This study investigates how migrating slugs of the cellular slime mold Dictyostelium discoideum orient toward environmental stimuli such as light and chemical gradients. The research tests the hypothesis that internal ammonia (NH₃) concentration governs orientation by affecting cell movement rates within the slug. Bonner manipulated internal NH₃ production through various treatments applied to slug tips: activated charcoal removed NH₃ and eliminated orientation, while papain protease increased proteolysis and NH₃ production causing slugs to turn away from the treated side. Conversely, protease inhibitors (antipain and phosphoramidon) reduced proteolysis and slowed cell movement, causing slugs to bend toward the treated side. Wounding and acidic conditions also triggered increased proteolytic activity and NH₃ production, resulting in slug orientation away from these sites. Ethanol, a known proteolysis inhibitor, similarly reduced cell speed. All treatments produced results consistent with the NH₃ orientation hypothesis, supporting the model that local variations in proteolysis-derived NH₃ create differential cell movement rates that enable slug orientation.

Key findings

• Internal ammonia concentration, produced through local proteolysis, governs slug orientation in Dictyostelium discoideum
• Increased proteolysis (via papain or wounding) raises NH₃ levels and causes slugs to move away from the treated side
• Protease inhibitors and ethanol reduce proteolysis, lower NH₃ production, and slow cell movement on the treated side
• Activated charcoal saturated with NH₃ loses its ability to attract slugs, demonstrating NH₃'s repellent function
• Acidic pH promotes endogenous protease activity and increases cell movement speed, supporting the proteolysis-NH₃ mechanism