Does Domestication Affect Mutual Partners?

Figure 1: Diagram of Mycorrhizal Fungi. (accessed 10/04/2014)

Multiple studies of the domestication process have been conducted on grain crops where above-ground traits (increasing yield quantity/size and prolonging fruiting season) are typically favoured over below-ground traits. Less is known about domestication in crops which require a mutualistic relationship with below-ground mycorrhizal fungi. Fruit crops (which constitute to a large part of the human diet) are enhanced by these root symbionts by allowing the plant to access nutrients as well as increasing disease resistance and adding protection from pathogens. Mycorrhizal fungi are beneficial and can potentially influence plant fitness, community structure, biodiversity, ecosystem productivity and variability (Fig 1). 

An important staple food crop across the Tropics and Oceania is breadfruit (Artocarpus altils). Over 2000-3000 years domestication has changed breadfruit significantly from its ancestor, breadnut (A. camansi) by extending fruiting season, creating fleshier fruit with fewer seeds and increasing fruiting loads (Fig 2). To be considered domesticated, it must be wholly dependent upon human intervention for dispersal, growth and reproduction, which can be seen in all cultivated seedless breadfruit species.

Figure 2: Change in Breadfruit morphology from wild breadnut (left) to modern seedless varieties (right). (accessed 10/04/2014)

 A study by Xing et al., (2012) followed the domestication process of breadfruit species (Artocarpus sp.) across a strong geographical gradient from west to east across the Melanesian and Polynesian islands with a focus on its mutualistic partner - arbuscular mycorrhizas (AM fungi). It was found that modern breadfruit species are less able to support AM fungi than wild ancestors. This is supported by a decrease in quantity and colonization rate across the domestication gradient from wild ancestors to modern species.

 

It is possible that a trade-off in resource allocation has occurred. Fruits are considered a strong sink for photosynthate, where sugar and other chemicals created from photosynthesis are primarily allocated for fruit production. As selection for high fruit yield in breadfruit has occurred, the amount of photosynthate may have been limited with fewer resources available for below ground root colonization and the maintenance of the mutualistic AM fungi relationship. 

The consequences of reduced AM fungi colonisation could result in higher disease vulnerability/reduced resistance to pathogens as well as less efficiency in accessing nutrients and water under stressful conditions. Breadfruit has the potential to be propagated at an industrial crop scale internationally in new environments using a single cultivated genotype. However there is much uncertainty in its success - especially in places that have nutrient limitations or drought stress. This study suggests that human-driven selection, as seen in cultivated breadfruit, can have unintended effects on below-ground mutualists, with potential impacts on the stress tolerance of crops and overall long-term food security.