Influence of extremophily and parasitism on the evolutionary dynamics of plant mitochondrial genomes.

摘要

Horizontal gene transfer (HGT) has heavily influenced the evolution of both prokaryotes and eukaryotes through the introduction of novel genes that provide new functions to organisms. This thesis describes the evolutionary and adaptive effects of HGT on the organellar genomes of algae and plants. Genomic sequencing has revealed that plant mitochondrial DNA (mtDNA) experiences more frequent HGT compared with chloroplast DNA (cpDNA). Many examples of plant mitochondrial HGT have reported genetic exchange between parasites and hosts, yet the mechanisms of transfer are poorly understood. It is speculated that the haustorial connection between parasite and host may allow the transfer of genetic material via vectors like bacteria, fungi or viruses. Here, we report two independent cases of HGT from parasites in different families to the common ancestor of their Plantago (Plantaginaceae) hosts. The first case involves massive transfer of 18 genes from the parasite Cuscuta (Convolvulaceae) and 3 genes from non-parasitic monocots into the mtDNA of Plantago macrorhiza and P. coronopus . The second study showed that three genes (atp1, atp9, ccmFn ) were acquired in the mtDNA of P. rigida and P. tubulosa from two parasites Bartsia and Castilleja. We also report an unprecedented recapture of genes via HGT, which have been previously lost from the mtDNA of most angiosperms. In red alga Galdieria sulphuraria, HGT has conferred adaptive benefits by acquiring bacterial genes that make it tolerant to extreme environments like high temperature, low pH, high salt and heavy metals. To determine the effects of HGT and extremophily on organellar genomes, we assembled and annotated the Galdieria mitochondrial and plastid genome. Although no evidence of HGT was detected, the genomes demonstrate other unusual characteristics. Its mtDNA seems to be the most affected, exhibiting genome size reduction, gene loss, high GC content and GC skew. On the other hand, the Galdieria cpDNA is typical of red algae, except for the unique retention of two big and many small inverted repeats (IRs). The extremophilic conditions may place additional mutational pressures on the mtDNA, whereas the presence of numerous IRs may shield the cpDNA from similar genomic stress.