Abstract
Isogenic populations of animals still show a surprisingly large amount of phenotypic variation between individuals. Using a GFP reporter that has been shown to predict longevity and resistance to stress in isogenic populations of the nematode Caenorhabditis elegans, we examined residual variation in expression of this GFP reporter. We found that when we separated the populations into brightest 3% and dimmest 3% we also saw variation in relative expression patterns that distinguished the bright and dim worms. Using a novel image processing method which is capable of directly analyzing worm images, we found that bright worms (after normalization to remove variation between bright and dim worms) had expression patterns that correlated with other bright worms but that dim worms fell into two distinct expression patterns. We have analysed a small set of worms with confocal microscopy to validate these findings, and found that the activity loci in these clusters are caused by extremely bright intestine cells. We also found that the vast majority of the fluorescent signal for all worms came from intestinal cells as well, which may indicate that the activity of intestinal cells is responsible for the observed patterns. Phenotypic variation in C. elegans is still not well understood but our proposed novel method to analyze complex expression patterns offers a way to enable a better understanding.
Highlights
In Rea et al [1], we have found that average activity of hsp-16.2 correlates well with lifespan in adult worms
We found clusters that were consistent with previous results based on average activity measurements from Rea et al [1], but showed a more complex structure, with the bright worms being assigned to one cluster and the dim worms being separated into two clusters with distinct expression patterns
We found that the vast majority of signal from the Phsp-16.2::gfp reporter transgene originated in the intestine cells, suggesting that the observed patterns are caused by intestinal cells
Summary
In Rea et al [1], we have found that average activity of hsp-16.2 correlates well with lifespan in adult worms. Within isogenic populations of adult C. elegans expressing Phsp-16.2::gfp and having developed in the same environment, worms with higher average GFP intensity after heat shock tend to live significantly longer than those with a lower average GFP intensity. There remains a suprisingly large amount of phenotypic variation in the expression of this protein which merits further study. The Copas Biosort worm sorter which we used for these experiments is unable to resolve intensity variation along the lateral axis. Intensity variation along the antero-posterior axis could potentially be measured. Dupuy et al [2] show just such an analysis for C. elegans promotor activity from early larvae to adult. As we wanted to study the intensity variation along both axes, we had to develop a different approach. The main idea was to use worm microscopy images to obtain intensity variation images automatically by developing appropriate image processing methods
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