Index >> Cytoplasmic or Extranuclear Inheritance >> Extra Nuclear Inheritance in Eukaryotes

Only in the F₂ generation does segregation in tile ratio of 3: 1 become apparent. Since the individuals of the F₂ generation had the genotypes-1S⁺S⁺, 2S⁺S ISS three quarters of them on the average, produce eggs developing into sinistral individuals.

Maternal effect in the direction of coiling of the shell in Limnaea (after SRB, Owen and Edgar, 1965).

It is easy to understand that the results of a reciprocal cross that is, of the fertilization of the eggs of a homozygous dextral individual (S⁺S⁺) by the sperm of a sinistral individual (S⁺S)-will lead to a somewhat different type of pedigree: the F₁ generation will be dextral (with genotype S⁺S) and the F₂ generation again all dextral (with genotypic ratio of 1S⁺S⁺: 2S⁺S: ISS). The F₂ generation will show segregation among broods, just as in the cross examined first.

The whole case become clear if it is realized that the type of cleavage (sinistral or dextral) depends on the organization of the egg which is established before the maturation divisions of the oocyte nucleus. The type of cleavage is, therefore, under the influence of the genotype of the material parent.

The sperm enters the egg after this organization is already established. Lastly, the direction of coiling of shell depends upon the orientation of the mitotic spindle of first cleavage of the zygote. If the spindle is tipped toward the Wt of the median line of the egg cell, the sinistral pattern will develop, conversely if the mitotic spindle is tipped toward the right of the median line of the cell, the dextral pattern will develop. The spindle orientation is thus controlled by the organization of ooplasm which becomes established during oogenesis and before fertilization.

(b) Maternal inheritance in Ambystoma-In axolotl (Ambystoma mexicanum) a lethal gene; "O" has been discovered which has a maternal effect. The gene 'O' is recessive, so that heterozygote individuals (with genotype+O) are completely normal. Homozygote animals (with genotype OO) produced by a cross between two heterozygote parents (+O) develop normally in the early stages but in later life show a slight retardation of growth. Their regeneration capacity is severly reduced so that amputated legs are not regenerated as would occur in normal axolotls. The homozygote (OO) males are sterile-their testes are poorly developed and spermatogenesis does not go beyond the spermatogonial stages. The homozygote (OO) females on the other hand, produce eggs capable of fertilization and normal cleavage, but at the on set of gastrulation the development is retarded and embryos normally die. This abnormal course of development is in no way affected by the genotype of the spermatozoon: the spermatozoon may carry the normal +gene or a mutant ethal O gene (if the male is heterozygote +O).

The arrest in development is in both cases exactly the same: this shows that the particular type of abnormal early development does not depend on the gene present in the cells of the developing embryo (+O), but exclusively on the OO genes of the mother (maternal effect). Briggs and Justus (1967) found that eggs of a OO female lack a protein-like substance the “corrective factor" which is necessary for normal development and is synthesized by normal genes + + of homozygous or heterozygous normal females (+ + or +O) during oogenesis.

Maternal Inheritance and Manifestation of the Ogene in the Axoloti (after Balinsky, 1970)

(c) Eye pigmentation in water fleas and flour moths-Like the Limnaea and Ambystoma mexicanum, the maternal effects has also been observed in at least two very different invertebrates, the water flea (Grammars) and the flour moth (Ephestia kuhniella). The normal colour of the eye in both invertebrates is dark due to the dominant gene (AA or KK) in which the dominant gene K directs the production of hormone-like substance called kynurenine which is involved in the pigment synthesis. The recessive mutants do not possess pigment in the eye (viz., kynurenineless) and have the genotype aa or kk.

When aa or kk female is crossed with heterozygous with Aa or Kk male, only half of the larvae show dark pigment in the eye. But, a -cross between Aa or Kk female and aa or kk male produces all larvae with dark eyes. On reaching the adult stage half of the off springs (those of the genotype aa) become light eyed. This indicate that some kynurenine diffuses from the Aa mother into all young (larvae), enabling them to manufacture pigment regardless of their genotype. The aa progeny however has no means of continuing the supply of kynurenine, with the result that their eyes eventually become light. This example suggests an ephemeral type of maternal effect.

(d) Male sterility in plants-In plants a most interesting example of maternal inheritance is associated with pollen failure. This occurs in many flowering plants and results in male sterility. In maize wheat, sugar beets, onion and some other crop plants fertility is controlled at least in part, by cytoplasmic factors. Male sterility is also controlled in some plants by nuclear genes.

Maternal Inheritance of Male Sterility in Maize, (after Gardner, 1968)

The classical example of maternal inheritance of male I sterility was discovered by Rhoades in maize. A particular male I Sterile variety produced only male sterile progeny when fertilized with Pollen from normal maize plants. The male sterile seed parent 1 plants were then backcrossed repeatedly with pollen fertile lines until all chromosomes from the male sterile line had been substituted for those of the male fertile line. Male sterility persisted, indicating that inheritance was maternal and was not controlled by chromosomal genes. When a small amount of pollen was obtained from the male sterile line making reciprocal crosses possible. Inheritance was maternal regardless of the direction in which the cross was made. Male sterility, in this case, was attributed to cytoplasmic plasmagenes transmitted by female gametes.

The cytoplasmic effect is not entirely independent, however, because specific nuclear genes (called restorer genes) are now known suppress maternally inherited male sterility in maize (Gardner, 1972) A single dominant chromosomal gene will restore pollen fertility in the presence of cytoplasm which ordinarily would result in sterility. Male sterility has some economic significance and has been used in the production of hybrids of maize, cucumber, onions, wheat, and other plants for the purpose of obtaining hybrid vigour.