Loss of developmental competence in axenic culture could involve selection - not direct selection if fruiting isn't occurring - but selection on traits that are pleiotropically related to fruiting.  However, we also have results (soon to be submitted) on phenotypic traits from the same mutation-accumulation study where selection is minimized, and we see the same kinds of increases in variance and declines in fitness that you see in such studies.  So it appears that, though the point mutation rate is very low, mutational degradation of phenotypic traits does occur.  We're not sure what the explanation is.  It could be mutations of other types such as indels but we haven't thought much about epigenetic changes.

Dave Q.



On Sep 23, 2012, at 5:43 AM, <[log in to unmask]> <[log in to unmask]> wrote:

> Interesting abstract by Gerda from David's group!
> How do you fit into these findings the relavively rapid loss of developmental competence in cultures that grow for some time in ax. medium?
> We assume (but have no experimental proof yet)that epigenetic mechanisms may play a substantial role in variability. Does anyone have data along these lines? It may be worth to take this into consideration.
> wolfgang
> 
> 
>> Whole Genome Sequencing of Mutation Accumulation Lines Reveals a Low Mutation Rate in the Social Amoeba Dictyostelium discoideum
>> Gerda Saxer, Paul Havlak, Sara A. Fox, Michael A. Quance, Sharu Gupta, Yuriy Fofanov, Joan E. Strassmann, David C. Queller
>> PLoS ONE, in press
>> Spontaneous mutations play a central role in evolution.  Despite their importance, mutation rates are some of the most elusive parameters to measure in evolutionary biology.  The combination of mutation accumulation (MA) experiments and whole-genome sequencing now makes it possible to estimate mutation rates by directly observing new mutations at the molecular level across the whole genome.  We performed an MA experiment with the social amoeba Dictyostelium discoideum and sequenced the genomes of three randomly chosen lines using high-throughput sequencing to estimate the spontaneous mutation rate in this model organism.  The mitochondrial mutation rate of 6.76x10^-9, with a Poisson confidence interval of 4.1x10^-9 - 9.5x10^-9, per nucleotide per generation is slightly lower than estimates for other taxa. The mutation rate estimate for the nuclear DNA of 2.9x10^-11, with a Poisson confidence interval ranging from 7.4x10^-13 to 1.6x0^-10, is the lowest reported for any eukaryote.  These results are consistent with low microsatellite mutation rates previously observed in D. discoideum and low levels of genetic variation observed in wild D. discoideum populations.  In addition, D. discoideum has been shown to be quite resistant to DNA damage, which suggests an efficient DNA-repair mechanism that could be an adaptation to life in soil and frequent exposure to intracellular and extracellular mutagenic compounds.  The social aspect of the life cycle of D. discoideum and a large portion of the genome under relaxed selection during vegetative growth could also select for a low mutation rate.  This hypothesis is supported by a significantly lower mutation rate per cell division in multicellular eukaryotes compared with unicellular eukaryotes.
>> Submitted by Gerda Saxer [[log in to unmask]]
>> ==============================================================
>> [End dictyNews, volume 38, number 24]
> 
> Wolfgang Nellen
> Abt. Genetik
> Univ. Kassel
> Heinrich-Plett-Str. 40
> 34132 Kassel
> Germany
>