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 .