Big leap for frog genetics
Along with their mammal, furry and feathered counterparts (human, chickens, rats and mice), frogs can now be added to the growing list of species whose genomes have been sequenced. The first amphibian genome sequence of the African clawed frog Xenopus tropicalis (X. tropicalis) has provided scientists with a complete picture of ancestral vertebrate genes, filling a major gap among the vertebrates sequenced to date. Findings from the study conducted by 48 international researchers are published in the journal Science. In the early 20th century, biologists discovered that X. tropicalis (a native of sub-Saharan Africa) was remarkably sensitive to human chorionic gonadotropin (HCG), a hormone produced during pregnancy. In the 1940s and 1950s, the species was widely used as an inexpensive pregnancy test in hospitals. A urine sample from a woman would be injected into the frog; if the woman were pregnant, the HCG in the urine sample would make the frog ovulate and produce eggs in less than 10 hours. With the high-quality draft sequence reported in the current study, we now know that the genome of X. tropicalis contains more than 20,000 genes, just a few thousand short of the roughly 23,000 genes in humans. At least 1,700 of the genes in the amphibian are similar to genes associated with disease in humans. Learning more about these uncanny similarities could help us better understand aspects of ourselves. According to leading author of the study Dr Uffe Hellsten of the Department of Energy Joint Genome Institute in the US, there is a remarkable similarity between regions of specific genes in the frog, chicken and human genomes, suggesting much of the structure or organisation of the chromosomes has remained intact over time. 'When you look at segments of the Xenopus genome, you literally are looking at structures that are 360 million years old and were part of the genome of the last common ancestor of all birds, frogs, dinosaurs and mammals that ever roamed the Earth,' explained Dr Hellsten. 'Chromosome archaeology helps to understand the history of evolution, showing us how the genetic material has rearranged itself to create the present day mammalian genome and present day amphibian genome.' The X. tropicalis genome sequence is expected to help researchers discover more about the amphibian's more popular cousin, Xenopus laevis (X. laevis), which is used in embryo development and cell biology studies. Co-author Dr Richard Harland of the University of California, Berkley in the US explained that X. tropicalis was among the last model organisms to be discovered. 'It will be tremendous to have a high quality sequence of X. tropicalis upon which to build the X. laevis sequence.' Dr Hellsten noted that the new genome sequence may even assist researchers with understanding the effect of endocrine disruptors (chemicals that mimic a frog's own hormones) at the molecular and genomic level. Found in lakes and streams worldwide, these chemicals may be part of the reason for the phenomenal drop in frog populations. 'Hopefully, understanding the effects of these hormone disruptors will help us preserve frog diversity and, since these chemicals also affect humans, could have a positive effect on human health,' added Dr Hellsten.
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