (Annelida, Polychaeta, Aeolosomatidae)
No full textSpermatogenesis, spermatozoa, seminal receptacles and the transformations of sperm after mating have been examined for the first time in the polychaete annelid Aeolosoma singulare from Lake Baikal. The structure of the spermatozoon in the coelomic cavity is very similar to that already described for A. litorale, in particular in the presence of a cytoplasmic droplet containing lysosome - like vesicles around the nuclear basis. However, a comparison of coelomic spermatozoa with those stored in the sperm receptacles reveals a progressive reduction of the cytoplasmic droplet. The cytoplasmic droplet can thus be considered as a sort of remnant of the spermiogenetic process in the mature spermatozoon. The seminal receptacles of A. singulare, like those of the other aeolosomatids, are epidermal formations; but in this species they have a multicellular organisation and penetrate the muscular layers to end deep inside the coelomic cavity. The spermatozoon of A. singulare was compared with those of polychaetes considered close to aeolosomatids and found to have characters in common only with potamodrilid
Comparative ultrastructural study of the cuticle and spermatozoa in Propappus volki Michaelsen 1916 (Annelida: Clitellata)
No full textThe ultrastructure of the cuticle and mature spermatozoa of the oligochaete Propappus volki Michaelsen, 1916 is described with the aim of providing additional data for clarifying the systematic position of the taxon. P. volki is a fresh-water species living in streams, and is easily recognized by its proboscis on the pre-segmental prostomium and, in mature specimens, by a clitellum covering the segments XII–XIV. The cuticle is composed of a proximal fibre zone and a distal layered epicuticle covered with membrane-bound epicuticular projections. The fibre zone consists of collagenous fibres in a matrix, arranged in either densely packed parallel layers with the fibres oriented in the same direction, or with more loosely distributed fibres, although with the same main orientation. The epicuticular projections are pyramidal with the base leaning on the outer surface of the epicuticle. The cuticle covering the proboscis differs in morphology from that of the rest of the worm; the fibre zone is composed of thin and short fibrils running in all directions, and the epicuticular projections are longer and more narrow than the projections in other regions of the worm. The spermatozoa are filiform cells formed, in sequence, by an acrosome, an elongated nucleus, a long midpiece, and a flagellum. The acrosomal tube is short and straight with a completely external acrosomal vesicle. Following the acrosome is a apically corkscrew-shaped and basally straight nucleus. The midpiece is twisted and formed by five mitochondria. The flagellum shows a prominent central sheath arrangement.
A comparison with ultrastructurally described cuticles and spermatozoa from other clitellate species reveals most
similarities with enchytraeids
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
The double sperm line in Isochaetides (Annelida, Clitellata, Tubificidae)
No full textSperm morphology and spermatogenesis were examined in the oligochaete annelid Isochaetides
arenarius , a species belonging to the subfamily Tubificinae inhabiting the sediments of Lake
Baikal. As all tubificines, Isochaetides produces two types of spermatozoa, named eusperm and parasperm.
The eusperm are the fertilizing male gametes and consist, in sequence, of an acrosome, a
nucleus, a mitochondrial mid-piece, and a tail. The parasperm have the same general architecture, but
differ in cytological details: the acrosome is shorter, devoid of a perforatorium, and the acrosome vesicle
has a different, simpler, shape. The nucleus is much shorter and rectilinear (the eusperm nucleus is
twisted). The mid-piece mitochondria are less numerous but their overall volume is larger. The flagellum
has a plasma membrane largely separated from the axoneme, and is devoid of glycogen granules.
After mating, the two sperm types gather in the spermathecae to form spermatozeugmata; in these
structures the parasperm form an external sheath involving the centrally located eusperm and their tails
are connected by conspicuous septate junctions. Parasperm nuclei are produced through a process of
fragmentation of the ‘spermatocytes’, whereas the flagellar basal bodies are produced by a process
similar to that giving rise to basal bodies in ciliated epitheli
The unique gravireceptor organs of Pleurodasys helgolandicus (Gastrotricha: Macrodasyida)
No full textThe drumstick-like organs of the marine interstitial gastrotrich Pleurodasys helgolandicus Remane, 1927 were studied using optical and electron microscopy, in
order to widen our knowledge of the sensory organs among gastrotrichs. The drumstick-like sensory organs are paired, symmetrical structures located on the dorsal side of the
post-cephalic region of the body. Each sensory organ is formed by an outgrowth of the lamellar exocuticle covering the body surface, and it consists of an elongated, cylindrical hollow stalk surmounted by a hollow bulbous tip, housing in its cavity a dense spherical body. Compared with the
three main sensory structures recognized among Gastrotricha,
i.e., mechanoreceptors, photoreceptors, and chemoreceptors, the organization of the drumstick-like sensory organ matches that of mechanoreceptors. In particular, we suggest that the drumstick-like sensory organs of P. helgolandicus are gravireceptor organs, analogous to the orthopteran “plumb bob-like” receptors. This is the first description of a putative gravireceptor organ among gastrotrichs
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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