Gravitational Biology 



lead to modeling of gravity-sensing systems. Models can be used to predict the 

 adaptive behavior of the sense organs under changed gravitational conditions, 

 such as would be experienced on Mars or in interstellar space. Such theoretical 

 work is useful in singling out the most important questions to ask in space and, 

 therefore, the kinds of experiments that are most critical to conduct on the Shuttle 

 or the Space Station. This kind of computer-based research should be extended to 

 include a comparative series from invertebrates to vertebrates. One reason for 

 doing so is to study the question of whether evolutionary advances from aquatic 

 to terrestrial forms, and from prostrate to upright posture with increased mobility, 

 are reflected in the functional organization of gravity sensors. 



The ground-based research under consideration in Space Biology leads naturally 

 to studies in weightlessness. Flight and ground-based experiments have shown 

 that jellyfish rotated on a clinostat (to produce an ambiguous gravity vector) 

 contain reduced numbers of statoliths, suggesting a role for gravity in their normal 

 development. Because animal gravity sensing and organs are functionally 

 organized as weighted neural networks and process information in parallel, they 

 are highly adaptive systems. Some aquatic species possibly adapt quite readily to 

 the space environment because of the buoyancy they experience in their everyday 

 lives on Earth. Terrestrial forms possibly will experience longer periods of 

 adaptation. An unanswered question is whether some species will begin to select 

 for some altered functional organization after multiple generations of exposure to 

 weightlessness, and another is whether progeny of these lines will readily readapt 

 to Earth's gravitational field when returned from their "normal" habitat on the 

 Space Station. 



The Effects of Gravity on Organismal Development 



As with mature organisms, developing individuals are exposed to a range of 

 environmental factors that exert a strong influence on bodily structure and 

 behavior. The major objective of research in this area is to understand the role of 

 gravity in reproduction, growth, development, and aging. 



Developmental Biology of Plants. A few space missions conducted by the 

 Americans and Soviets have carried plant experiments that demonstrated a variety 

 of responses by plants to space flight. The exposure of plants to the space 

 environment seems to alter the character and rate of cell differentiation, 

 accelerating it in some species and apparently slowing it in others. Carrot cells 

 cultured aseptically on defined media develop somatic embryos during space flight 

 as well as on the ground. 



The Soviets have grown Arabidopsis, a small plant, in space from seeds and 

 brought it through a complete life cycle to produce fertile seeds. As they matured, 

 these plants grew slower, were smaller in size at maturity, and produced fewer 

 leaves and seeds than did ground-based controls. Current research projects 

 include investigation into the effects of gravity on plant cells and embryos; the role 

 of calcium in the regulation of plant development; the genetic basis of 

 gravitropism; the effects of gravity on chromosomes, cell and tissue competence, 



105 



