The Structural Differences Between Animal and Plant Mitochondrial Genomes -- Two Evolutionary Scenarios

The function of mitochondria is very conserved in almost all eukaryotes,while the sizes and structures of mitochondrial genomes are very diverse among animals,plants,protozoa,algae and fungi.The causes for these differences and the corresponding evolutionary paths are yet to be found.Animal mitochondrial genomes are very small and compact,with a size of about 16 kb.As"an extreme example of genetic economy",introns and non-coding repeated sequences are absent,and the arrangement of genes is relatively stable.On the other hand,higher plant mitochondrial genomes are very large,ranging from 186 kb to 2400 kb.Their genomes are complex and heterogeneous,with many repeated sequences and introns present.On the whole,animal and plant mitochondrial genomes represent the two extremes of this organelle genome in eukaryotes.As for the mitochondrial genomes of protozoa,algae and fungi,their sizes and structures are somewhere in between these two extremes.Although heterogeneous within group,repeated sequences may be found in mitochondrial genomes of protozoa and fungi,The existence of introns are common in fungal mitochondrial genomes,even in the smallest ones,several introns are present.Among these three kinds of lower eukaryotes,when the mitochondrial genome is relatively small,its structure is rather like that of animals than plants.Conversely,if the genome is relatively large,it resembles plants rather than animals.With the analysis of the structural character of mitochondrial genomes in different eukaryotes,it seems that there are also two evolutionary scenarios for mitochondrial genomes similar to those for nuclear (nucleoid) genomes proposed by the author.As a proposed pattern for the origin and evolution of repeated sequences,gene and genome structure (Zhang,1990),repeated sequences may have an ancient origin,dating back to the early stages of biological evolution.The most primitive"genes" and "genomes" would be composed of repeated sequences.With this basis,evolution would lead to a kind of genome consisting of early split genes and repeated sequences.Two possibilities for the further evolution of this kind of genome would lead to either small genome organisms or large genome organisms with different genomic character.If an organism evolved to have a small genome,it must first contain the genes with higher functions for its efficient survival,leaving little or no room for repeated sequences and introns (they were lost in the course of evolution).On the contrary,if an organism evolved to have a large genome,it may contain both the necessary genes and repeated sequences and introns.The repeated sequences and introns would be then"relics"of the primitive genomes,they would continue to play structural and evolutionary roles in modern genomes containing them.Modern prokaryotes would be the representatives of "small genome"evolutionary scenario,while the "large genome"scenario would be represented by modern eukaryotes.With the proposed pattern and the theory of endosymbiotic origin of mitochondria,it can be assumed that the ancestor of mitochondria possessed already a genome of considerable size,with repeated sequences and introns present.This kind of primitive mitochondrial genome evolved either to "small genome"mitochondria,introns and repeated sequences being lost,as in the case of animals as well as other eukaryotes with small mitochondrial genomes;or to "large genome"mitochondria as those of plants and other eukaryotes with large mitochondrial genomes.These two evolutionary scenarios can explain well the structural character of mitochondrial genomes in different kinds of eukaryotes.The repeated sequences in plant,protozoan and fungal mitochondrial genomes,and the introns in plant and fungal mitochondrial genomes,can be considered as "relics"of the ancestor of mitochondria.Their existence would not be a conflict with,but rather a support to the endosymbiont theory.In this context,a general pattern for the origin and evolution of nuclear (nucleoid) genomes and mitochondrial genomes (organelle genomes) can be summarized as follows (s,s as "small genome"scenario with repeated sequences and introns lost;1,1 as"large genome" scenario).