Molecular Evolution ofpsbD Gene in Ferns：Selection Pressure and Co-evolutionary Analysis

The D2 protein of the PhotosystemII(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 environmentsꎬ 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 selectionꎬ 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 ̄R168ꎬ D2 ̄H245 and D2 ̄M272 of tree ferns were involved in the same co ̄evolution network.