Little Ainslie E F, Currie Cameron R
Department of Bacteriology, University of Wisconsin-Madison, Madison, WI 53706, USA.
Biol Lett. 2007 Oct 22;3(5):501-4. doi: 10.1098/rsbl.2007.0253.
The fungus-growing ant-microbe mutualism is a classic example of organismal complexity generated through symbiotic association. The ants have an ancient obligate mutualism with fungi they cultivate for food. The success of the mutualism is threatened by specialized fungal parasites (Escovopsis) that consume the cultivated fungus. To defend their nutrient-rich garden against infection, the ants have a second mutualism with bacteria (Pseudonocardia), which produce antibiotics that inhibit the garden parasite Escovopsis. Here we reveal the presence of a fourth microbial symbiont associated with fungus-growing ants: black yeasts (Ascomycota; Phialophora). We show that black yeasts are commonly associated with fungus-growing ants, occurring throughout their geographical distribution. Black yeasts grow on the ants' cuticle, specifically localized to where the mutualistic bacteria are cultured. Molecular phylogenetic analyses reveal that the black yeasts form a derived monophyletic lineage associated with the phylogenetic diversity of fungus growers. The prevalence, distribution, localization and monophyly indicate that the black yeast is a fifth symbiont within the attine ant-microbe association, further exemplifying the complexity of symbiotic associations.
种植真菌的蚂蚁与微生物的共生关系是通过共生关联产生的生物复杂性的经典例子。蚂蚁与它们为获取食物而培育的真菌有着古老的专性共生关系。这种共生关系的成功受到专门消耗培育真菌的真菌寄生虫（埃氏菌属）的威胁。为了保护它们营养丰富的“花园”免受感染，蚂蚁与细菌（假诺卡氏菌属）形成了第二种共生关系，这些细菌产生抑制“花园”寄生虫埃氏菌属的抗生素。在这里，我们揭示了与种植真菌的蚂蚁相关的第四种微生物共生体的存在：黑色酵母菌（子囊菌门；瓶梗霉属）。我们表明，黑色酵母菌通常与种植真菌的蚂蚁相关联，在它们的地理分布范围内都有出现。黑色酵母菌生长在蚂蚁的表皮上，特别定位于培养共生细菌的部位。分子系统发育分析表明，黑色酵母菌形成了一个与种植真菌蚂蚁的系统发育多样性相关的衍生单系谱系。其普遍性、分布、定位和单系性表明，黑色酵母菌是切叶蚁 - 微生物共生关系中的第五种共生体，进一步例证了共生关联的复杂性。
