Introduction
The plant family Vitaceae is well-known for its economically important fruit crop, the grape (Vitis vinifera ). It comprises 16 genera with over 950 species and is classified into five tribes (Wen et al., 2018b). Many species in the family are dominant climbers in tropical/temperate forests, savannas, and mountains (Kubitzki et al., 2007), representing one of the earliest diverged lineages in the major Rosid clade of eudicot plants (Zeng et al., 2017; Zhang et al., 2016) (Figure 1a and 1b). The grapevine (PN40024) was the first fruit crop whose genome was decoded (Jaillon et al., 2007).
Cissus L. is the largest genus in Vitaceae, comprising over 300 species (Wen et al., 2018), and the only genus of the tribe Cisseae Rchb. Unlike grapevines which are mostly distributed in temperate regions, Cissus mainly occur in the seasonal arid regions of the tropics and subtropics (DeSanto and Bartoli, 1996). Species in this genus exhibit considerable variations in both chromosome number (2n = 24–66) and genome size (1C = 0.38–1.03 pg) (Chu et al., 2018). Morphological modifications such as succulent leaves or stems have arisen in some Cissus species in the face of drought stress (Figure 1a) (Griffiths and Males, 2017). Therefore, these groups provide an opportunity to investigate the strategies of plant adaptive evolution on drought tolerance. Crassulacean acid metabolism (CAM) is a water use efficient adaptation of photosynthesis that has evolved independently many times in diverse lineages of flowering plants (Niechayev et al., 2019). The genomes of CAM plants, including pineapple (Ananas comosus ), orchid (Phalaenopsis equestris ),Kalanchoe fedtschenkoi , Dendrobium catenatum , D. officinale, and Sedum album, was available (Cai et al., 2015; Ming et al., 2015; Wai et al., 2019; Yan et al., 2015; Yang et al., 2017; Zhang et al., 2016). Comparative analyses between Kalanchoë fedtschenkoi and non-CAM species identified the convergence in protein sequence in nocturnal CO2 fixation and carbohydrate metabolism (Yang et al., 2017). CAM is also widespread in Cissus(DeSanto and Bartoli, 1996; Olivares et al., 1984; Sayed, 2001; Ting et al., 1983), enabling us to dissect the convergent evolution of CAM in the plant kingdom.
Cissus rotundifolia Lam. are mainly distributed in the tropical savannas of Eastern Africa. Their leaves are consumed as a local traditional food (Al-Bukhaiti et al., 2019). It has a relatively small genome with 1C = 0.38 pg (Chu et al. , 2018). To understand the adaptive strategies of the genus Cissus in the harsh climate, we generated and compared the draft genome of C. rotundifolia with that of V. vinifera to uncover how the genome evolved and genes regulated underpinning its arid adaptation. Further, we conducted extensive transcriptome comparisons to characterize the evolution of CAM in C. rotundifolia .