In this article we will discuss about:- 1. Subject Matter of Cytoplasmic Inheritance 2. Maternal Effects of Cytoplasmic Inheritance 3. Infective Particles.
Subject Matter of Cytoplasmic Inheritance:
The inheritance of characters controlled by the genes is principally related to the chromosomes of the nucleus. These genes, which earlier designated as factors by Mendel, follow a distinct pattern of inheritance as enunciated in Mendel’s laws.
Though this is the general rule for majority of characters, certain qualities do not show Mendelian law of inheritance. Such behaviour has been well exemplified in a number of plant and animal systems. Evidence of non-Mendelian inheritance is best reflected in reciprocal crosses in certain species.
Reciprocal crosses in plants provide good evidence of such non-Mendelian behaviour where the progeny follows the character of the maternal parent to whichever way the cross is made. The nucleus of both male and female parents are inherited by the progeny in both the crosses. The difference lies only in the cytoplasm, which is contributed by the female parent only.
As such, resemblance of the progeny with the female parent is attributed to the nature of the cytoplasm.
Once the influence of cytoplasm was realised, the concept of cytoplasmic genes, the then termed as plasma-genes, was accepted. However, in later years it has been clearly established that chloroplastid and mitochondria too contain DNA, the genetic material. The principal sources of cytoplasmic inheritance now are being attributed to cell organelles, the mitochondria and chloroplastid, both of them having DNA.
Maternal Effects of Cytoplasmic Inheritance:
Inheritance depending indirectly on nuclear genes and involving no known cytoplasmic hereditary units is described as maternal effects. Such cases of maternal effects can be distinguished from those, where extra-chromosomal or cytoplasmic hereditary units are present and function either independently or in collaboration with nuclear genetic system.
Coiling of Shell in Snail:
One of the earliest and best known examples of a maternal effect is that of the direction of coiling in shells of the snail (Limnaea peregra). Some strains of the species have dextral shells, which coil to the right; others have sinistral shells, which coil to the left. This direction of coiling is genetically controlled. The dextral-coiling depends upon the dominant allele D, and sinistral coiling depends upon recessive allele d, so the dextral is DD and sinistral is dd.
When crosses were made between females coiled to the right (DD) and males coiled to the left (dd), the F1 snails were all coiled to the right. The usual 3:1 ratio was not obtained in the F2 because the phenotype of dd is not expressed. Instead, the pattern determined by the mother genes (DD) was expressed in the F1 and the F2 mother genotype was expressed in the F1.
When dd individuals were inbred, only progeny that coiled to the left were produced. When DD or Dd were inbred, however, they produced offsprings that all coiled to the right (Figs. 10.6 & 10.7).
This is a case of delayed effect of genotype. The phenotype in progeny obtained from reciprocal crosses (♀ DD x♂ dd : ♀ dd x ♂ DD) is determined by the genotype and not by the phenotype of the female parent. In reciprocal crosses, it is evident that the genotype Dd (F1) can be dextral as well as sinistral depending upon the genotype of the female parent.
Similarly, dd can be dextral if genotype of female parent carries dominant allele (Dd). Phenotype of female parent does not have any effect on phenotype of progeny. It is the genotype of female parent which is really decisive.
Pigmentation in Flour Moth:
In flour moth, pigmentation in body is controlled by a dominant gene A, responsible for production of kynurenine, a pigment precursor.
Crossing between pigmented heterozygote (Aa) and non-pigmented homozygote (aa) is expected to show segregation of progeny into 1 pigmented (Aa): 1 non-pigmented (aa). But reciprocal crosses reveal that when mother is pigmented (Aa), all progeny larvae are pigmented. However, when larvae mature, only half of them (Aa) becomes pigmented and the rest half (aa) becomes non-pigmented.
These homozygous (aa) pigmented larvae have kynurenine (received from mother through egg. cytoplasm) in early stages of development, but are incapable of synthesizing it due to absence of dominant allele and subsequent loss of pigmentation occurs in adult moth (Fig. 10.8).
Infective Cytoplasm Hereditary Particles:
There are cases, where cytoplasmic inheritance depends on extra-chromosomal particles which are not essential for cell function and, therefore, may be present or absent. Such dispensable particles are not only inherited but are also infective, since they can be introduced into new hosts without the need of actual process of reproduction.
Further, the presence and reproduction of these infective particles may also depend on nuclear genes.
Kappa Particles in Paramoecium:
Kappa particles are found in certain killer strains of Paramoecium and are responsible for the production of substance called paramecin, which is toxic to certain strains not possessing kappa (sensitive strain). The production of kappa particles is dependent on a dominant allele K, so that killer strains are KK or Kk and the sensitive strains are ordinarily kk.
In absence of dominant allele K, kappa particles cannot multiply and in absence of kappa particles, dominant allele K cannot produce them de novo. Consequently sensitive strains with genotypes KK or kk can be obtained. These will not carry any kappa particles. However, killer strains with genotype kk cannot be obtained, because even if kappa particles are present, these would be lost in absence of dominant allele.
If Paramoecium clones with genotype KK or Kk are allowed to multiply asexually at such a fast rate, that division of kappa particles cannot keep pace with division of the cells, kappa particles will be lost. Consequently, sensitive strains with dominant genotype (KK, Kk) having no kappa particles would be obtained.
If the killer (KK) and sensitive (kk) strains are allowed to conjugate, all exconjugants (the cells separating after conjugation) will have same genotype (Kk). Phenotypes depend upon the duration for which conjugation is allowed.
If conjugation does not persist long enough for exchange of cytoplasm, heterozygote (Kk) exconjugants will only have parental phenotypes, i.e., killers remain as killers and sensitive as sensitive (Fig. 10.9). But, if conjugation persists, sensitive strain will receive kappa particles and will become killer, so that exconjugants will be killers having genotype Kk (Fig. 10.10).