dictyNews

Electronic Edition

Volume 45, number 24

September 20, 2019



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Abstracts

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Many roads lead to Rome: neutral phenotypes in microorganisms



Vidyanand Nanjundiah



Centre for Human Genetics, Bangalore 560004, India





Journal of Experimental Zoology B, accepted



John Bonner pointed out that microorganisms differ in several ways, 

some of which may reflect neutral phenotypic evolution. For making 

his case, Bonner referred to inter-species differences and morphological 

traits. Here we consider intra-species differences and physiological traits.

As a case-study, we examine the production of an extracellular cyclic 

AMP phosphodiesterase in the cellular slime mould Dictyostelium 

discoideum. Temporal profiles of phosphodiesterase activity differ 

significantly between wild-type strains. From that we argue that the 

inference drawn initially from studies on a single wild-type, namely that 

the profile displayed by it pointed to an adaptive role, was mistaken. We 

generalise the conclusion to suggest that physiological differences 

exhibited by microorganisms of the same species may, but need not, 

reflect adaptations to different environments. Rather, the differences are 

related to the fact that microorganisms live in groups whose composition 

can vary between homogeneous (clonal) and heterogeneous (polyclonal). 

More than one physiological profile is consistent with the normal 

development of the group in a given environment; the alternatives are

neutral.  When studying microbial physiology and behaviour, it is 

expected that the observations are made on a clonal population; genetic 

(and so phenotypic) heterogeneity is carefully guarded against. As the

example from D. discoideum shows, an unintended consequence of 

overlooking phenotypic heterogeneity is that one can fall into the trap of 

accepting a seemingly plausible, but possibly erroneous, adaptive 

explanation for a “normal” wild-type phenotype.  





submitted by:  Vidya Nanjundiah [[log in to unmask]]

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Structure of Dictyostelium discoideum telomeres. Analysis of possible 

replication mechanisms.



J. Rodriguez-Centeno, C. Manguán-García, R. Perona and L. Sastre





PlosOne, accepted



Telomeres are nucleo-protein structures that protect the ends of eukaryotic 

chromosomes. They are not completely synthesized during DNA replication 

and are elongated by specific mechanisms. The structure of the telomeres 

and the elongation mechanism have not been determined in Dictyostelium 

discoideum. This organism presents extrachromosomal palindromic 

elements containing two copies of the rDNA, also present at the end of the 

chromosomes. In this article the structure of the terminal region of the rDNA 

is shown to consist of repetitions of the A(G)n sequence where the number 

of Gs is variable. These repeats extend as a 3’ single stranded region. The

G-rich region is preceded by four tandem repetitions of two different DNA 

motifs. D. discoideum telomere reverse transcriptase homologous protein 

(TERTHP) presented RNase-sensitive enzymatic activity and was required 

to maintain telomere structure since terthp-mutant strains presented 

reorganizations of the DNA terminal regions. These modifications were 

different in several terthp-mutants and changed with their prolonged culture 

and subcloning. However, the terthp gene is not essential for D. discoideum 

proliferation. Telomeres could be maintained in terthp-mutant strains by 

homologous recombination mechanisms such as ALT (Alternative 

Lengthening of Telomeres) or HAATI (heterochromatin amplification-

mediated and telomerase-independent). In agreement with this hypothesis, 

the expression of mRNAs coding for several proteins involved in homologous 

recombination was induced in terthp-mutant strains. Extrachromosomal rDNA 

could serve as substrate in these DNA homologous recombination reactions.





submitted by: Leandro Sastre [[log in to unmask]]

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[End dictyNews, volume 45, number 24]