Genomic approaches in the identification of hypoxia biomarkers in model fish species

Eutrophication leading to hypoxic water conditions has become a major problem in aquatic systems worldwide. Monitoring the levels and biological effects of lowered oxygen levels in aquatic systems may provide data useful in management of natural aquatic environments. Fishes represent an economically important resource that is subject to hypoxia exposure effects. Due to the extreme diversity of fish species and their habitats, fishes in general have evolved unique capabilities to modulate gene expression patterns in response to hypoxic stress. Recent studies have attempted to document quantitative changes in gene expression patterns induced in various fish species in response to reduced dissolved oxygen levels. From a management perspective, the goal of these studies is to provide a more complete characterization of hypoxia responsive genes in fish, as molecular indicators (biomarkers) of ecosystem hypoxic stress.The molecular genetic response to hypoxia is highly complex and overlaps with other stress responses making it difficult to identify hypoxia specific responses using traditional single gene or low throughput approaches. Therefore, recent approaches have been aimed at developing functional genomic (e.g. high density microarray and real-time PCR) and proteomic (two-dimensional fluorescence difference in gel electrophoresis coupled with mass spectrometry based peptide identification) technologies that employ fish species. Many of the fish species utilized in these studies do not have the advantages of underlying genome resources (i.e., genome or transcriptome sequences). Efforts have attempted to establish correlations between discreet molecular responses elicited by fish in response to hypoxia and changes in the genetic profiles of stressed organs or tissues. Notable progress in these areas has been made using several different versions of either cDNA or oligonucleotide based microarrays to profile changes in gene expression patterns in response to hypoxic stress.Due to these efforts, hundreds of hypoxia responsive genes have been identified both from laboratory reared aquaria fish and from feral fish derived from both fresh and saltwater habitats. Herein, we review these reports and the emergence of hypoxia biomarker development in aquatic species. We also include some of our own recent results using the medaka (Oryzias latipes) as a model to define genetic profiles of hypoxia exposure.