chapter 18
We have talked about plasmids, e.g. F, and also viral inclusions like phage P1 of E. coli which in the lysogenic state does not integrate into the E. coli chromosome, but remains as a circular plasmid-like inclusion in the cytoplasm. These are both examples of genetic material which is extrachromosomal, i.e. genetic material which doesn=t form part of the main chromosome or chromosomes of the organism and usually doesn=t replicate or divide in the same manner.
The main categories of extrachromosomal genetic material include:
1) mitochondrial (DNA)
2) plastid (chloroplast, DNA)
3) plasmid (DNA)
4) endosymbionts & viruses (includes DNA, RNA and double-stranded RNA)
Detection of extrachromosomally-inherited traits
Extrachromosomally determined characteristics can be detected by their non-Mendelian behaviour with respect to the major genomic components.
Fig: reciprocal cross of horse and donkey
The reason for such a reciprocal difference would seem to be that the mother supplies far more cytoplasm to the zygote than does the male parent and also that, certainly in interspecific hybridizations as we have here, the sperm mitochondrial DNA would appear to be lost after fertilization. So differences in reciprocal crosses could be accounted for by extranuclear genetic material.
In general though, maternal effects are not transmitted past a single generation and may be distinguished from extranuclear inheritance by backcrossing.
This was originally applied to fungi but applicable in any situation in which heterokaryons can be formed. In fungi, hyphae from different strains (same species) can fuse so that the different nuclei are resident in a single cytoplasm and this would be a heterokaryon. Such heterokaryons can be forced, or balanced, by for example, the nutritional selection imposed on MM by different auxotrophic markers in the two strains. If the selection is removed by subculturing the heterokaryon onto complete medium, then the component homokaryons will segregate out.
Now if a particular characteristic is determined by nuclear genes, then it will segregate out with the same nucleus that it went in with as there is no possibility for genetic exchange between the nuclei.
Fig
Alternatively, if the characteristic is cytoplasmically determined by extranuclear genetic elements, then reassociation can occur among the segregant homokaryons.
Fig
In this case, oligomycin resistance segregates independently of the nuclear genotype and can be assumed to be determined by cytoplasmic genes.
This is the essence of the Jinks heterokaryon test: a genetic trait whose mode of transmission (nuclear or cytoplasmic) is put into a heterokaryon with known nuclear markers. If the trait shows different segregation patterns from the nuclear markers, it can be assigned to a cytoplasmic location.
[In Aspergillus nidulans, nuclear and mitochondrial oligomycin resistant mutants have been found. The normal products of both these types of genes are different but both are involved in a mitochondrial (ATP-ase).]
A heteroplasmon is the situation where two distinct cytoplasms are mixed but both the nuclei are not. This can be achieved by several methods:
(a) microinjection of one cytoplasm into another cell or organism.
(b) protoplast fusion in which the protoplasts from one parental strain are anucleate.
(c) infection - certain extrachromosomal factors may be infectiously transmissible.
If any of these methods result in a heritable change in the recipient, then the genetical trait transferred must have been cytoplasmically located.
If all the nuclear chromosomes are well characterized genetically in an organism e.g. the genomes of maize and Drosophila. Then in crosses, if a heritable trait shows no linkage with any of the markers available on the nuclear chromosomes then it is likely that this trait is extrachromosomally located e.g. cytoplasmically determined male sterility in maize associated with mtDNA. CO2 sensitivity in Drosophila (sigma virus, probably ssRNA) and Asex ratio@ strains of Drosophila (reduced number of males). (The latter is due to the presence of spiroplasma microorganisms - like spirochaetes - which are 4 to 5 mm long, in SR strains and it is thought that DNA viruses harboured by these organisms are the causative agents, infecting and killing male zygotes.)
Most extrachromosomal genetic traits are determined by mtDNA and chloroplast DNA.
Chloroplast genomes are circular DNA molecules which are similar in size throughout the plant kingdom, ~ 120-300 kb in size and highly conserved.
Fig: cpDNA genome
Chloroplast genetic systems are closely related to genomes of eubacteria like E. coli. E.g. (1) organization of the rRNA transcription unit is the same as in E. coli, (2) chloroplast ribosomes will work with E. coli enzymes to produce proteins, (3) chloroplast ribosomal subunits will combine with E. coli ribosomal subunits to produce functional proteins, (4) chloroplast genes can be transcribed and translated in E. coli to produce chloroplast proteins.
However, chloroplast genes contain introns. One possible theory is that early life-forms possessed introns but these have been lost in present day eubacteria, although introns are still present in some genes of archaebacteria (these are thought to be more primitive life forms).
Mitochondrial genomes vary enormously in size between different organisms:
Animals circular 14-18kb (human, 16 569 bp, completely sequenced)
higher plants circular, 100-150 kb
Fungi circular, 30-80 kb
Protozoa circular (some linear), some minicircles as little as 1 kb
Algae some linear e.g. Chlamydomonas, 15 kb
A lot of the size variation is attributable to introns (no introns in
human mtDNA).
This template created by the Web Diner.