Why establish taxa using molecular and ultrastructural characters?
- One goal of taxonomy is to establish classification of organisms based upon their natural relationships. Classically, similarity in morphology and development was considered reflective of the relationship among organisms. The idea was that the more similar organisms appeared and developed, the more closely related they were. This idea is easy to understand. For example, you are more closely related to your parents and brothers and sisters than you are to others. Consequently, you typically look more similar to them than to other people.
- However, with chytrids, morphology is convergent; that is organisms that are not related may have similar morphologies. This is because of how they have evolved.With limited morphological features on which selection can act, organisms in similar habitats exposed to similar selective pressures may evolve similar adaptive structures.
- Thus, morphology can be deceiving of phylogenetic relationships. As a result, it is necessary to use characters other than morphological characters as primary characters to reveal relationships.
- Phylogenetic position based on analyses of gene sequences is congruent with ultrastructural character suites of zoospores. Concordance between molecular-based phylogeny and comparative ultrastructural analyses confirm their reliability in revealing relationships among chytrids.
What is our current understanding of the relationships among the Rhizophydiales?
- Our current phylogenetic hypothesis of relationships within the Rhizophydiales is based on comparative zoospore ultrastructure and gene sequence analyses. This tree reflects our contemporary knowledge of relationships within the Rhizophydiales.
- The Rhizophydiales are terminal representatives in the evolutionary radiation of chytrids. Molecular analyses include taxa in Rhizophydiales with character states such as operculation, multiple rhizoids, and endobiotic thallus development, features not clasically associated with Rhizophydium and which indicate morphological character convergence among different lineages in Chytridiomycetes.
How does this view of relationships impact taxonomy in the Rhizophydiales?
- We now define families based on molecular monophyly and zoospore ultrastructure. We use fourteen ultrastructural characters with multiple character states for each character.
- To describe species within this framework molecular characters and ultrastructural chacters are used first to define relationships. Then thallus morphological chacters is used to distinguish species from among monophlytic groups of isolates.
- A member of the Rhizophydiales, Kappamyces, was the first chytrid genus to be delineated based on combined molecular phylogeny and distinct zoospore architecture.
- We can key organisms to families and genus using ultrastructural characters. Thus, if one has a rhizophydialean chytrid in culture and observes its zoospore with electron microscopy, than it can be placed into a family following this key.
Are all members of the Rhizophydiales established?
- Recent molecular phylogenetic analyses have revealed remarkable diversity among the Rhizophydiales. The effort to fully characterize the Rhizophydiales is an on-going project because of the great genetic diversity in this order.
- Before extensive species level studies can lead to description of species, it has been necessary to establish the "backbone" phylogenetic relationships within the order to assure that all members considered members are indeed correctly placed in the order.
- Consequently, there is still much more work needed to reveal the abundant diversity that exists in this order.