Figure 2.

Deduced evolutionary history of venom-related gene families in the Protobothrops flavoviridis genome. Through two rounds of whole-genome duplication, an original set of 18 genes (shown in a grey box in the left column) became 72 (four copies each). Then, a single copy of each family was likely co-opted to develop toxic functions, resulting in one toxic copy (SV) (shown in a pale orange box in the middle column) and three non-toxic (NV) paralogs (shown in a light blue box in the middle column). Then the 18 venom protein families experienced different numbers of additional gene duplications. Eleven families (Vespryn, 5Nase, DDPase, Hyal, NGF, VEGF, LAAO, PDE, PLB, BNP and GPCase) retained more or less the original configuration, with a single SV copy and one to eleven NV copies (Category I, shown in light blue and pale orange boxes in the right column). Three families (3FTX, APase and CRISP) have experienced stochastic gene losses and gains, resulting in moderately diverse configurations with two to four SV copies and two to ten NV copies (Category II, shown in blue and orange boxes in the right column). Four families of major protein components in the venom (MP, SP, CTLP and PLA2) have experienced repeated duplication, resulting in complex configurations with 9–11 SV genes and 31–57 NV genes (Category III, shown in deep blue and red boxes in the right column). As shown in Fig. 3, SV genes in Category III also show accelerated evolution.