Question

The most recent hybrid classification of Phylum Nematoda by De Ley and Blaxter in 2002 proposes

• A. Two classes
• B. Three classes
• C. Four classes
• D. Five classes

Hint:

De Ley and Blaxter's classification simplified the high-level taxonomy of nematodes by grouping them into just two main evolutionary lineages (classes): Enoplea and Chromadorea.
This replaced the older Secernentea/Adenophorea system.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
Nematode systematics underwent a major revision with the introduction of molecular phylogenetic data.
De Ley and Blaxter (2002) proposed a hybrid classification system that integrated classic morphological characteristics with molecular data, primarily from the small subunit (18S) ribosomal DNA.

Step 2: Detailed Explanation:
Historically, the Phylum Nematoda was divided into two classes based on the presence or absence of phasmid sensory organs: Secernentea (with phasmids) and Adenophorea (without phasmids, further subdivided into Enoplia and Chromadoria).
However, molecular analysis of 18S rDNA demonstrated that Adenophorea was a paraphyletic group.
To reflect these evolutionary relationships, De Ley and Blaxter (2002) proposed a revised classification system that divided the Phylum Nematoda into two classes:
1. Class Enoplea: This class corresponds to the traditional subclass Enoplia.
It is characterized by a relatively simple esophagus, the presence of metanemes, and the lack of a highly modified terminal bulb.
It includes many free-living marine nematodes, animal parasites (like Trichinellida), and plant parasites (such as Dorylaimida).
2. Class Chromadorea: This class combines the traditional subclass Chromadoria and the class Secernentea.
It is characterized by a more complex esophagus, often with a distinct terminal bulb, and highly specialized cuticle modifications.
It includes a wide range of free-living, plant-parasitic (Tylenchida), and animal-parasitic nematodes.
This hybrid system reduced the class-level divisions of the phylum to these two monophyletic classes.
Therefore, the correct option is (A).

Step 3: Final Answer:
The hybrid classification system proposed by De Ley and Blaxter (2002) divides the Phylum Nematoda into two classes: Enoplea and Chromadorea.