Structure of Organelle Genomes
Maternal Inheritance Overheads
Snail shell coiling and maternal effects
The embryo is formed when a female gamete unites with a male gamete. In the vast majority of species, the female gamete is physically larger than the male gamete and provides the cytoplasm for the developing embryo. Within this cytoplasm are factors that were released by the nuclear genes of the female. Those factors may have specific effects upon the developing embryo. The female cytoplasm also contributes the mitochondria for all species as well as the chloroplast for plant species. These two organelles contain DNA and control certain traits in the offspring. Those phenotypes that are controlled by nuclear factors found in the cytoplasm of the female are said to express a maternal effect. Those phenotypes controlled by organelle genes exhibit maternal inheritance.
The classic phenotype which exhibits maternal effects is coiling direction of snail shells. The coiling phenotype that is seen in the offspring is controlled by the genotype of the mother. The following crosses were made between pure line snails, and the following results were seen. By convention, the female is always given first.
These results at first glance appear to be at odds with Mendel's laws. First, the F1 phenotype is not the same for both crosses. With other experiments, the results of reciprocal crosses (complementary crosses were the phenotypes of female and male are reversed in the initial parental cross) were equivalent, but with this experiment it appears that the female controls the phenotype. Yet, the F2 appears to contradict this hypothesis because the left- and right-coiled F1 individuals produced all right progeny. Furthermore, the 3:1 Mendelian ratio is not seen in the F2, but rather appears in the F3 generation.
How can this result be explained? First, let's look for results that are familar. The F3 ratio of 3 right:1 left for both crosses suggests that right-coiled shells are dominant to left-coiled shells. If this is the case, then we can assign the following genotypes to the pure lines:
- Right-coiled shell: s+s+
- Left-coiled shell: ss
The next observation is that the phenotype of the F1 generation is always that of the female parent. One hypothesis would suggest that the genotype of the female controls the genotype of its offspring. Can these result be confirmed in the subsequent generations? If the genotypes we assigned to the parents are correct, then the genotype of F1 individuals from each cross are s+s (from s+s+ x ss and ss x s+s+). If the female genotype does control the phenotype of its offspring, then we would predict that all the F2 snails would have right coils. This is the exact result that is seen. But what would the genotypes of the F2 snails be? If we intermate snails with the genotype s+s the genotypic ratio should be 3 s+_ to 1 ss. These genotypes would not be expressed as a phenotype until the F3 generation. These are the results that were obtained. A general conclusion from all traits that express a maternal effect is that the normal Mendelian ratios are expressed one generation than expected.
Copyright © 1998. Phillip McClean