Asymmetry switched in snail
Scientists have found a new way to manipulate the direction of snail shell coiling, altering the animal's left-right asymmetry. The research, published linkurl:online;http://www.nature.com/nature/journal/vaop/ncurrent/index.html today (November 25) in Nature, may offer clues as to how "handedness" develops in invertebrates, which could improve scientists' understanding of the mechanics that drive cell regeneration and embryonic development.
'Left-handed' and 'right-handed' shells of L. stagnali
Scientists have found a new way to manipulate the direction of snail shell coiling, altering the animal's left-right asymmetry. The research, published linkurl:online;http://www.nature.com/nature/journal/vaop/ncurrent/index.html today (November 25) in Nature
, may offer clues as to how "handedness" develops in invertebrates, which could improve scientists' understanding of the mechanics that drive cell regeneration and embryonic development.
|'Left-handed' and 'right-handed'|
shells of L. stagnalis
Image: Kuroda laboratory
"This paper illustrates how chirality" -- the inability of a structure to be superimposed on its mirror image -- "arises," said linkurl:Michael Levin,;http://ase.tufts.edu/faculty-guide/fac/mlevin11.biology.htm a developmental biologist at Tufts University, who was not involved in the study. This new research, he said, shows that snails are a useful way to track early embryonic-stage signaling for handedness.
The left-right asymmetry of an animal's internal organs and physical appearance are important evolutionary traits that often make tasks easier, such as an owl's unbalanced ears for better hearing, but researchers have only recently begun to decipher the mechanisms underlying it. Most studies have focused on what dictates asymmetry in late-stage development, suggesting, for example, that the structural patterning happens when an embryo shifts from a spherical ball of cells into a multi-layered organism. But linkurl:Reiko Kuroda,;http://www.pwri.go.jp/icharm/activities/act_e/act20090402prof_kuroda.html a biochemist at the University of Tokyo, and colleagues demonstrate that a snail shell's coil orientation is determined much earlier -- at the eight-cell embryonic development stage.
Using two glass rods, Kuroda's team inverted the layout of the early stage cells in Lymnaea stagnalis
, essentially creating a mirror image of their original grouping pattern. As the snails developed into adulthood, their shells coiled in the opposite direction than expected based on their genetic code. Simply put, genetically "right-handed" snails developed "left-handed" shells, and vice versa, said Kuroda.
Kuroda's team also found that altering the layout of the eight-cell stage completely reversed the expression patterns of the nodal gene -- the gene that dictates the direction in which the shell will spiral under normal conditions. The gene is the regulating factor of the Nodal signaling pathway, which scientists have previously identified as the system that dictates organ laterality and handedness as an embryo develops into a multi-layered organism. The pathway is also responsible for determining the direction of embryonic axes and stimulating the creation of the germ cell layers.
In previous studies, scientists were able to determine an animal's handedness by looking at how the nodal gene was expressed. But Kuroda and her colleagues' results indicate that the decision of which way the shell will coil originates upstream of the Nodal signaling pathway and not in the pathway itself.
This altered handedness, however, did not pass from generation to generation; offspring of reversed-coiled snails reclaimed the pre-manipulated orientations of their ancestors, suggesting the physical manipulations by researchers did not affect the genetically programmed structural pattern.
"Putting a finger on the actual onset of handedness in invertebrates may shed light on how it is determined in other, more complex organisms," said Kuroda, which will allow scientists to determine the mechanics driving asymmetric evolutionary traits.
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[31st August 1998]