The melding of morphology and molecular phylogeny: Insight into ancestral prokaryotic phenotypes

Abstract

Phylogenetic trees constructed from small subunit ribosomal RNA sequences of the Bacteria were examined from the perspective of morphological characters. Several patterns emerge. Spirochaetal morphology is not exhibited in the gram positive lineage and may be indicative of the inability of these bacteria to assume this shape due to the thicker murein layer. Phylogenetic subgroupings which are heterogenous with respect to cell shape typically exhibit a gradation of shapes including rods with the end result frequently being coccal symmetry. Groupings containing the coccal phenotype are in contrast invariably morphologically homogenous. Thus once a bacterial lineage reaches the coccal phenotype it seems to lose it ability for further morphological evolution. Filamentous forms are found throughout the Bacteria but are also morphologically homogenous. Thus filaments appear to represent "end-state" morphology as well and may be an intermediate step to a very primitive multi-cell morphology. An examination of the best established microfossils to date show coccoid and filamentous forms akin to those seen in extant cyanobacteria with essentially no morphological change over approximately 3 billion years. The hypobradytely exhibited by the cyanobacteria lineage may in part be explained due to the evolutionary end-states as demonstrated in this study. There is an overwhelming preponderance of rod shaped bacteria in the deepest branches of the Bacteria such that one concludes that the most likely phenotype of the early ancestor was a rod. While scant, one of the oldest microfossil records revealThe Melding of Morphology arts evidence of ancient bacteria exhibiting a typical rod morphology. Cell morphology likely was of considerable significance in the earliest stages of evolution. The likely inefficient metabolism of the earliest prokaryotes and possibly incomplete pathways for cell wall synthesis may have necessitated larger cell size. Maximal near surface contact with the outside environment would be a special advantage offered by the rod-shaped morphology. On one hand the rod shape may simply be the easiest solution to problems in cell division. The necessity of renewing the cell wall may be simplified by taking advantage of the bilateral symmetry obtained with rod shaped cells. Melding morphology with 16S rRNA molecular phylogeny allows for examination of the contribution of classical morphological characters with respect to bacterial evolution. It is clear from this work that while we can not strictly classify bacteria based on cell shape, their morphology does indeed offer some insight. The extension of this study to include the Archaea may offer evidence of what the last common ancestor must have "looked like", a necessity to understanding the simplest life form.