Question

Legumes form intimate associations with bacteria called rhizobia that integrate into the plant root system. The relationship is mutualistic because the rhizobia fix nitrogen for the plant (this is costly for the bacteria), while the plant provides resources necessary for bacterial growth such as oxygen and carbon. Since fixing nitrogen is costly, a rhizobial genotype that receives carbon and oxygen without fixing nitrogen (or fixing less nitrogen) would be expected to have an advantage. Kiers et al. demonstrated that bacteria that fix less nitrogen are also supplied with less oxygen. This is an example of ________ and could contribute to the ________ of this mutualism.

1) punishment; evolutionary stability
2) vertical transmission: evolutionary stability
3) punishment; evolutionary instability
4) vertical transmission; evolutionary instability
5) horizontal transmission; evolutionary stability

Answer 1

The answer is 'punishment; evolutionary stability,' as this describes how legumes reduce oxygen supply to rhizobia as a sanction for fixing less nitrogen, which maintains the mutualistic relationship's stability.

Step-by-step explanation:

The interaction described between rhizobia and legumes is an example of mutualism due to the reciprocal benefits each organism receives: legumes obtain nitrogen and rhizobia receive carbon and oxygen. However, when rhizobia provides less beneficial nitrogen fixing, the legumes respond by supplying less oxygen, a mechanism that can be considered a form of punishment. This response helps maintain the evolutionary stability of the mutualistic relationship by discouraging exploitative behavior by the bacteria.

Rhizobia fixes nitrogen, which is costly in terms of energy and resources, providing a valuable nutrient to legumes. Legumes, in turn, supply rhizobia with carbon and a protected environment to thrive in. Rhizobia that fixes less nitrogen than expected might gain an immediate advantage by preserving resources, but the legumes police this behavior by restricting the supply of oxygen to less cooperative bacterial genotypes. This sanction discourages uncooperative behavior by the bacteria and contributes to the long-term stability of this mutualistic interaction. Such mechanisms are essential to maintaining the evolutionary balance and functionality of mutualistic relationships, ensuring that both parties continue to benefit and cooperate.