In this article we will discuss about the Inheritance of Shell Coiling in Snail.
One of the classical examples of intricate relationship between maternal genotype and egg cytoplasm “phenotype” was studied in snails by Sturtevant. He showed that there are two strains of water snails (limnaea peregra) that differ each other in the direction of coiling of shell.
Looking into the opening of the shell it can be seen that in one strain the shell always coils to the left (sinistral) whereas in the other strain the shell always coils to the right (dextral).
In the cross dextral ♀x sinistral ♂ all the F, progeny have dextial coils implying that dextral is dominant over sinistral. However, in the F1, x F1, cross (i.e., inbreeding), all the F1 snails are also dextral. The reciprocal cross (dextral ♂ x sinistral ♀) produces F1, progeny that are all left coiler. In this case F1, x F1, cross also yields only dextral coils.
From these experiments it becomes clear that coiling of snails is not determined by individuals’ own genes but by those of mother.
The offsprings whose mothers are either homozygous or heterozygous for right coiling are right coilers even if they are homozygous for sinistrality (left coiling). In the same way offspring of left coiling mother are left coilers even if they carry dominant genes for right coiling.
The F2 females of either cross (right coiler) when mated with males of any genotype produce at an average right coilers and left coilers in 3: 1 ratio. But 3: 1 ratio appears in F3 and not in F2. If F2 males are mated with homozygous right coiling females, there is no segregation and all their progenies are right coilers, but if they are mated with homozygous left coiling female’s only left coilers are produced.
It is clear then that the paternal (male) genotype is not crucial in determining the phenotype of the offspring. It becomes clear when it is assumed that the genes for left coiling pattern (S+) and that the phenotypic expression of individuals is determined by the genotype of their mothers. The crosses illustrating the maternal determination of shell coiling in snails are shown in the Fig 18.4.
Further investigations suggest that coiling depends upon the early cleavages in the zygote. If the spindle is tilted to left of the median line of zygote, the successive cleavages will produce a spiral to left and if the orientation of spindle is tilted to the right of the median line of zygote a dextral pattern will follow.
The spindle orientation is governed by the genotype of oocyte from which the egg develops. However, the exact mechanism of spindle orientation is not known (Fig. 18.5).