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Non-systemic metamorphosis: millipede gonopods as a model system
Full text linkArthropod post-embryonic life is characterized by periodical moult cycles. These events are essential during individual growth, given that a rigid exoskeleton wraps them. When a moult event corresponds with a relatively sudden and conspicuous change in animal’s structure, is called metamorphosis; this phenomenon is characterized by a relatively high level of processes like apoptosis and cellular proliferation. For this reason, the study of the processes involved during metamorphosis has become a very important subject in evolutionary developmental biology (evo-devo) of arthropods. The principal limit of these researches, nevertheless, is that they focus on a small number of species and to draw general conclusions based uniquely on data collected from a few model systems can be risky.
Recent studies about metamorphosis underpin on species characterized in most of the cases by a systemic metamorphosis. These are mainly holometabolous insects, in which the transition to the adult is rich of changes, affecting the whole body, although often with different degrees of intensity. Most of the arthropods, exhibits minor post-embryonic changes. Less usual is a situation in which a sudden developmental change affects one or a few non terminal segments, that initially are identical to those that precede and follow them along the body. This kind of post-embryonic transformation, confined in a well defined region of the body, is defined in this thesis with the term non-systemic metamorphosis. It can be observed during the post-embryonic development of males of helminthomoph millipedes.
In males helminthomorph millipedes, during post-embryonic life, a sudden changes occurs that involves only one or two pair of non terminal appendages, initially not different from the others and part of a long homonomous series. In the adults these appendages, localized in the seventh ring, are modified into gonopods, that are used as claspers or to transfer sperm during the copula. During the first stages of development the seventh ring is identical to the others and it brings two pair of conventional walking legs; in one of the following stage the legs are substituted by extremely reduced structures (called squamiform appendages), that subsequently turn into gonopods.
In this thesis I investigated the metamorphosis of the eighth and ninth pair of legs, that correspond to the seventh ring of the body (a ring is a morphological unit that generally is considered as two conventional segments) in two blanulids (Nopoiulus kochii (Gervais, 1847) and Blaniulus guttulatus (Bosc, 1792)), in one nemasomatid (Nemasoma varicorne C.L. Koch, 1847) and in one polydesmid (Oxidus gracilis (C.L. Koch, 1847)) millipedes. Special attention was paid to the last stage in which males maintain the eighth (and eventually the ninth) pair of locomotory appendages, the stages in which these are substituted by squamiform appendages and the first stage with gonopods.
I performed investigations on the external morphology, using optical and scanning electron microscopy. Internal anatomy was described through paraffin sections. The endoskeletal structures associated with gonopods, was studied using confocal laser scanning microscopy taking advantage of the autofluorescence of the cuticle.
As documented here for the first time the external modifications caused by the non-systemic metamorphosis of diplopods are associated with a huge rearrangement of internal anatomy. The degree of intensity of internal changes is different in the four species. In the two blaniulids, the consequences that gonopods have in the general trunk architecture are bigger than in the others, in particular because of the voluminous endoskeletal structures. In N. varicorne and in O. gracilis the anatomy of the trunk is less affected. In O. gracilis it was also possible to perform some observations during the last moulting period: it is during this step that squamiform appendages become gonopods undergoing larger modifications.
I also present an hypothesis on the basic mechanisms that characterize gonopod formation in millipedes, according to which the changes in the seventh ring depend from the presence of a segmental marker produced during embryonic development, that is activated only much later, at the beginnig of non-systemic metamorphosis.Di norma gli artropodi, nella loro vita postembrionale, affrontano periodicamente cicli di muta per potersi accrescere, essendo dotati di un esoscheletro rigido che racchiude il corpo dell’animale. Quando la muta coinvolge improvvisi e vistosi cambiamenti nella struttura dell’animale si parla di metamorfosi, un fenomeno caratterizzato da numerosi eventi di apoptosi e proliferazione cellulare. Proprio per questa ragione, lo studio dei processi coinvolti nella metamorfosi è diventato un tema ricorrente ed importante per la biologia evoluzionistica dello sviluppo (evo-devo) degli artropodi. Il principale limite di queste ricerche, tuttavia, riguarda il fatto che sono limitate ad un ristretto numero di specie e trarre conclusioni generali basandosi unicamente sui dati raccolti da pochi sistemi modello può rivelarsi rischioso.
Gli studi più recenti sulla metamorfosi sono stati svolti su specie caratterizzate per la maggior parte da una metamorfosi di tipo sistemico. Si tratta principalmente di insetti olometaboli, nei quali il passaggio allo stadio adulto è costellato da cambiamenti che coinvolgono tutto il corpo dell’animale, eventualmente con gradi di intensità diversi. Tuttavia nella maggior parte degli altri artropodi i cambiamenti postembrionali sono notevolmente meno estesi, come avviene, ad esempio, negli insetti emimetaboli. Una situazione sicuramente meno frequente è quella in cui ad essere investite da un brusco cambiamento nello sviluppo siano uno o pochi segmenti non terminali, inizialmente uguali ai moduli che li precedono e che li seguono lungo l’asse principale del corpo. Questo tipo di trasformazione postembrionale, confinata in un distretto del corpo ben circoscritto, è definita, in questa tesi, metamorfosi non sistemica e si può trovare nello sviluppo postembrionale dei maschi dei millepiedi elmintomorfi.
