微生物ゲノムの進化

The core genome comprises genes shared by all strains of a species, encoding essential functions for survival and basic cellular processes. The pan-genome includes every gene found across all strains, combining core genes with strain-specific genes acquired through horizontal gene transfer or other mechanisms. Together, they reveal genomic diversity within a species.

Microbes primarily gain genes through horizontal gene transfer, which introduces new traits enabling antibiotic resistance, occupation of new ecological niches, or metabolism of novel substrates. These acquired genes can be neutral, beneficial, or harmful. Natural selection retains beneficial genes while removing those offering no advantage through genomic deletions.

Gene deletions are common in microbial genomes because many acquired genes offer no benefit and may be removed through genomic deletions or genetic drift. This process helps maintain streamlined, efficient genomes. Natural selection retains only genes essential for survival or those providing competitive advantages in specific environments.

Approximately 30 to 60 universally conserved gene families are shared across all bacterial and archaeal genomes. These genes encode core functions such as DNA replication, transcription, translation, and basic metabolic pathways, reflecting deep evolutionary constraints and fundamental biological requirements for life.

Comparative genomics analyzes genes across multiple strains to identify core genes shared by all strains and strain-specific genes unique to certain lineages. This approach reveals how gene acquisition, loss, and selective retention shape microbial genomes, demonstrating both remarkable adaptability and fundamental constraints imposed by essential biological requirements.

Yes, in some species the pan-genome continuously expands as new sets of genes are gained through horizontal gene transfer or other mechanisms. However, expansion depends on ecological niches demanding broader metabolic capabilities. Many acquired genes are eventually removed if they provide no survival advantage, maintaining genomic balance.

Natural selection acts as a filter on microbial genomes by retaining beneficial genes while removing neutral or harmful acquired genes through deletions. All genes essential for basic survival are preserved as part of the core genome. This selective pressure balances genomic innovation with the maintenance of critical cellular functions.