Preface [Hot topic: Xenopus Genomics (Guest Editor: Gerard J.M. Martens)]

E-ISSN： 1875-5488|4|8|i-ii

ISSN： 1389-2029

Source： Current Genomics, Vol.4, Iss.8, 2003-11, pp. : i-ii

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Abstract

During the last century amphibians have served as excellent models for developmental studies, in particular concerning early embryonic development. Since the 1950s (when they were first used in human pregnancy tests), the South-African claw-toed frog Xenopus laevis has been the most popular amphibian model organism. X. laevis is readily maintained in the laboratory, is commercially available, and can be induced to ovulate and mate at any time of the year (most frog species are seasonal breeders). Thousands of the relatively large and robust Xenopus eggs can be produced following a simple injection of the mammalian hormone chorionic gonadotropin. Xenopus embryos efficiently translate injected (synthetic) mRNAs, are transparent, develop externally in a simple salt solution, and are characterized by identifiable blastomeres, a reliable fate map, and ease of microinjection, micromanipulation, surgical intervention (grafting), labelling and culturing in vitro. Furthermore, it has recently become possible to generate hundreds of stable, non-mosaic transgenic Xenopus embryos in a cost-effective and efficient way in a single day [1]. This allows the experimenter to combine the aforementioned traditional advantages of X. laevis with the ability to express a gene at any time and in any place, not only for developmental studies but also to examine cell biological and biochemical processes. X. laevis has therefore, now been added to the list of (transgenic) model systems used for functional analyses (C. elegans, Drosophila, zebrafish and the mouse). Xenopus tropicalis is a close, diploid relative of the pseudotetraploid X. laevis, and has a short generation time making it attractive for creating permanent transgenic lines and performing molecular-genetic studies. The prospect of being able to perform genetic screens and make mutations in known genes will add an important dimension to the Xenopus system. Most anatomical and functional features as well as regulatory pathways are highly conserved between Xenopus and mammals, including humans. Therefore, the majority of information revealed from studies on this lower vertebrate will apply to mammalian systems. For these reasons, interest in Xenopus as a genomics system has increased, as is apparent from recent community-wide initiatives for the generation of large-scale Xenopus expressed sequence tags (ESTs) and genomic sequencing, expression profiling and genetic resources. These research efforts are often coordinated by the Xenopus Initiative of the US National Institutes of Health (NIH) that originates from the Human Genome Project via the NIH Non-Mammalian Models Initiative [2]. A search of Xenopus in the PubMed database revealed citations for over 28,000 articles (for comparison: mouse, 680,000 and zebrafish, 3,600). Nevertheless, in recent years Xenopus has not experienced the great increase in broad interest as seen for other vertebrate model organisms, such as the mouse and zebrafish. This is somewhat surprising since these models have a number of disadvantages when compared to Xenopus. For instance, mice produce fewer eggs, have no external embryonic development, it is not possible to generate genetic mouse chimeras (via tissue transplantation), and mouse transgenesis is cumbersome and costly. Zebrafish have a duplicated genome that is not only more divergent from mammals but the duplicated zebrafish genes have also diverged more from each other (a disadvantage for knock-down approaches), since the duplication occurred >400 million years ago [3]. In contrast, X. tropicalis is a true diploid and the genome duplication in X. laevis occurred ∼30 million years ago [4]. Furthermore, both mouse and zebrafish transgenesis are less efficient than in Xenopus and often mosaic, necessitating the selection of transgenic lines before analysis. Given these facts, it seemed appropriate to pay more attention to Xenopus as a vertebrate model organism and thus, to bring together the latest development