Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)




Professor Jean E. M. Westermann


The nature of the rodlet cell of teleost fish has been the subject of speculation since its first published description in 1892. Research since then has described the rodlet cell morphologically and/or histochemcally, both at the light and electron microscopic levels. However, whether the rodlet cell is a "parasite" or a normal cell has been a subject of debate, and there has been no consensus on its affinity or function.

While the detailed morphology of the rodlet cell is known, no previous study has considered the ecological parameters of the fish with respect to the presence of rodlet cells. In this investigation, the available literature on rodlet cells was examined, and the 114 teleost species reported to contain rodlet cells classified for geographic range, habitat and food choices. There appeared to be no geographical or habitat pecularities in this group, but a comparison of the food choices of these 114 species with a known, diverse, geographically circumscribed population of 326 species (Wheeler, 1969) indicated that the former had proportionally 60 times more species which were listed as omnivorous, 6 times as many which fed on bottom detritus and 3 times as many which were herbivorous than the latter. If the rodlet cell represented a parasitic organism, such information could have implications for its mode of dissemination.

This investigation has been the first to employ modern molecular biological techniques to explore the nature of the rodlet cell. The DNA of the rodlet cell nucleus has been compared qualitatively, using hybridization in situ, and quantitatively, using two methods of microdensitometry; with cells known to contain the fish genome but no rodlets. Because the DNA of rodlet cell nuclei hybridizes to the same extent to genomic DNA of the fish as does the DNA of the fish's own cells, and because there appears to be no statistically significant difference in the quantity of DNA in rodlet cell nuclei compared to fish cells of the same species, the rodlet cell itself has been concluded to be of teleost origin.

However, earlier observations in this laboratory had shown a light microscopic "RNA-type" histochemical reaction in the rodlet cores, after both acridine orange and methyl green-pyronin staining methods; furthermore, the cores were Feulgen-negative. The procedures differentiate DNA from RNA in most situations. The nuclease-gold technique of Bendayan (1981) was used in electron microscopic histochemistry to confirm the presence of nucleic acid(s). The rodlets were found to contain DNA, but no RNA. Further studies using S1 nuclease, which is specific for single-stranded DNA, indicated that the DNA of the rodlet labels also with S1-gold. The DNA of the rodlet, in all cases, is confined to the periphery of the rodlet core, and the DNA appears to be spirally distributed and of differing concentration along the length of the core. Because the rodlet core failed to react to the Feulgen procedure and failed to hybridize to genomic DNA, it must contain non-genomic DNA which is in a different conformation from the DNA found in the nucleus of the rodlet cell. Arguments are presented which consider the possible origins of rodlet DNA, and conclude that the rodlet DNA is exogenous, thus supporting the hypothesis that the rodiet, at least, is a "parasite".

This study represents the first time that the rodlet cell and the rodlet itself have been regarded as separate entities. It is postulate that the rodlet-containing cell itself is a teleost cell and the rodlet an exogenous DNA-containing structure. The phylogenetic affinity of the rodlet is unknown, as is the precise nature of the rodlet DNA; speculation includes both single-stranded and Z-DNA. Directions for future research are considered.

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