Table_4_No Evidence for Single-Copy Immune-Gene Specific Signals of Selection in Termites.xls

Selection pressures from pathogens appear to play an important role in shaping social evolution. Social behavior, in particular brood care, is associated with pathogen pressure in wood-dwelling “lower” termites. Yet, generally pathogen pressure is predicted to be low in wood-dwelling termite species that never leave the nest except for the mating flight. In comparison, pathogen pressure is predicted to be higher in species that leave the nest to forage, and thus constantly encounter a diversity of microbes from their environment. We hypothesized that such differences in predicted pathogen pressure are also reflected by differences in the intensity of natural selection on immune genes. We tested this hypothesis in a phylogenetic framework, analyzing rates of non-synonymous and synonymous substitutions on single-copy immune genes. Therefore, we leveraged recent genomic and transcriptomic data from eight termite species, representing wood-dwelling and foraging species as well as 14 additional species spanning the winged insects (Pterygota). Our results provide no evidence for a role of pathogen pressure in selection intensity on single-copy immune genes. Instead, we found evidence for a genome-wide pattern of relaxed selection in termites.

病原体施加的选择压力，似乎在社会演化的塑造过程中发挥着关键作用。在栖息于木材的"低等"白蚁（wood-dwelling "lower" termites）类群中，社会行为（尤其是育幼行为（brood care））与病原体压力（pathogen pressure）存在显著关联。然而一般而言，仅在婚飞（mating flight）阶段离巢的木材栖息白蚁物种，其所处环境的病原体压力被认为较低。相较而言，离巢觅食的白蚁物种会持续接触环境中多样的微生物，因此其所处环境的病原体压力被认为更高。我们据此提出假说：预测的病原体压力差异，也会体现在免疫基因所受自然选择强度的差异之上。我们依托系统发育框架（phylogenetic framework）检验了这一假说，对单拷贝免疫基因（single-copy immune genes）的非同义替换（non-synonymous substitution）与同义替换（synonymous substitution）速率进行了分析。为此，我们利用了近期获得的基因组与转录组数据（genomic and transcriptomic data），这些数据涵盖8个白蚁物种（包含木材栖息类与觅食类类群）以及另外14个覆盖有翅昆虫（有翅亚纲Pterygota）的物种。我们的研究结果并未发现证据支持病原体压力对单拷贝免疫基因的选择强度存在影响。相反，我们的研究结果显示，白蚁类群存在全基因组范围的放松选择（relaxed selection）模式。