Xenopus genetics and genomics

This issue of Mechanisms of Development addresses recent advances in Xenopus genomics and genetics, a topic whose time has undoubtedly come. Xenopus has long been a favourite species of the cell, molecular and (especially) developmental biologist. There are good reasons for this. Xenopus embryos are large and have reliable and accurate fate maps: they are therefore excellent for grafting and micromanipulation studies. Xenopus embryos are also available in large numbers, thus providing enough material for biochemical experiments. It is a simple matter, furthermore, to over-express specific gene products in the embryo, in particular regions if so desired, by microinjection of synthetic mRNA. In addition to these 'traditional' advantages there are the more recent developments of transgenesis (Kroll and Amaya, 1996) and of a GAL4 over-expression system (Chae et al., 2002; Hartley et al., 2002). It is also possible to inhibit zygotic gene function by means of dominant-negative approaches (Amaya et al., 1991; Conlon et al., 1996) and, most recently, by use of antisense morpholino oligonucleotides (Heasman et al., 2000). Together, these features have made Xenopus a remarkably productive model system, and one that has yielded insights into problems as diverse as axis determination, embryonic induction, morphogenesis, gene regulation, chromosome structure and regulation, cell cycle control, and signal transduction. It is not the intention of this Introduction to discuss such achievements in any detail, but it is noteworthy that these enormous contributions to cell, molecular and developmental biology, even the most recent ones, were made with essentially no recourse whatsoever to modern genomic information. Indeed, compared with organisms such as humans, mouse, Drosophila, Caenor-habditis elegans and even zebrafish, Xenopus was rather slow to take advantage of sequencing technologies. Perhaps this is because Xenopus laevis is an organism that is not well suited to genetic studies or indeed to sequence assembly: its generation time is long and its genome 'pseudotetraploid'. Or perhaps the 'traditional' advantages of Xenopus were so great the need for a sequencing project was less keenly felt by the community. Whatever the reason, one of the catalysts for change was the adoption and promotion, particularly by Zimmerman, of Xenopus tropicalis as a new model system for the genetic analysis of development. Use of this species addresses several of the points above. In particular, X. tropicalis has a short enough generation time that it can serve as a 'genetic' organism, and it is diploid, making genetic analyses and assembly of …