What causes a helix in nature to appear with either a dextral (right-handed, or clockwise) twist or a sinistral (left-handed, or counterclockwise) twist is one of the most intriguing puzzles in the science of form. The phenomenon above is quite puzzling
Most spiral-shaped snail species are predominantly dextral. The snali usually a dexstous
But at one time, handedness (twist direction of the shell) was equally distributed within some snail species that have become predominantly dextral or, in a few species, predominantly sinistral. Not a so clear sentence to understand. At the same time, the shape of the shell in the snail is usually of the right but in some of sub-species are towards the left
What mechanisms control handedness and keep left-handedness rare?Why is so on the left and why it is a minority phenomenon
It would seem unlikely that evolution should discriminate against sinistral snails if sinistral and dextral snails are exact mirror images, for any disadvantage that a sinistral twist in itself could confer on its possessor is almost inconceivable. Basically the sentence above tells us that the evolution process is even because right and left are mirror of one another
But left- and right-handed snails are not actually true mirror images of one another. Actually left and right are NOT mirror
Their shapes are noticeably different. They are different in shape
Sinistral rarity might, then, be a consequence of possible disadvantages conferred by these other concomitant structural features. The left direction could be the result of certain disadvantages that as it turns out is the result of certain fatures
In addition, perhaps left- and right-handed snails cannot mate with each other, having incompatible twist directions. The cannot match each other
Presumably, an individual of the rarer form would have relative difficulty in finding a mate of the same hand, thus keeping the rare form rare or creating geographically separated right- and left-handed populations.Maybe, mating two individuals of the same side is difficult and the result is that the left side is keeping a rare feature and the right and left snails are separated.
But this evolutionary mechanism combining dissymmetry, anatomy, and chance does not provide an adequate explanation of why right-handedness should have become predominant. All the above, however, does not explain why the right side feature is predominant respect the left one
It does not explain, for example, why the infrequent unions between snails of opposing hands produce fewer offspring of the rarer than the commoner form in species where each parent contributes equally to handedness. rarely the left and right shell feature mate. And why does this union produces more right side shells than left side.
Nor does it explain why, in a species where one parent determines handedness, a brood is not exclusively right- or left-handed when the offspring would have the same genetic predisposition. Why in those couples where the shell on one parent is the relevant, the result is left direction or right without where it (the brood) should be mainly right.
In the European pond snail Lymnaea peregra, a predominantly dextral species whose handedness is maternally determined, a brood might be expected to be exclusively right- or left-handed—and this often occurs. However, some broods possess a few snails of the opposing hand, and in predominantly sinistral broods, the incidence of dextrality is surprisingly high.This particular species in Europe where the individuals are usually right; the brood is not a mix. Or they are right or left where the mother is right and the offspring should be right. However, the sinistral feature is the most relevant
Here, the evolutionary theory must defer to a theory based on an explicit developmental mechanism that can favor either right- or left-handedness. Some sort of mechanism that favorites the right side or left side should be implied
In the case of Lymnaea peregra, studies indicate that a dextral gene is expressed during egg formation; i.e., before egg fertilization, the gene produces a protein, found in the cytoplasm of the egg, that controls the pattern of cell division and thus handedness. For example, in the above case, a protein is the culprit of the right direction of the shell.
In experiments, an injection of cytoplasm from dextral eggs changes the pattern of sinistral eggs, but an injection from sinistral eggs does not influence dextral eggs. An experiment was performed to confirm thew hypothesis above
One explanation for the differing effects is that all Lymnaea peregra eggs begin left-handed but most switch to being right-handed. An explanation could be that the direction began left and than shift to the right
Thus the path to a solution to the puzzle of handedness in all snails appears to be as twisted as the helix itself.We do not know yet for sure. It is a conundrum
_________________