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A maternal effect is a situation where the phenotype of an organism is determined not only by the environment it experiences and its genotype, but also by the environment and phenotype of its mother. In genetics, maternal effects occur when a mutant organism shows not the mutant phenotype, but rather the phenotype expected from the genotype of the mother, often due the mother supplying mRNA or proteins to the egg. Maternal effects can also be caused by the maternal environment independent of genotype, sometimes controlling the size, sex, or behaviour of the offspring. It has been proposed that maternal effects are important for the evolution of adaptive responses to environmental heterogeneity.

Maternal effects and evolution

A maternal effect, in genetics, is the phenomenon where the genotype of a mother is expressed in the phenotype of its offspring, unaltered by paternal genetic influence. The phenotype of an individual therefore reflects the genotype of its mother, rather than the genotype of the individual.

This maternal effect is usually attributed to maternally-produced molecules, such as mRNAs, that are deposited in the egg cell. Maternal effect genes often affect early developmental processes. An example in Drosophila melanogaster morphogenesis is axis formation (See Below).

Another mechanism for the specific expression of genes from one parent is stable epigenetic modification of germ line genes in one of the sexes. This phenomenon is termed genomic imprinting, but is not a form of a maternal or parental effect.

"Maternal effect" should not be confused with maternal inheritance, in which some aspect of an offspring's genotype is inherited solely from the mother. This is often attributed to maternal inheritance of mitochondria or plastids, each of which contains its own genome. Maternal inheritance is distinct from maternal effect inheritance because in maternal inheritance the individual's phenotype reflects its own genotype, rather than the genotype of a parent.

Dorsal-ventral Axis

Formation of the Dorsal-Ventral Axis is dependent on a maternally synthesized transcription factor known as dorsal protein. The production of dorsal protein is stimulated by the localization of the embryonic nuclei. The nuclei secretes a protein called Gurken. Gurken inhibits the production of PIPE protein by interacting with Torpedo receptor on flanking oocyte follicle cells. PIPE positive cells are able to secrete dorsal protein and form the ventral side of the egg, while PIPE negative cells do not secrete dorsal protein and form the dorsal side of the egg.

Dorsal induces the transcription of two genes twist & snail while repressing zerknullt & decapentaplegic.

Intra-membranous dorsal receptor proteins, known as Toll receptors are responsible for transporting dorsal protein into the embryonic nuclei. These Toll receptors are the product of Toll gene, and are uniformly spaced across the embryoinic plasma-membrane.

Since dorsal protein is secreted by PIPE positive-ventral follicular cells of an egg, dorsal protein enters the embryo to the ventral side. Once transported into the nuclei, dorsal protein is most concentrated at the ventral side of the embryo.

This process sets up a gradient differential between the ventral and dorsal side of an immature embryo, the repression or induction of these four genes are differentially regulated. For example;

• At the ventral end of the embryo, blastoderm nuclei exposed to high concentrations of dorsal protein induce the transcription of twist and snail while repress zerknullt & decapentaplegic
• In the middle of the embryo, blastoderm nuclei exposed to mild concentrations of dorsal protein don't express any genes
• At the dorsal end of the embryo, blastoderm nuclei exposed to little or no dorsal protein express only zerknult & decapentaplegic.

Anterior-Posterior Axis

The formation of the anterior-posterior axis in Drosophila is created by the regional synthesis of transcription factors encoded by the hunchback & caudal genes. These genes are transcribed among nurse cells of the maternal germ line that support the growth and development of an oocyte. Maternal transcripts of the hunchback and caudal genes are transported into the oocyte to become uniformly distributed in the cytoplasm.

Although hunchback and caudal genes are evenly transcribed, their translation is regulated so that the hunchback protein is more concentrated at the anterior determination of the oocyte while the caudal protein is accumulated more in the posterior. The "bicoid" and "nanos" proteins described below are the translational regulators. Hunchback and caudal proteins act as transcription factors of many genes involved in the differentiation of an embryo along the anterior-posterior axis.

Bicoid and nanos RNAs are synthesized in the nurse cells of the maternal germ line and are transported into the oocyte.

Functions of nanos

• A transcriptional regulator - binding to the 3'OH untranslated region of hunchback RNA and causes the degradation of the RNA.

Functions of bicoid

• Acts as a transcription factor to stimulate synthesis of RNAs from several genes including hunchback. These RNAs are translated into proteins that control the formation of the anterior structures of the embryo.
• Inhibits transcription of caudal RNA by binding to sequences located in the 3'OH termini untranslated regions.

Mutations in maternal effect genes

Maternal gene mutations are inherited from the mother's genotype. Crossing a homozygous mutant female with a homozygous wild-type male will produce an inviable progeny. This lethal effect is strictly maternal.

Paternal effect genes

In contrast, a paternal effect is when a phenotype results from the genotype of the father, rather than the genotype of the individual. The genes responsible for these effects are components of sperm that are involved in fertilization and early development. An example of a paternal-effect gene is the ms(3)sneaky in Drosophila, males with a mutant allele of this gene produce sperm that are able to fertilize an egg, but the snky-inseminated eggs do not develop normally. However, females with this mutation produce eggs that undergo normal development when fertilized.

See also

Regional_specification#Drosophila