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The molecular basis of development of the sword, asexual selected trait in the genus Xiphophorus

The molecular basis of development of the sword, asexual selected trait in the genus Xiphophorus


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OFFEN, Nils, 2008. The molecular basis of development of the sword, asexual selected trait in the genus Xiphophorus [Dissertation]. Konstanz: University of Konstanz

@phdthesis{Offen2008molec-6747, title={The molecular basis of development of the sword, asexual selected trait in the genus Xiphophorus}, year={2008}, author={Offen, Nils}, address={Konstanz}, school={Universität Konstanz} }

terms-of-use application/pdf 2011-03-24T17:28:54Z The molecular basis of development of the sword, asexual selected trait in the genus Xiphophorus eng 2008 Molekulare Grundlagen der Entwicklung des Schwertes, eines sexuell selektierten Merkmales im Genus Xiphophorus Offen, Nils 2011-03-24T17:28:54Z Offen, Nils The sword, a colourful extension of the ventral caudal fin of male swordtails of the genus Xiphophorus was one of Darwin's chosen examples for his idea of sexual selection. Experiments in X. helleri have shown that (1) the total length of the sword is an important criterion during mate choice and (2) the females have a preference for a specific pattern of differently coloured stripes. Besides its role in the process of sexual selection the sword has an interesting evolutionary history. Only males of swordtail species develop a sword, whereas males of platyfish, another group within the genus Xiphophorus, are swordless. One scenario suggests a sworded common ancestor of all Xiphophorus species and a secondary loss of the sword in platyfish during evolution. It remains elusive which molecular events preceded the loss of the sword in platyfish, since the genetic network that controls sword development is poorly understood. Data from interspecies crosses suggests that multiple loci control sword development. In addition, testosterone was identified as sufficient factor to induce sword development in immature fish, which indicates that sword development is controlled by androgen signalling. Recent work also identified the homeobox transcription factor msxC as another potential candidate, since it was shown to be up-regulated in growing sword rays. Up-regulation of msxC has also been found in the developing gonopodium, the modified male anal fin that is also induced by exogenous testosterone. The gonopodium is evolutionary older than the sword and it was assumed that the genetic network controlling gonopodium development was partly co-opted for the sword.<br />In chapter I we focussed on Fgf signalling that has been shown to regulate msxC expression during caudal fin regeneration. Both sword development and fin regeneration are characterized by elevated outgrowth of fin rays, which is likely controlled by a conserved genetic network. We showed that fgfr1 is specifically up-regulated in developing swords, which presents first evidence that fgfr1 is involved in sword development. A similar pattern was also observed in the developing gonopodium. fgfr1 is spatial-temporally co-expressed with msxC both in the sword and the gonopodium, which might indicate a putative interaction between both genes. Interestingly, in the ventral caudal fin rays of testosterone treated platyfish, fgfr1 and msxC are only up-regulated after prolonged hormone treatment. This points towards a disruption between the fgfr1/msxC network and its regulation by testosterone as a likely developmental cause for sword-loss in platyfish. Finally, we demonstrated that fgfr1 and msxC activation is correlated with fin ray growth rates by employing the X. maculatus brushtail mutant that exhibits excessive growth of the median caudal fin rays.<br />Only a subset of the genes involved in sword development can be targeted by candidate gene approaches (e.g. as performed in chapter I), because prior knowledge about gene function is needed to select appropriate candidates. In chapter II we employed the suppression subtractive hybridisation (SSH) technique to bypass this limitation, because this method can be applied to isolate genes that are differentially expressed in swords and gonopodia compared to juvenile fins without ab initio knowledge of gene identity or function. In this study we identified 128 different sequences with significant similarity to known genes. We showed that four of these sequences with similarity to rack1, dusp1, klf2 and tmsb a-like are specifically up-regulated in induced swords and/or gonopodia. In parallel, we also showed that these genes are strongly expressed during fin regeneration. Therefore these four genes are interesting candidates to further analyse their role in both sword development and fin regeneration.<br />The anal fin of male Xiphophorus fishes is modified into an intromittant organ, the gonopodium. The gonopodium is formed during sexual maturation by a subset of three anal fin rays, the 3-4-5 complex. These three rays are modified in terms of ray length, segment thickness and different distal structures like blades, claws, spines, hooks and serraes. Therefore, the mature gonopodium exhibits a strong proximo-distal polarity due to the smaller terminal segments and terminal structures. Gonopodium development is thought to proceed in two phases. During the first phase, low levels of testosterone promote ray outgrowth, whereas high levels of testosterone induce the formation of terminal structures during the second phase. Shh, androgen and probably Fgf signalling are involved in gonopodium development.<br />In chapter 3 we tested the role of retinoic acid (RA) signalling during gonopodium development, for two reasons. RA signalling is essential for paired appendage development in vertebrates and it provides positional information along the proximodistal axis in developing and regenerating limbs. Therefore, RA signalling might either play a general role in gonopodium development or specific role in establishing the proximo-distal polarity within the gonopodium. RA, a small lipophilic, diffusible molecule is synthesised by retinaldehyde dehydrogenases (Aldh1as) and stimulates gene expression through binding to two types of receptors, retinoic acid receptors (RARs) and retinoic X receptors (RXRs). In this study we showed that aldh1a2, a RA synthesising enzyme, and two RA receptors, rarg-a and rarg-b, are expressed in developing gonopodia. Inhibiting RA synthesis with DEAB increases the length of newly formed terminal segments, whereas the segment length decreases when RA signalling is overactivated by exogenous all-trans RA. Both the expression and the functional data present first evidence that RA signalling is involved in gonopodium development. Finally, we showed that androgen receptors b (arb), a putative regulator up-stream of RA signalling is co-expressed with aldh1a2 in the distal mesenchyme of the gononopodial rays. Interestingly, developing swords lack the distal expression domain of both aldh1a2 and arb, whereas the two rars are similarly expressed in developing swords and gonopodia. This might point towards an interaction between these two genes.

Dateiabrufe seit 01.10.2014 (Informationen über die Zugriffsstatistik)

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