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- Vårdal, Hege, 1972-
(författare)
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From Parasitoids to Gall Inducers and Inquilines : Morphological Evolution in Cynipoid Wasps
- 2004
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- One of the large lineages of parasitic wasps, the Cynipoidea, exhibits three distinctly different life modes. Slightly more than half of the about 3000 species are parasitoids in insect larvae, whereas the remaining species are associated with plants, either as gall inducers or as inquilines (guests feeding on plant tissue in galls). The main focus of this thesis has been to identify morphological changes associated with the shifts between life modes. Particular attention was paid to structures believed to be important in gall initiation. Comparative anatomical studies of the egg, larva and venom apparatus were performed, including representatives of parasitoids, gall inducers and inquilines. Examination of gross morphology and ultrastructure revealed that the eggs of the gall inducers are larger and surrounded by a thicker shell than the parasitoid eggs. These differences may be related to the fact that the gall inducer egg contains sufficient egg yolk for the embryo during the entire egg period, whereas the parasitoid egg often absorbs nutrients through the eggshell. Furthermore, the gall inducer egg is probably more exposed to desiccation and therefore a thicker and more resistant eggshell is crucial. Comparing the terminal-instar larvae of about 30 species of parasitoids, gall inducers and inquilines, extensive morphological variation was found, particularly in the head and mouthpart features. The variation was summarized in 33 morphological and one life-history character and parsimony analyses were performed. The resulting phylogenetic estimates were largely in accordance with previous analyses of adult morphology and molecular data. The larval data point to a single origin of the inquilines, in agreement with adult morphology but in conflict with molecular data. The venom apparatus was found to be quite uniform in structure among a sample of 25 species of cynipoid species. It consists of a very short venom duct, a reservoir and a single unbranched venom gland. With few exceptions, the venom apparatus is conspicuously larger relative to the female metasoma in the gall inhabiting species than in the parasitoids. We found little evidence of anatomical structures that could facilitate chemical communication between the gall-inducer embryo and the surrounding plant tissue through the thick eggshell. On the other hand, the enormous venom glands and reservoirs, which are apparently not used for defence, suggest that the adult female plays a significant role in gall induction by injecting secretions into the host plant when laying eggs.
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| 2. |
- Wallberg, Andreas
(författare)
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The Dawn of a New Age : Interrelationships of Acoela and Nemertodermatida and the Early Evolution of Bilateria
- 2009
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Deciphering the rapid emergence of bilaterian animals around the time of the Cambrian Explosion and reconstructing the interrelationships of animal groups have long been two of the most elusive problems in Zoology. This thesis concerns the phylogenetic interrelationships within and among Acoela and Nemertodermatida, two groups of small worms that are believed to be basal bilaterians and which may provide important clues for understanding the early evolution of animals. In addition to trying to resolve the phylogenetic positions of these groups, major focus is put on inferring how ancestral animals might have looked, given the phylogenetic hypotheses put forward. The data used to infer phylogenies include nuclear ribosomal DNA, the mitochondrial COI gene and microRNAs. Based on phylogenetic analyses of a large number of 18S SSU ribosomal DNA sequences, it is proposed that Cnidaria is the sister taxon to Bilateria. Poor taxon sampling is suggested to be one of the reasons for why earlier assessments of the interrelationships among the most basal animal groups have yielded many conflicting results using the same gene. Analyses of new 18S SSU rDNA and 28S LSU rDNA sequences from six of the nine known species of nemertodermatids corroborate earlier indications that Acoela and Nemertodermatida are not sister taxa, as once thought. Being separate basal bilaterian animal groups, it is suggested that the last common ancestor of all bilaterians shared much of their comparatively simple morphology. Many methods are deployed to assess whether the phylogenetic results are mainly due to long-branch attraction, but no indication of this artifact is detected. The first comprehensive phylogenetic framework of Acoela is reconstructed from the 18S SSU, 28S LSU and COI genes, in combination with morphological data. The ancestral acoel worm is reconstructed using Bayesian methods and morphological observations in extant species. Two indeces, posterior similarity and reconstruction signal, are implemented to assess how similar different species are to the last common ancestor of all acoels and illustrate how clearly different characters or nodes are reconstructed. It is suggested that the ancestral acoel looked much like extant species of Diopisthoporus. The phylogenetic positions of Acoela and Nemertodermatida are assessed using new data on microRNAs in the acoel Hofstenia miamia and the nemertodermatid Meara stichopi. Acoela and Nemertodermatida are again found to be basal bilaterians, in congruence with earlier results. Using the work-flow and indeces developed earlier, it is concluded that the bilaterian ancestral microRNA repertoire can not yet be reconstructed with high confidence. All papers stress the importance of inclusive taxon sampling for making generalized inferences about ancestral features in animals.
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