Drago, Leandro
Non-systemic metamorphosis: millipede gonopods as a model system [Tesi di dottorato]

Arthropod 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.

In relazione con http://paduaresearch.cab.unipd.it/1933/
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BIO/05 - Zoologia


Tesi di dottorato. | Lingua: | Paese: | BID: TD13052410