Name: how dna defines microbial species
Uploaded: 2026-03-12T13:57:40.000Z
Duration: 1 min 22 s
Channel: JoVE

系統発生種の概念は、共通の祖先を共有し、独自の遺伝的特徴を持つ最小の生物群として種を定義します。微生物は無性生殖を行い、形態的多様性は限定的で、水平的な遺伝子移動を行います。したがって、従来の種の定義は使えません。系統発生種の概念は、遺伝子配列や全ゲノムを比較して進化的関係を示すため、微生物の分類に適しています。例えば、2つのDNA配列の16S rRNA遺伝子配列が約98.7%の類似性を持つ場合、一般的には同じ種に属すると考えられます。平均ヌクレオチド同定(ANI)はゲノム全体を比較し、より正確な方法と考えられています。95〜96%のANI閾値は、95〜96%のゲノム類似性を示し、種レベルの同一性として広く受け入れられています。系統発生的種の概念は通常、生態学的特徴や代謝的特徴と組み合わせて微生物種をより正確に定義します。

微生物学における系統発生的種の概念

Phylogenetic Species Concept in Microbiology

系統発生的種概念(PSC)は、進化的関係に基づいて種を区別するための枠組みであり、共通の祖先や診断可能な遺伝的特徴を重視しています。形態学的または生物学的な種の概念とは異なり、PSCは無性生殖の蔓延、形態的分化の最小限、原核生物間の広範な水平遺伝子移動により、従来の生殖基準や表現型基準がしばしば不足する微生物分類学において特に有利です。微生物学におけるPSCの基盤の一つは、遺伝配列の比較解析を行い、系統関係を推定することです。16SリボソームRNA(rRNA)遺伝子は、細菌や古細菌にまたがる存在と、可変な断片に散在する保存領域から、歴史的に一次分子マーカーとして機能してきました。16S rRNA遺伝子配列の約98.7%の類似度は、2つの生物が同一種に属するかどうかを示す閾値として一般的に用いられます。しかし、この閾値は決定的なものではなく、ゲノムの多様性や進化的分岐を完全に捉えきれない可能性があります。解像度と精度を高めるために、平均ヌクレオチド同定(ANI)を用いた全ゲノム比較が標準となっています。ANIは、2つのゲノム間で共有されるすべての直系遺伝子対の平均同一性を測定します。95〜96%のANIの閾値は、種レベルの近縁性を示す指標として広く受け入れられており、単一遺伝子比較よりも細かいスケールの指標を提供します。ANI解析は、細菌集団における隠蔽種の同定や複雑な分類学的関係の解明に特に効果的です。遺伝的指標は基本的な分類を提供しますが、生態学的役割や代謝能力を取り入れることは、包括的な種の定義に不可欠です。生息地特異性、代謝経路、環境相互作用などの形質はゲノムデータを補完し、機能的・生態学的に異なる微生物種の同定を支援します。したがって、表現型や生態学的情報と統合することで、PSCは微生物多様性の堅牢かつ多次元的な理解を可能にします。

The phylogenetic species concept (PSC) is a framework used to delineate species based on evolutionary relationships, emphasizing shared ancestry and ...

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phylogenetic-species-concept-in-microbiology

Education

JoVE Core

Microbiology

微生物学

Unit

ユニット

Microbial Evolution

微生物の進化

KEY TERMS AND CONCEPTS

主要用語と概念

how dna defines microbial species

The phylogenetic species concept helps define microbial species by looking at DNA and evolutionary relatedness. It focuses on shared ancestry and genetic traits that can be diagnosed in different organisms. This approach is especially useful in microbiology, where other species concepts often do not work well.

Traditional morphological and biological species concepts rely on body form or reproduction. Those methods can fall short for microbes because many reproduce asexually, show little visible difference, and exchange genes through horizontal gene transfer. As a result, scientists use sequence data to compare microbes and trace their phylogenetic relationships.

The 16S ribosomal RNA, or 16S rRNA, gene has long been a key marker in this work. It is found in bacteria and archaea and contains conserved regions mixed with variable segments. A 16S rRNA similarity of about 98.7% is often used to place two organisms in the same species, although this cutoff is not perfect because it may miss deeper genomic diversity or evolutionary divergence.

