Mikheyev A S, Mueller U G, Boomsma J J
Section of Integrative Biology, 1 University Station C0930, University of Texas, Austin, TX 78712, USA.
Mol Ecol. 2007 Jan;16(1):209-16. doi: 10.1111/j.1365-294X.2006.03134.x.
Switching of symbiotic partners pervades most mutualisms, despite mechanisms that appear to enforce partner fidelity. To investigate the interplay of forces binding and dissolving mutualistic pairings, we investigated partner fidelity at the population level in the attine ant-fungal cultivar mutualism. The ants and their cultivars exhibit both broad-scale co-evolution, as well as cultivar switching, with short-term symbiont fidelity maintained by vertical transmission of maternal garden inoculates via dispersing queens and by the elimination of alien cultivar strains. Using microsatellite markers, we genotyped cultivar fungi associated with five co-occurring Panamanian attine ant species, representing the two most derived genera, leaf-cutters Atta and Acromyrmex. Despite the presence of mechanisms apparently ensuring the cotransmission of symbiont genotypes, different species and genera of ants sometimes shared identical fungus garden genotypes, indicating widespread cultivar exchange. The cultivar population was largely unstructured with respect to host ant species, with only 10% of the structure in genetic variance being attributable to partitioning among ant species and genera. Furthermore, despite significant genetic and ecological dissimilarity between Atta and Acromyrmex, generic difference accounted for little, if any, variance in cultivar population structure, suggesting that cultivar exchange dwarfs selective forces that may act to create co-adaptive ant-cultivar combinations. Thus, binding forces that appear to enforce host fidelity are relatively weak and pairwise associations between cultivar lineages and ant species have little opportunity for evolutionary persistence. This implicates that mechanisms other than partner fidelity feedback play important roles in stabilizing the leafcutter ant-fungus mutualism over evolutionary time.
共生伙伴的转换在大多数互利共生关系中普遍存在，尽管存在一些似乎能确保伙伴忠诚的机制。为了研究维系和解除互利共生配对的各种力量之间的相互作用，我们在切叶蚁 - 真菌栽培互利共生关系中，从种群层面研究了伙伴忠诚度。蚂蚁及其栽培真菌既表现出广泛的共同进化，也存在栽培真菌的转换现象，通过蚁后传播母体菌圃接种物以及清除外来栽培真菌菌株，维持了短期的共生体忠诚。我们使用微卫星标记，对与五种同时出现的巴拿马切叶蚁物种相关的栽培真菌进行基因分型，这些切叶蚁代表了两个进化程度最高的属，即切叶蚁属（Atta）和顶切叶蚁属（Acromyrmex）。尽管存在明显确保共生体基因型共同传播的机制，但不同种类和属的蚂蚁有时会共享相同的菌圃基因型，这表明存在广泛的栽培真菌交换现象。就宿主蚂蚁物种而言，栽培真菌种群在很大程度上是无结构的，遗传变异中只有10%可归因于蚂蚁物种和属之间的划分。此外，尽管切叶蚁属和顶切叶蚁属之间存在显著的遗传和生态差异，但属的差异在栽培真菌种群结构中几乎没有造成任何变异，这表明栽培真菌交换使得可能促使形成共同适应的蚂蚁 - 栽培真菌组合的选择力量相形见绌。因此，看似确保宿主忠诚的维系力量相对较弱，栽培真菌谱系与蚂蚁物种之间的成对关联几乎没有机会实现进化上的持久性。这意味着除了伙伴忠诚反馈之外的其他机制在进化时间尺度上对稳定切叶蚁 - 真菌互利共生关系起着重要作用。
