Question

Given below are two statements:
Statement (I): SSU rDNA is a good phylogenetic marker for elucidating species-level phylogenetic relationships within a genera.
Statement (II): SSU rDNA phylogenetic marker is the most reliable and appropriate phylogenetic marker for nematode molecular taxonomy.
In light of the above statements, choose the most appropriate answer from the options given below:

• A. Both Statement (I) and Statement (II) are true
• B. Both Statement (I) and Statement (II) are false
• C. Statement (I) is true but Statement (II) is false
• D. Statement (I) is false but Statement (II) is true

Hint:

Remember the general rule for molecular taxonomy:
- SSU (18S) is highly conserved and best for deep, high-level phylogenetics (orders, classes).
- LSU (28S D2-D3) and ITS regions are more variable and better suited for species-level diagnostics and taxonomy within a genus.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Molecular taxonomy and phylogenetics of nematodes rely on sequencing specific regions of ribosomal DNA (rDNA) and mitochondrial DNA (mtDNA).
Different genetic regions evolve at different rates, making them suitable for resolving phylogenetic relationships at different taxonomic levels (such as phylum, class, order, family, genus, or species).

Step 2: Detailed Explanation:
Let us analyze the two statements:
Statement (I) asserts that the Small Subunit ribosomal DNA (SSU or 18S rDNA) is a good marker for species-level phylogenetic relationships within a genus.
This is incorrect.
The SSU rDNA is highly conserved across the Phylum Nematoda due to strong functional constraints on the ribosome.
Because it accumulates mutations very slowly, there is often little to no sequence variation in the SSU region among closely related species within the same genus.
Therefore, SSU rDNA lacks the resolving power needed for species-level identification and phylogenetics.
Instead, faster-evolving regions, such as the Internal Transcribed Spacers (ITS-1 and ITS-2), the D2-D3 expansion segments of the Large Subunit (LSU or 28S rDNA), or mitochondrial genes like Cytochrome Oxidase I (COI), are used for species-level molecular taxonomy.
Thus, Statement (I) is false.
Statement (II) states that the SSU rDNA is the most reliable and appropriate phylogenetic marker for nematode molecular taxonomy.
This is also incorrect.
While SSU rDNA is highly valuable for high-level systematics (resolving relationships at the level of families, orders, and classes, such as the five major clades proposed by Blaxter et al.), it cannot be considered the "most reliable and appropriate" marker for nematode molecular taxonomy as a whole.
Because molecular taxonomy focuses heavily on diagnosing and differentiating species and genera (the levels of greatest practical importance in agricultural nematology), a single conserved marker like SSU is insufficient.
A robust molecular taxonomy system must use a combination of SSU, LSU, ITS, and mitochondrial markers to address different taxonomic levels.
Since both statements are incorrect, the correct option is (B).

Step 3: Final Answer:
Both Statement (I) and Statement (II) are false because SSU rDNA is too conserved to resolve species-level relationships, and no single marker is sufficient for all levels of nematode molecular taxonomy.