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The worlds commercial production of natural rubber,an essential industrial raw material,is solely from one tropical tree species,Hevea brasiliensis (para rubber tree),owing to the high yield and good quality.A reference genome has been long awaited for this species to address important scientific questions about the biology of rubber trees,such as the physiological functions of rubber-producing laticifers and the mechanisms underlying ethylene stimulation of latex production.Here we present a high-quality genome assembly of this species (1.37 Gb,scaffold N50=1.28 Mb) that covers 93.8% of the genome (1.47 Gb) and harbors 43,792 predicted protein-coding genes.A striking expansion of the REF/SRPP gene family and its divergence into several laticifer-specific isoforms appear crucial for rubber biosynthesis.The REF/SRPP family has isoforms with sizes comparable to,or larger than SRPP1 (204 AA) in 17 other plants examined,but no isoforms with similar size to REF1 (138 AA),the predominant molecular variant in latex (cytoplasm of laticifers).The emergence of REF1,a protein located on the surface of large rubber particles that account for 93% of rubber in the latex despite their constituting only 6% of total rubber particles,becomes a pivotal event in rubber tree evolution and is largely responsible the modern rubber trees capacity for high rubber production.The stringent control of ethylene synthesis under active ethylene signaling and response in laticifers resolves a longstanding mystery of ethylene stimulation in rubber production.Our study which includes the re-sequencing of five other Hevea cultivars and extensive RNA-seq data provides a valuable resource for functional genomics and tools for breeding elite Hevea cultivars.