Abstract: The D2 protein of the Photosystem Ⅱ (PSⅡ) reaction center complex is encoded by the chloroplast gene psbD.To gain a better understanding of the adaptive radiation of core leptosporangiate ferns in low-light environments,we sequenced psbD in twelve species and collected eight published sequences to represent extant ferns in order level.We then evaluated the selection pressure of D2 protein based on ω values (nonsynonymous synonymous substitution ratio).The results of statistical tests based on different models indicated that most sites and branches were under strong negative selection,but tree ferns exhibited a slower evolutionary rate and higher ω value compared with other ferns.Co-evolution analysis based on different approaches showed that the D2-R168,D2-H245 and D2-M272 of tree ferns were involved in the same co-evolution network.