Nei maschi dei millepiedi elmintomorfi, durante il processo di crescita postembrionale, avviene un brusco cambiamento che coinvolge solo uno o due paia di appendici non terminali, inizialmente non diverse da tutte quelle che le precedono e le seguono, nel mezzo di una lunga serie omonoma. Negli adulti, infatti, queste appendici, localizzate nel settimo anello del tronco, sono modificate in gonopodi, i quali sono usati come organi di presa o veicoli per il trasferimento dello sperma durante la copula. Osservando nel dettaglio lo sviluppo del settimo anello possiamo notare che: durante i primi stadi di sviluppo è identico agli altri e porta due paia di zampe convenzionali; in un qualche stadio successivo le zampe vengono sostituite da strutture estremamente ridotte (appendici squamiformi) che, successivamente, diverranno gonopodi.
Lo scopo di questa tesi è quello di indagare la metamorfosi dell’ottavo e del nono paio di zampe, corrispondenti al settimo anello del tronco (unità morfologica generalmente equivalente a due segmenti convenzionali) dei seguenti diplopodi: due blaniulidi Nopoiulus kochii (Gervais, 1847) e Blaniulus guttulatus (Bosc, 1792), uno julide nemasomatide Nemasoma varicorne C.L. Koch, 1847 ed un polidesmide Oxidus gracilis (C.L. Koch, 1847). In particolare, ho analizzato l’ultimo stadio in cui i maschi presentano l’ottavo (ed eventualmente il nono) paio di appendici locomotorie, gli stadi in cui queste sono sostituite da appendici squamiformi e il primo stadio che presenta i gonopodi.
Gli studi effettuati hanno previsto principalmente indagini di morfologia esterna, basate sulla microscopia ottica e sulla microscopia elettronica a scansione, mentre l’anatomia interna è stata descritta principalmente attraverso l’utilizzo di protocolli istologici. Le strutture endoscheletriche associate ai gonopodi sono state studiate attraverso l’uso del microscopio confocale a scansione laser, sfruttando l’autofluorescenza della cuticola.
Dai risultati ottenuti emerge che le modificazioni esterne dovute alla metamorfosi non sistemica nei diplopodi sono associate a un notevole riarrangiamento dell’anatomia interna, documentato per la prima volta con questa tesi. Il grado di intensità dei cambiamenti interni è diverso nelle quattro specie: i due blaniulidi risentono maggiormente dell’impatto che i gonopodi hanno nell’architettura generale del tronco, in particolare a causa delle voluminose strutture endoscheletriche. Al contrario in N. varicorne, ed in particolare in O. gracilis, l’anatomia del tronco viene modificata in modo meno significativo. Nel polidesmide è stato anche possibile effettuare alcune osservazioni durante l’ultimo periodo di muta, durante il quale avvengono le maggiori modificazioni trasformando le appendici squamiformi in gonopodi.
Viene inoltre presentata un’ipotesi sui meccanismi di base che caratterizzano la formazione dei gonopodi nei millepiedi, secondo la quale i cambiamenti del settimo anello dipendono dalla presenza di un marcatore segmentale prodotto durante lo sviluppo embrionale, che viene attivato solo molto più tardi, quando iniziano i cambiamenti dovuti alla metamorfosi non sistemica
Nonsystemic Metamorphosis in the Development of Millipede Gonopods
No full textAmong millipedes, adult males of the Helmintomorpha possess gonopods
on the VII body ring. These specialized appendages differentiate in a very
advanced phase of postembryonic development, changing from legs, to
squamiform appendages, to gonopods. This transformation involves variable
numbers of moults in the different species and is limited to one or
very few nonterminal serially homologous elements in an otherwise morphologically
homogeneous series of trunk rings. Here, we describe the
changes affecting this area through three subsequent developmental stages.
Our study, based on Nopoiulus kochii (Gervais, 1847), included observations
on external morphology and internal anatomy as well. By this double
approach, we discovered a dramatic remodeling of the endoskeletal,
muscular, and nervous system. Most evident is the considerable reduction
of the ring lumen caused by gonopod growth. This affects the course of
the ventral nerve cord that is displaced dorsally comparing with immature
stages. The complexity of the gonopod endoskeletal component is studied
using confocal laser scanning microscopy. This technique permits us to
describe their fine morphology in situ, avoiding the problems of accidental
damages deriving from the dissection. Thus, we could note two gonopod
apodemes that surround the digestive tract ventrally and laterally and offer
insertion to two muscles, which are present only in the adults. This data
can be the starting point from which the morphogenetic processes
involved in the formation of the gonopods can be investigated
Non-systemic metamorphosis in male millipede appendages: long delayed, reversible effect of an early localized positional marker?