To improve species resolution, scientists also use whole-genome comparisons such as Average Nucleotide Identity, or ANI. ANI measures the average identity of orthologous gene pairs shared by two genomes. A value of 95 to 96% ANI is widely accepted as a sign of species-level relatedness, and it can help reveal cryptic species and resolve difficult bacterial relationships.

Genetic data are not the only part of species definition. Ecological roles and metabolic capabilities also matter in a fuller view of microbial diversity. Habitat specificity, metabolic pathways, and environmental interactions can help distinguish microbes that are genetically close but functionally different. When genomic, phenotypic, and ecological information are combined, the phylogenetic species concept gives a stronger picture of microbial species.

How DNA Defines Microbial Species

origin of cellular life

How Early Cells Got Their Start

Early cellular life began under the harsh conditions of early Earth, where simple chemicals could form more complex organic molecules. Experimental evidence supports the idea that RNA nucleotides, amino acids, and lipids could arise spontaneously in this environment. Volcanic activity, strong UV radiation, and an atmosphere with little or no free oxygen likely helped drive these reactions. Hydrothermal vents on the ocean floor are considered one likely setting for the first life forms. These...

origin of photosynthesis

How Photosynthesis Changed Earth

Photosynthesis changed Earth’s atmosphere and made complex life possible. It began about 3.4 to 3.8 billion years ago, when the first photosynthetic organisms used light energy to build organic compounds. These early organisms were anoxygenic phototrophs, which means they made food from light but did not release oxygen. Early photosynthesis used hydrogen sulfide (H2S) or ferrous iron (Fe2+) as electron donors instead of water. Because of that, it did not produce molecular oxygen (O2) as a...

evolutionary processes in microbes

How Microbes Gain New Traits

Microbes gain new traits through fast genetic change. Their short generation times help evolution happen quickly. Mutation, horizontal gene transfer, recombination, and genetic drift all contribute to this process. Together, these mechanisms help microbial populations adjust to changing environments. Horizontal gene transfer, or HGT, moves genes between different species. It happens in three main ways: conjugation, transformation, and transduction. Conjugation requires direct cell-to-cell...

evolution of new traits in microbes

Microbial Adaptation in Changing Environments

Microbial adaptation in changing environments can happen very quickly. Microorganisms often evolve fast because they have large population sizes and short generation times. In the lab, these changes can sometimes be measured within days. Natural selection acts on standing genetic variation, which is the genetic diversity already present in a population. When conditions change, helpful traits can be kept and increased because they improve fitness. This lets microbes respond to new pressures...

evolution of microbial genome

Microbial Core and Pan-Genome Changes

Microbial genomes change through gene gain and gene loss. These changes help microbes adapt to new environments and to other organisms they live with. In many species, genome evolution is fast and ongoing. A key idea is the difference between the core genome and the pan-genome. The core genome is the set of genes shared by all sampled strains of a species. These genes support basic cell functions. For example, Salmonella enterica has about 2,800 core genes, although the exact number can change...

microbial phylogeny

Reading Microbial Family Trees

Reading microbial family trees helps scientists study how microorganisms are related. These trees are important in microbial ecology and taxonomy. They use molecular sequences such as DNA, RNA, or proteins to compare bacteria and archaea and trace genetic changes over time. A phylogenetic tree has tips, branches, and nodes. The tips are the species that are sampled or still alive. The branches show evolutionary paths, and branch length reflects the number of genetic changes between nodes.

phylogenetic species concept in microbiology

The phylogenetic species concept helps define microbial species by looking at DNA and evolutionary relatedness. It focuses on shared ancestry and genetic traits that can be diagnosed in different organisms. This approach is especially useful in microbiology, where other species concepts often do not work well. Traditional morphological and biological species concepts rely on body form or reproduction. Those methods can fall short for microbes because many reproduce asexually, show little...

key terms and concepts

Learn how microbial species are classified with DNA evidence, from 16S rRNA and ANI thresholds to ecology, metabolism, and shared ancestry.

How DNA Defines Microbial Species | JoVE K12

JoVE Core Microbiology provides an engaging introduction to the microbial world and its influence on health, industry, and the environment. This comprehensive collection covers key concepts such as microbial structure, metabolism, genetics, and ecology. It introduces essential laboratory techniques and microscopy methods through visually engaging animations. Corresponding scientist-in-action videos bridge classroom concepts with real-world research and applications.

microbiology

How DNA Defines Microbial Species

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