For most animals, the left half of the body is pretty much a mirror image of the right half, at least on the outside. One exception to this bilateral symmetry is found in snails, whose shells spiral to either the right or the left. A new study overturns a long-held theory about how these asymmetries develop, and the researchers say their work could help explain left-right asymmetries in other animals, such as the position and shape of the heart and liver in humans and other vertebrates.
In most snail species, shells in all individuals spiral either to the left or to the right. Some species, however, such as Lymnaea stagnalis, exhibit both leftward (sinistral) and rightward (dextral) spirals. For more than a century, researchers thought that these mirror-image snails resulted from mirror-image development patterns. That thinking was based on microscopic observations of the embryos. At the third cleavage, when the embryo goes from having four cells to eight, the quartet of micromeres, the budding daughter cells, are rotated slightly counterclockwise from their sister cells in embryos that develop into sinistral snails and clockwise in their dextral cousins.
Contrary to this conventional wisdom, different mechanisms produce dextral and sinistral individuals in Lymnaea stagnalis, reports a group led by biochemist Reiko Kuroda of the University of Tokyo. By snapping 3D images of fluorescently labeled embryos, Kuroda's team found that in dextral snails, the spindles--tubular structures holding the chromosomes--are already spiraled and the cell boundaries seem twisted at an early stage of the third cleavage. Meanwhile, sinistral embryos don't show these signs of rotation. For these embryos, rotation sets in later, the team reports 24 August in Current Biology.
The group found that for species of snails that are dominantly sinistral, early embryo cell division is a mirror image of what happens in the dominantly dextral Lymnaea stagnalis. Kuroda says her team is closing in on the gene or genes controlling handedness in Lymnaea stagnalis. The implications could go far beyond snails, she says. "These sorts of [developmental] dynamics are not only seen in snails, they are involved in all cell cleavage."
Lisa Nagy, a developmental biologist at the University of Arizona, Tucson, praises the group's work as "an exciting and elegant new study" that overthrows a long-standing developmental model. But she cautions that it is "too soon to tell whether the mechanisms of left-right asymmetry are conserved between snails and any other metazoans."