Maternal effects

Maternal effects are the influences of a mothers genotype on the phenotype of her offspring. It results from the asymmetric contribution of the female parent to the development of zygotes.
In terms of chromosomal genes, both male and female parents contribute equally to the zygote. The female parent contributes to the zygotes initial cytoplasm and organelles. Sperm rarely contribute anything other than chromosomes. Therefore zygotic development begins within a maternal medium and hence the maternal cytoplasm directly affects zygotic development.

1. MATERNAL EFFECTS: SHELL COILING IN SNAILS & PIGMENTATION IN MOTHS Presented by: Vishakha Chodankar M.Sc Part- I Semester- II Paper- III

2. What are Maternal Effects? Maternal effects are the influences of a mothers genotype on the phenotype of her offspring. It results from the asymmetric contribution of the female parent to the development of zygotes. In terms of chromosomal genes, both male and female parents contribute equally to the zygote. The female parent contributes to the zygotes initial cytoplasm and organelles. Sperm rarely contribute anything other than chromosomes. Therefore zygotic development begins within a maternal medium and hence the maternal cytoplasm directly affects zygotic development .

3. SHELL COILING IN SNAILS ( Limnaea peregra )

4. Limnaea coiling Snails are coiled either- Dextrally (opening from right hand side) Sinistrally (opening from left hand side) Dextral coiling is dominant These snails are hermaphroditic and may undergo either cross- or self- fertilization, providing a variety of types of matings.

5. Fig.1: Inheritance of coiling in snail Limnaea perega

6. When a dextral snail provides the eggs ( DD ) and a sinistral snail provides the sperm( dd )(left half fig.1) the F 1 offsprings are dextral( Dd ). When F 1 are self-fertilized, all the offsprings are dextrally coiled. When the F 2 are self-fertilized, one-fourth produce only sinistral offsprings and three-fourths produce only dextral offsprings . When a sinistral snail provides the eggs ( dd ) and a dextral snail provides the sperm( DD )(right half fig.1) the genotype of F 1 offsprings is same ( Dd ) but are coiled sinistrally as in the female parent. The later results are same as in the first case. The explanation is that the genotype of maternal parent determines the phenotype of the offspring with dextral dominant.

7. Why does this pattern occur? This occurs due to a process called spiral cleavage. Spiral cleavage takes place in the zygotes of molluscs and also some other invertebrates.

8. The orientation of spindle in the first cleavage division after fertilization determines the direction of coiling. Spindle orientation is controlled by maternal genes acting on developing eggs in ovary. This influences cell divisions following fertilization and establishes the permanent adult coiling pattern. Dextral allele ( D ) produces an active gene product that causes right hand coiling. If ooplasm of dextral eggs injected into uncleaved sinistral eggs, they cleave in dextral pattern . In converse experiment, the sinistral ooplasm has no effect when injected into dextral eggs. Thus the sinistral allele is the result of a classic recessive mutation that encodes an inactive gene product.

9. Conclusion The females that are either DD or Dd produce oocytes that synthesize the D gene product which is stored is ooplasm. Even if the oocyte contains only the d allele (in female with Dd genotype) following meiosis and is fertilized by a d- bearing sperm the resulting ‘ dd ’ snail will be dextrally coiled.

10. PIGMENTATION IN MOTHS ( Ephestia kuehniella )

11. Ephestia Pigmentation Fig.2: Maternal influence in the inheritance of eye pigment in the meal moth Ephestia kuehniella

12. The wild type larva of Ephestia kuehneilla has a pigmented skin and brown eyes as a result of the dominant gene A . The pigment is derived from a precursor molecule, kynurenine , which is a tryptophan derivative. A mutation, a , interupts the synthesis of kynurenine . Homozygosity may result in red eyes and little pigmentation in larvae. The results of the cross ( Aa x aa ) are illustrated in fig.2 When the male is heterozygous parent a 1:1 brown to red-eyed ratio is observed in larvae. When the female is heterozygous for A gene, all larvae are pigmented and have brown eyes, in spite of half of them being aa . As these larvae develop into adults one half of them gradually develop red eyes, reestablishing the 1:1 ratio.

13. The explanation for these results is that the Aa oocytes synthesize kynurenine or an enzyme necessary for its synthesis and accumulate it in the ooplasm prior to the completion of meiosis . When the mothers were Aa , in the aa progeny , this pigment is distributed in the cytoplasm of the cells of the developing larvae and hence they develop pigmentation and brown eyes. In these progeny, however, the pigment is eventually diluted among many cells and depleted, resulting in the conversion to red eyes as adults . The Ephestia example demonstrates the maternal effect in which a cytoplasmically stored nuclear gene product influences the larval phenotype and, at least temporarily , overrides the genotype of the progeny.

14. Thank you!