The Genetics of Shell Coiling

or: Why Are Snail Kings Especially Rare?

Genetic terms.

Looking at the frequency of right and left handed shells in Roman snails, numbers alone (there may only be one snail king among some ten thousand Roman snails) lead to the assumption that shell coiling in snails must be a dominant-recessive pattern of heredity following Gregor Mendel (1822 - 1884). Alas, some crossing experiments may yield surprising results, besides not explaining the especially low numbers of sinistral shells in Roman snails, which have given those mutants the special name "snail kings".

The original assumption would be that right handed (dextral) snails have no more than one quarter of left handed (sinistral) offspring. Actually it may happen, that right handed snails have exclusively left handed offspring, which cannot be explained with a dominant-recessive pattern of heredity. On the other hand, left handed snails mating ought to produced exclusively left handed offspring, because in a dominant-recessive pattern of heredity, they only have the allele of sinistrality to inherit. But it is very well possible that two snail kings mating can produce exclusively right handed offspring.

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How can this be explained?

The American geneticist Alfred Henry Sturtevant in 1923 found the explanation examining the pond snail Radix labiata: Heredity of shell coiling in snails actually does have a dominant-recessive pattern, but with some changes from the normal: Heredity takes place in a so-called matrocline fashion. That means that not the genotype of the snail itself determines the way its shell is coiled, but only the genotype of the egg cell it comes from, which again is to be derived from the mother's genotype. The reason is that snails are Spiralia - their cells do not divide in a symmetrical but in an asymmetric fashion. The direction in which this asymmetric cell division takes place and as a late consequence also the direction in which the snail's shell then is coiled, are determined by genetic information in the egg cell's cytoplasm, based on the mother's genotype the egg cell comes from.

The following crossing diagram is supposed to clarify this. To understand it, it is necessary to look not at each mother's coiling direction, but on the egg cell's genotype.

  The left snail is a homozygous snail king exclusively with the genetic information
for a left handed shell (L) in egg cells as well as sperm cells.

The right snail also is a homozygous one with the wild type information for a
right handed shell (R) in egg cells as well as sperm cells.

   
  The left snail has developed from an egg cell with the allele L, so it is left
handed, another snail king. But this one is not homozygous any more, it
carries the alleles R and L. In a normal dominant-recessive heredity pattern,
a snail with both alleles should express the dominant one, which would be
R.

The right snail has developed from an egg cell with the allele R and so is right
handed. This snail also is not homozygous any more, carrying the alleles L and
R.

   
  Here, from one of the snail king's egg cells with the allele R, a right-handed
snail has developed, which by chance also carries is homozygous, exclusively
carrying the allele R.

Contrary to that, from the egg of the right handed snail, a snail king has hatched,
because the egg carried the allele L.

How the crossing experiments proceed may be left to the reader - so it might be interesting to crossbreed the left snail in the second row and the right snail in the third row. Both are snail kings, but what would the offspring look like? Left handed or right handed?

The solution to this question.

Snail kings - Left handed Helix snails.