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Australian, US Researchers Develop Crocodile Genetic Linkage Map

NEW YORK (GenomeWeb News) – A group of Australian and American researchers has generated a genetic linkage map for the saltwater crocodile, Crocodylus porosus, according to a study appearing online today in BMC Genomics.

The team assessed more than 200 microsatellite markers in dozens of crocodiles to come up with the most refined saltwater crocodile genetic map yet, including 14 linkage groups. The map is expected provide the basis for more extensive characterization of the crocodile genome and comparative genomics research. If so, researchers say, it may lead to a better understanding of crocodile and animal biology as well as improved crocodile farming programs.

"We have generated the first linkage map for a crocodilian, or indeed any other non-avian reptile," senior author Chris Moran, an animal geneticist at the University of Sydney, and his co-authors wrote.

Crocodiles belong to a group of reptiles related to the dinosaurs that eventually gave rise to birds. Their ancestors are thought to have diverged from the group leading to mammals about 300 million years ago.

But while genome sequencing efforts have been aimed at unraveling the genomes of some bird and reptile species, the authors noted, the crocodile group is poorly understood genetically. In 2006, Australian researchers published a paper identifying four crocodile genetic linkage groups involving ten loci. For the latest study, though, Moran and his colleagues developed a complete genetic linkage map including more recently identified microsatellite markers.

"[T]he generation of a comprehensive genetic map for a crocodilian will provide a valuable scaffold for genome sequence assembly," they explained, "and will be of intrinsic value to comparative mapping efforts aimed at understanding the molecular evolution of reptilian, as well as other amniote genomes."

Moran and his colleagues looked at 203 microsatellite markers in 83 crocodiles from five crocodile families from the Darwin Crocodile Farm in northern Australia. The team followed these families over two generations, integrating information from crocodile pedigree records.

Overall, the team identified 14 linkage groups including 180 loci. So far, though, these linkage groups haven't been mapped to crocodile chromosomes. The remaining 23 loci were unlinked.

The researchers noted that the female crocodiles had recombination maps that were almost six times larger than male recombination maps. While the reason for this difference is unclear, they speculated that it might relate to the sex determination process in crocodiles, which is based on temperature.

Those involved in developing the crocodile genetic map say it should provide the foundation for genome sequencing efforts, comparative genomics studies involving other crocodylian species, and other efforts aimed at uncovering the genetic basis of crocodile traits. As such, they added, the map may have implications for farmed crocodiles, providing genetic information that could improve the selection of desirable traits.

"As researchers continue to work towards elucidation of the crocodilian genome, this first generation map lays the groundwork for more detailed mapping investigations, as well as providing a valuable scaffold for future genome sequence assembly," they wrote.

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