Model Organisms

Over four decades of space biology research has seen an impressive diversity of organisms studied in space, generating a wealth of valuable data. But, why so many?

Determining the most appropriate subject for studying a particular biological question in space is not always straightforward. Scientific objectives must be reconciled with the operational and logistical constraints such as size, mission duration, and maintenance requirements. Different classes of organism have been used to study different biological areas. Bacteria and cell cultures are useful for studying genetic changes in response to microgravity radiation, as they are relatively easy to maintain and have short life cycles. Aquatic species such as sea urchins serve as models of fertilization and embryonic development. Insects have been used to study circadian rhythms while mammals such as rats are used frequently to address human adaptations to spaceflight and its health implications. Many species of plant have also been studied to investigate altered growth patterns of roots and shoots in response to gravity. Other, more exotic species were chosen because they have physiological systems already well studied by biologists. For example, jellyfish are excellent subjects for research on gravity-sensing mechanisms because their specialized gravity-sensing organs, or statoliths, are already well characterized.

Figure 2-06. Various images of the yeast Saccharomyces cerevisiae. It is the common yeast used in baking and brewing. Yeasts are used as model systems for the understanding of both applied and fundamental aspects of cellular biology-.

However, current trends are towards consolidation, and recent advances in cell and molecular biology have led to a more focused research strategy. This has involved the use of a smaller selection of highly characterized organisms, known as model organisms, to study the underlying mechanisms of adaptations to the space environment. There is also a desire to conduct detailed "reference studies" using different multiple model organisms and conditions that can be used to more accurately assess the biological risks of long-term spaceflight, in preparation for human missions beyond Earth orbit (Souza et al. 2000).

Researchers selected a small assortment from tens of millions of possibilities because they have common attributes as well as unique characteristics. They are practical: A model must be cheap and plentiful; be inexpensive to house; be straightforward to propagate; have short gestation periods that produce large numbers of offspring; be easy to manipulate in the lab; and boast a fairly small and (relatively) uncomplicated genome.

Model organisms have emerged out of the genome-sequencing projects of the late 1990s. These relatively simple organisms historically used for research are now understood at the genetic level and are providing new insights into the essential biological properties (genes, proteins, metabolic pathways) that they share with each other and, more importantly, with humans (Bahls et al. 2003). For example, because its genetic blueprint is known and gene changes can be detected, the nematode roundworm Caenorhabditis elegans is a good model for studies of the effects of radiation and weightlessness on genetic material. Complete genetic sequences, or genomes, are also known for the fruit fly Drosophila melanogaster, the yeast cell Saccharomyces cerevisiae, and the bacterium Escherichia coli (see Figure 106).

The genome sequence for the mouse, a valuable mammalian model, also was recently completed and extends the list of model organisms accessible for modern biological research. There are also lab rats of a new field, nicknamed "space genomics", in which scientists study the way weightlessness and space radiation affect an organism's genes. Since diverse organisms share many of the same genes, such studies may give scientists a better understanding of how space travel may affect human genes.

Selection of living organisms as models to work with depends largely upon the experimental purpose. For space biology, the constraints of spaceflight must also be considered. Model organisms believed to have the necessary characteristics are briefly described in the following sections.

0 0

Post a comment