Fish microsporidia: fine structural diversity and phylogeny.

Area of Science:

• * Parasitology
• * Microbiology
• * Fish Health

Background:

• * Microsporidia are obligate intracellular parasites with diverse life cycles and host-parasite interactions.
• * Fish microsporidia can induce host cell degradation or form xenomas, complex structures with varying cell walls.
• * Understanding these structures is crucial for parasite identification and host-parasite relationship assessment.

Purpose of the Study:

• * To review the structural diversity of fish microsporidian life cycle stages and host-parasite interfaces.
• * To discuss the implications of these structures for parasite classification and phylogeny.
• * To propose a tentative classification of fish-infecting microsporidia based on available data.

Main Methods:

• * Literature review of existing studies on fish microsporidian morphology and life cycles.
• * Analysis of host-parasite interface structures, including xenoma formation and spore envelopes.
• * Review of molecular phylogenetic data, particularly ribosomal DNA (rDNA) sequences.
• * Evaluation of structural characteristics for taxonomic purposes.

Main Results:

• * Microsporidian development in fish hosts involves diverse structural adaptations, including xenoma formation and varied envelope types during merogony and sporogony.
• * Nucleospora exhibits a unique sporogony process, distinguishing it from other fish microsporidia.
• * Molecular phylogenetic analysis of rDNA suggests most fish microsporidia form a cohesive clade, with Nucleospora outside this group.
• * A tentative classification into five groups is proposed, integrating morphological and molecular data, with some groups representing families and others clades.

Conclusions:

• * Structural diversity in fish microsporidia is significant but shares common features with microsporidia from other hosts.
• * While structures are relevant for identification and host relations, rDNA molecular phylogeny is more informative for higher-level classification.
• * The proposed five-group classification provides a framework for understanding fish microsporidian diversity and evolution.