Phylogenetic relationships within the invertebrata in relation to the structure of septate junctions and the development of ‘occluding’ junctional types

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Abstract
The structures of 13 variants of invertebrate septate junction are reviewed on the basis of freeze-fracture, lanthanum tracer and thin-section studies. In addition, a simple type of occluding junction in the phylum Porifera, a variation of tight junction in the phylum Tunicata and the vertebrate tight junction are covered. All the junctions considered form a belt around the apical circumference of cells lining a lumen or an exterior surface. The large number of these junctions now recognized permits discussion relating to invertebrate classification and suggested phylogenetic relationships, and to the development of intercellular junctions. The relationships revealed are discussed under three headings: Coelenterates and lower invertebrates, Proterostomia (the annelid, molluscan and arthropod lineage) and the Deuterostomia (the echinoderm and chordate lineage). It is proposed that the pleated septate junction of the lower invertebrates resembles that of the hydrozoan rather than anthozoan Coelenterates. This lower invertebrate pleated septate junction occurs in several lower invertebrate phyla including the Annelida (of the proterostome lineage), but also occurs in the Sipunculoidea, a group supposedly on the deuterostome lineage. The proterostome line includes the molluscs and the arthropods, which have the molluscarthropod pleated septate junction. Several variations of the smooth septate junction are also seen in Arthropoda. Among the deuterostomes the Chaetognatha have both a paired septate junction and a pleated junction and are therefore considered to be not very far removed from the Sipunculoidea. The echinoderms and hemichordates also have double-septum septate junctions. In addition however, these two phyla have anastomosing septate junctions that are very similar, varying only in their final configuration. Of the two, the echinoderm anastomosing septate junction most closely resembles the tight junction seen in the tunicates, and the Hemichordata are therefore considered to be a lateral development from the main line of chordate evolution. The tunicates have a tight junction similar to that seen in vertebrates; it is however more ‘leaky’ and has distinctive freeze-fracture characteristics. In the phylum Porifera a form of simple parallel membrane junction appears to serve an occluding function. This junction has regular intercellular spacing in the absence of any septa and it is suggested that the spacing in septate junctions is probably not dictated by the septa. This interpretation is reasonable particularly when the diversity of septal types in conjunction with stable intercellular spacing is considered. Finally, a theory is put forward suggesting that in evolution a change from the septate to the tight junction could simply involve a modification of a ‘membrane spacing factor’, which allows the membranes of adjacent cells to come together at intervals, in the normal tight junction pattern.