Surface morphology of Saccinobaculus (Oxymonadida): implications for character evolution and function in oxymonads

Area of Science:

• Protistology
• Microbiology
• Evolutionary Biology

Background:

• Oxymonads are symbiotic flagellates found in the hindgut of arthropods.
• The genus Saccinobaculus inhabits the wood-feeding cockroach Cryptocercus punctulatus.
• Previous studies relied on light microscopy, limiting detailed surface morphology analysis.

Purpose of the Study:

• To investigate the surface ultrastructure of Saccinobaculus using advanced electron microscopy techniques.
• To identify novel morphological characters for phylogenetic analysis of oxymonads.
• To infer evolutionary relationships and character evolution within the Oxymonadidae family.

Main Methods:

• Scanning electron microscopy (SEM) for external surface topography.
• Transmission electron microscopy (TEM) for internal ultrastructure and fine details.
• Comparative analysis with existing molecular phylogenies of oxymonads.

Main Results:

• Discovered surface concavities, a glycocalyx, and coated pinocytotic vesicles in Saccinobaculus.
• Identified unique, unidentified organelles in one Saccinobaculus species.
• Observed the absence of extracellular structures, holdfasts, and ectobiotic bacteria in Saccinobaculus, unlike related taxa.
• Concavities support a clade including Saccinobaculus, Pyrsonymphidae, and Oxymonadidae.
• Glycocalyx is a synapomorphy for all oxymonads; ectobiotic bacteria are ancestral but lost in Saccinobaculus.
• Holdfast structures evolved multiple times independently.

Conclusions:

• Saccinobaculus surface morphology provides new characters for oxymonad phylogeny.
• Concavities may be involved in solute accumulation and internalization.
• The motility and plasticity of Saccinobaculus likely prevent ectobiotic bacterial colonization.