Classifying eukaryotic viruses – Key criterion: For viruses infecting eukaryotes, which characteristic is most important for formal classification schemes?

Introduction:
Virus taxonomy integrates multiple attributes, but one overarching feature anchors classification for eukaryotic viruses: the chemical nature of the genome and structural components. Recognizing this helps interpret families, Baltimore classes, and diagnostic algorithms.

Given Data / Assumptions:

• Viral genomes vary: DNA vs RNA, single- vs double-stranded, segmented vs non-segmented, positive vs negative sense.
• Structural proteins and envelopes also differ chemically.
• Taxonomy aims for evolutionary and mechanistic coherence, not only clinical features.

Concept / Approach:
The chemical nature of virion constituents—especially genome type and replication strategy—forms the backbone of formal classification (e.g., Baltimore system categories I–VII). Morphology and host range inform lower-level distinctions but are not the primary axis.

Step-by-Step Solution:
Step 1: Prioritize genome chemistry (DNA/RNA, sense, strandedness).Step 2: Incorporate envelope presence, capsid symmetry, and replication mode.Step 3: Recognize host preference and disease severity as variable phenotypes not reliable for taxonomy.Step 4: Choose the option emphasizing chemical nature.

Verification / Alternative check:
ICTV and Baltimore classification frameworks consistently center on genome chemistry and replication strategy for high-level grouping.

Why Other Options Are Wrong:

• Host preference/Disease severity: epidemiologic or clinical traits, not fundamental taxonomy.
• Morphology alone/Physical nature alone: helpful but insufficient for accurate classification.

Common Pitfalls:
Overvaluing disease outcomes or host range; ignoring replication chemistry when thinking about taxonomy.

Final Answer:
Chemical nature of virion constituents (type of nucleic acid and related properties).