Full text linkAbstract Background The development of specialized appendages involved in sperm transfer in the males of julid millipedes is an extreme case of specialized, complex structures differentiating in a very advanced phase of post-embryonic development. Here, a non-systemic metamorphosis affects the external morphology and the internal anatomy of a trunk double segment only. Presentation of the hypothesis We hypothesize that during early (possibly embryonic) development a segmental marker is produced that remains unexploited throughout late embryonic and early post-embryonic development, until, activated by a systemic signal, it finally determines the release of a segmentally localized but anatomically major change. Testing the hypothesis Key to testing the hypothesis are (1) the identification of both the putative segmental marker involved in the localization of the legs to be eventually metamorphosed into gonopods and the systemic signal activating it, (2) the identification of the cell population from which the gonopods are built, and (3) a longitudinal study of the marker's expression throughout late embryonic and, possibly, post-embryonic development. Implications of the hypothesis Proving the validity of this hypothesis would demonstrate the existence of a cryptic developmental module that will be activated only months, or years, after it has been first laid down during early development. This study also opens a window onto the very poorly explored domain of late expression of developmental genes and molecular control of late developmental events.</p
Fig. 4 in An unusually elongate endogeic centipede from Sardinia (Chilopoda: Geophilidae)
No full textFig. 4. Forcipules of Endogeophilus ichnusae gen. et sp. nov. and the two most similar geophilid species. Left part of the forcipular segment, ventral view, in adult females. A. E. ichnusae gen. et sp. nov., holotype, ♀, original drawing. B. Geophilus electricus (Linnaeus, 1758), original drawing from a representative specimen, ♀ (see 'Material and methods'). C. Galliophilus beatensis Ribaut & Brolemann, 1927, holotype, ♀, redrawn from Brolemann (1927). Scale bars = 200 µm.Published as part of Bonato, Lucio, Zapparoli, Marzio, Drago, Leandro & Minelli, Alessandro, 2016, An unusually elongate endogeic centipede from Sardinia (Chilopoda: Geophilidae), pp. 1-19 in European Journal of Taxonomy 231 on page 11, DOI: 10.5852/ejt.2016.231, http://zenodo.org/record/384044
Structural aspects of leg-to-gonopod metamorphosis in male helminthomorph millipedes (Diplopoda)
Full text linkBackground: In the adult males of helminthomorph millipedes, one or two pairs of legs in the anterior part of the trunk are strongly modified into sexual appendages (gonopods) used for sperm transfer during the copula. Gonopods differentiate in an advanced phase of post-embryonic development, in most cases as replacement for the walking legs of the seventh trunk ring, as these first regress to tiny primordia, to eventually develop into gonopods at a subsequent stadium. These extremely localized but dramatic changes have been described as a non-systemic metamorphosis. In the present study we describe morphological and anatomical changes of trunk ring VII associated with non-systemic metamorphosis in four helminthomorph species.
Results: As documented here for the first time by means of traditional histology methods and new techniques based on confocal laser scanning microscopy, the external odifications caused by non-systemic metamorphosis are associated to a huge rearrangement of internal anatomy, mostly due to the development of gonopod apodemes and extrinsic muscles.
Conclusions: Internal changes in the seventh trunk ring, locally leading to the dorsal displacement of the ventral nerve cord and the digestive tract, are modulated in a taxon-specific manner, and are very conspicuous in the
blaniulids Nopoiulus kochii and Blaniulus guttulatus, with likely major functional consequences
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
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
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Evolution of Strigamia centipedes (Chilopoda): a first molecular assessment of phylogeny and divergence times
Full text linkWe present a first phylogenetic and temporal framework, with biogeographical insights, for the centipedes of the genus Strigamia, which are widespread predators in the forest soils of the Northern Hemisphere and comprise the evo-devo model species Strigamia maritima. The phylogeny was estimated by different methods of maximum likelihood and Bayesian inference from sequences of two mitochondrial (16S, COI) and two nuclear (18S, 28S) genes, obtained from 16 species from all major areas of the global range of the genus and encompassing most of the overall morphological and ecological diversity. Divergence times were estimated after calibration upon the fossil record of centipedes. We found that major lineages of extant species of Strigamia separated most probably around 60 million years (Ma) ago. The two most diverse lineages diversified during the last 30 Ma and are today segregated geographically, one in Europe and another in Eastern Asia. This latter region hosts a hitherto underestimated richness and anatomical diversity of species, including three still unknown, yet morphologically well distinct species, which are here described as new: Strigamia inthanoni sp. n. from Thailand, Strigamia korsosi sp. n. from the Ryukyu Islands and Strigamia nana sp. n. from Taiwan. The northern European model species S. maritima is more strictly related to the Eastern Asian lineage, from which it most probably separated around 35 Ma ago before the major diversification of the latter
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