Seal J N, Schiøtt M, Mueller U G
Department of Biology, University of Texas at Tyler, 3900 University Blvd, Tyler, TX 75799, USA Integrative Biology, University of Texas at Austin, 1 University Station C0930, Austin, TX 78712, USA
Centre for Social Evolution, Department of Biology, University of Copenhagen, Universitetsparken 15, DK-2100 Copenhagen, Denmark.
J Exp Biol. 2014 Jul 15;217(Pt 14):2540-7. doi: 10.1242/jeb.098483. Epub 2014 May 6.
Fungus-gardening insects are among the most complex organisms because of their extensive co-evolutionary histories with obligate fungal symbionts and other microbes. Some fungus-gardening insect lineages share fungal symbionts with other members of their lineage and thus exhibit diffuse co-evolutionary relationships, while others exhibit little or no symbiont sharing, resulting in host-fungus fidelity. The mechanisms that maintain this symbiont fidelity are currently unknown. Prior work suggested that derived leaf-cutting ants in the genus Atta interact synergistically with leaf-cutter fungi (Attamyces) by exhibiting higher fungal growth rates and enzymatic activities than when growing a fungus from the sister-clade to Attamyces (so-called 'Trachymyces'), grown primarily by the non-leaf cutting Trachymyrmex ants that form, correspondingly, the sister-clade to leaf-cutting ants. To elucidate the enzymatic bases of host-fungus specialization in leaf-cutting ants, we conducted a reciprocal fungus-switch experiment between the ant Atta texana and the ant Trachymyrmex arizonensis and report measured enzymatic activities of switched and sham-switched fungus gardens to digest starch, pectin, xylan, cellulose and casein. Gardens exhibited higher amylase and pectinase activities when A. texana ants cultivated Attamyces compared with Trachymyces fungi, consistent with enzymatic specialization. In contrast, gardens showed comparable amylase and pectinase activities when T. arizonensis cultivated either fungal species. Although gardens of leaf-cutting ants are not known to be significant metabolizers of cellulose, T. arizonensis were able to maintain gardens with significant cellulase activity when growing either fungal species. In contrast to carbohydrate metabolism, protease activity was significantly higher in Attamyces than in Trachymyces, regardless of the ant host. Activity of some enzymes employed by this symbiosis therefore arises from complex interactions between the ant host and the fungal symbiont.
菌圃昆虫是最复杂的生物之一，因为它们与专性真菌共生体及其他微生物有着广泛的共同进化历史。一些菌圃昆虫谱系与同谱系的其他成员共享真菌共生体，因此呈现出扩散的共同进化关系，而其他一些谱系则很少或没有共生体共享现象，从而形成宿主 - 真菌的专一性。目前尚不清楚维持这种共生体专一性的机制。先前的研究表明，切叶蚁属（Atta）中进化而来的切叶蚁与切叶蚁真菌（Attamyces）协同相互作用，其真菌生长速率和酶活性高于培养来自Attamyces姐妹进化枝的真菌（所谓的“Trachymyces”）时，后者主要由非切叶的Trachymyrmex蚂蚁培养，而Trachymyrmex蚂蚁相应地构成切叶蚁的姐妹进化枝。为了阐明切叶蚁中宿主 - 真菌专一性的酶学基础，我们在德州切叶蚁（Atta texana）和亚利桑那Trachymyrmex蚁（Trachymyrmex arizonensis）之间进行了相互真菌转换实验，并报告了转换和假转换菌圃消化淀粉、果胶、木聚糖、纤维素和酪蛋白的测量酶活性。当德州切叶蚁培养Attamyces真菌时，菌圃的淀粉酶和果胶酶活性高于培养Trachymyces真菌时，这与酶的专一性一致。相比之下，当亚利桑那Trachymyrmex蚁培养这两种真菌中的任何一种时，菌圃的淀粉酶和果胶酶活性相当。虽然切叶蚁的菌圃并非已知的纤维素重要代谢者，但亚利桑那Trachymyrmex蚁在培养任何一种真菌时都能够维持具有显著纤维素酶活性的菌圃。与碳水化合物代谢不同，无论宿主蚂蚁如何，Attamyces中的蛋白酶活性都显著高于Trachymyces。因此，这种共生关系所使用的一些酶的活性源于宿主蚂蚁和真菌共生体之间的复杂相互作用。
