SEARCH

SEARCH BY CITATION

Abstract

  1. Top of page
  2. Abstract
  3. References

New technologies promise to revolutionize the field of molecular ecology. This technological progress comes with its own set of challenges. Among the most important ones is the analysis and interpretation of the data in a way that tells us about the molecular causes of the phenotype of interest and its consequences. In this issue, Whitehead et al. (2010) reveal part of the mechanistic basis of evolved pollution tolerance by studying the developmental and transcriptional response of tolerant and sensitive fish embryos to polychlorinated biphenyls (PCBs), a pollutant commonly found in coastal waters of the United States. By integrating their gene expression profiling data with phenotypic data on individuals along with what is known about pathways by which this pollutant acts in zebrafish and mammals, they are able to suggest detailed mechanisms that have evolved to allow a fish population to adapt to a very damaging pollutant and develop normally.

Human impacts on biodiversity have been hastened over the centuries. The pace at which our environment changes makes us wonder whether natural populations harbour enough genetic variation and of the right kind to adapt to the new conditions imposed by our activities (Collins & Bell 2004). Industrial pollution has been changing many habitats in unforeseen ways. From the early fifties to the late seventies, the General Electric Company alone discharged more than one million pounds of polychlorinated biphenyls (PCBs) in the Hudson River (http://www.epa.gov/hudson/background.htm), a compound that would later become infamous for its carcinogenic properties and effects on the vertebrate immune, reproductive, nervous, endocrine and cardiovascular systems (Carpenter 2006). Many of the Great Lakes and coastal waters of the United States have been contaminated with this compound. Surprisingly, the 50 years or so that PCBs have been abundant in these regions has been enough for tolerance to evolve in many of the most polluted sites (Nacci et al. 2010). One of the species that has independently and repeatedly developed tolerance to PCBs and other pollutants is the killifish Fundulus heteroclitus (Fig. 1). This evolutionary tour de force made the killifish a model in ecotoxicology (Burnett et al. 2007).

image

Figure 1.  Atlantic killifish Fundulus heteroclitus. Photo by Andrew Whitehead.

Download figure to PowerPoint

To determine how fish could become adapted to such a harmful chemical, Whitehead et al. (2010) compared the transcription profiles of fish embryos from a tolerant population to the profiles of fish from a sensitive population, both found on the east coast of the USA. They exposed groups of fish from each population to different doses of PCBs, after growing them for two generations in the laboratory to avoid intergenerational effects that could result from the transfer of contaminants from mother to embryos or other environmental differences that could affect gene expression. They contrasted the developmental abnormalities that accumulate as a function of PCB dose in the sensitive population to those observed in the tolerant population. Using microarrays designed to study gene expression differences in Fundulus, they then compared the transcription profiles of each treatment group for about 6000 transcripts. From a broad analysis of the data, they show that the sensitive population showed a large transcriptional response to all PCB doses and that the number of genes involved and the magnitude of the response were dose dependent. In contrast, the tolerant population had a 37-fold lower response in terms of the number of genes differentially regulated when exposed to PCBs.

A major challenge in the analysis of how gene expression profiles associate with a trait of interest is to untangle the transcription differences that are causal of the phenotype and those that are downstream consequences. This task necessitates going beyond the mere description of gene lists and functions. Because genes and their products act in integrated molecular networks, one way to achieve this goal is to map the expression phenotypes on known pathways and networks and then ask how these can result in the phenotype we are interested in (Brown et al. 2008). The pathway onto which PCB acts in vertebrates has been identified and involves the aryl hydrocarbon receptor (AHR) (Rowlands & Gustafsson 1997; Carpenter 2006). With this architecture in mind, Whitehead and colleagues show that sensitive fish activate this pathway, whereas tolerant fish do not show this response. Expression changes in this pathway therefore allow narrowing down on the causal mechanisms.

Another key aspect of Whitehead and colleagues’ experiment is the care that was taken to obtain separate developmental and gene expression information for each fish. Each individual was assessed on a microarray rather than using a pool of embryos exposed to a given dose, which enables the correlation of individual developmental phenotypes in addition to simply linking it to the population and dose used. This approach helps support the hypothesis that a certain molecular pathway is involved, as any among-individual variation in the developmental response for a given PCB dose can be also analysed at the molecular level. Whitehead and colleagues found that an individual’s abnormalities were better predictors of gene expression profiles than were the PCB doses. Importantly, this was even more apparent in the tolerant population. Therefore, individuals from a tolerant background that developed abnormally at very high doses showed gene expression profiles that were closer to that of a sensitive individual at a lower dose. This individual-based approach makes the design and achievement of a microarray experiment much more complicated, lengthy and costly, but the benefits are outstanding, as illustrated by this study. This approach is particularly relevant in the study of the molecular basis of adaptations in wild populations where individual variation is abundant.

One of the limitations of the study by Whitehead et al. is that only two populations were compared, making it difficult to determine whether the transcriptional responses and the model they propose can be generalized to all tolerant populations of killifish. One can only hope that the study is extended to other populations, as the present study has firmly established a system with which long-lasting evolutionary questions can be addressed at the level of molecular networks. For example, are all tolerant populations achieving the same normal phenotypes by inactivating the same pathway? If yes, is it caused by a single mutation that spread to all populations or mutations that appeared independently? Answering these questions may reveal whether the architecture of molecular networks serve not only as a canvas for the interpretation of gene expression data but also as a blueprint for evolution, by limiting the space of possible paths by which populations can evolve in response to drastic selection pressure.

References

  1. Top of page
  2. Abstract
  3. References