Prokaryotic or gan isms are ex posed in the course of evo lu tion to var i ous im pacts, result ing of ten in dras tic changes of their ge nome size. De pending on cir cum stances, the same lin eage may di verge into spe cies hav ing sub stan tially re duced genomes, or such whose genomes have un der gone con sid er able en large ment. Ge nome re duc tion is a con se quence of ob li gate intracellular life style ren der ing nu mer ous genes ex pend able. An other con se quence of intracellular life style is re duc tion of ef fec tive pop u la- tion size and lim ited pos si bil ity of gene ac quire mentvia lat eral trans fer. This causes a state of re laxed se lec tion re sult ing in ac cu mu la tion of mildly del e te ri ous mu ta tions that can not be cor rected by re com bi na tion with the wild type copy. Thus, gene loss is usually irreversible. Additionally, constant environment of the eukaryotic cell renders that some bac te rial genes in volved in DNA re pair are ex pand able. The loss of these genes is a prob a ble cause of mutational bias re sult ing in a high A+T con tent. While causes of genome reduction are rather indisputable, those resulting in genome ex pan sion seem to be less ob vi ous. Pre sum ably, the ge nome en large ment is an indirect consequence of adaptation to changing environmental conditions and requires the ac qui si tion and in te gra tion of nu mer ous genes. It seems that the need for a great number of capabilities is common among soil bacteria irrespective of their phylo gen etic re la tion ship. How ever, this would not be pos si ble if soil bac te ria lacked in dig e nous abil i ties to ex change and ac cu mu late ge netic in for ma tion. The lat ter are con sid er ably fa cil i tated when house keep ing genes are phys i cally sep a rated from adaptive loci which are useful only in certain circum stances.
JavaScript jest wyłączony w Twojej przeglądarce internetowej. Włącz go, a następnie odśwież stronę, aby móc w pełni z niej korzystać.