Dearborn Donald C, Page Symmantha M, Dainson Miri, Hauber Mark E, Hanley Daniel
Department of Biology Bates College Lewiston ME USA.
College of Veterinary Medicine Midwestern University Glendale AZ USA.
Ecol Evol. 2017 Oct 16;7(22):9711-9719. doi: 10.1002/ece3.3508. eCollection 2017 Nov.
Oviparous animals have evolved multiple defenses to prevent microbes from penetrating their eggs and causing embryo mortality. In birds, egg constituents such as lysozyme and antibodies defend against microbial infestation, but eggshell pigments might also impact survival of bacteria. If so, microbes could exert an important selective pressure on the evolution of eggshell coloration. In a previous lab experiment, eggshell protoporphyrin caused drastic mortality in cultures of Gram positive, but not Gram negative, bacteria when exposed to light. Here, we test this "photodynamic antimicrobial hypothesis" in a field experiment. In a paired experimental design, we placed sanitized brown, protoporphyrin-rich chicken eggs alongside white eggs that lack protoporphyrin. We deployed eggs for 48 hr without incubation, as can occur between laying and incubation, when microbial infection risk is highest. Eggs were placed on the open ground exposed to sunlight and in dark underground storm-petrel burrows. We predicted that the proportion of Gram-positive bacteria on brown eggs should be lower when exposed to sunlight than when kept in the dark, but we expected no such difference for white eggs. Although our data revealed variation in bacterial community composition, the proportion of Gram-positive bacteria on eggshells did not vary by egg color, and there was no interaction between egg color and location. Instead, Gram-positive bacteria were proportionally more common on eggs on the ground than eggs in burrows. Overall, our experiment did not support the photodynamic antimicrobial hypothesis. The diverse range of avian egg colors is generated by just two pigments, but over 10 hypotheses have been proposed for the evolution of eggshell color. If our results are generalizable, eggshell protoporphyrin might not play a substantial role in defending eggs against microbes, which narrows the field of candidate hypotheses for the evolution of avian eggshell coloration.
卵生动物已经进化出多种防御机制，以防止微生物穿透它们的卵并导致胚胎死亡。在鸟类中，诸如溶菌酶和抗体等卵成分可抵御微生物侵扰，但蛋壳色素也可能影响细菌的存活。如果是这样，微生物可能会对蛋壳颜色的进化施加重要的选择压力。在之前的一项实验室实验中，蛋壳原卟啉在光照下会导致革兰氏阳性菌而非革兰氏阴性菌的培养物出现严重死亡。在此，我们在一项野外实验中对这一“光动力抗菌假说”进行了测试。在配对实验设计中，我们将经过消毒的富含原卟啉的褐色鸡蛋与缺乏原卟啉的白色鸡蛋放置在一起。我们将鸡蛋放置48小时不进行孵化，这在产卵和孵化之间是可能发生的，此时微生物感染风险最高。鸡蛋被放置在露天地面接受阳光照射以及黑暗的地下海燕洞穴中。我们预测，暴露在阳光下时，褐色鸡蛋上革兰氏阳性菌的比例应低于处于黑暗环境中的比例，但我们预计白色鸡蛋不会出现这种差异。尽管我们的数据揭示了细菌群落组成的差异，但蛋壳上革兰氏阳性菌的比例并未因鸡蛋颜色而有所不同，并且鸡蛋颜色和放置位置之间没有相互作用。相反，地面上的鸡蛋上革兰氏阳性菌的比例相对高于洞穴中的鸡蛋。总体而言，我们的实验不支持光动力抗菌假说。鸟类蛋壳的多种颜色仅由两种色素产生，但关于蛋壳颜色的进化已提出了超过10种假说。如果我们的结果具有普遍性，蛋壳原卟啉可能在保护鸡蛋免受微生物侵害方面并不起重要作用，这缩小了关于鸟类蛋壳颜色进化的候选假说范围。
