Genomic and morphological analyses of Sapindaceae species

Lychee (Litchi chinensis), longan (Dimocarpus longan), longli (Dimocarpus longan subsp. malesianus and Dimocarpus confinis), and rambutan (Nephelium spp.) are closely related, commercially important fruits in the Sapindaceae family. Longli fruits morphologically resemble those of both longan and lychee, displaying a yellow-brown pericarp, and small, sharp exocarp protuberances. These similarities have led to the assumption that longli is the result of intergeneric hybridization between the lychee and longan. Scanning electron microscopy, flow cytometry and multiple molecular markers have been used to test this hypothesized intergeneric origin of longli. As a result, longan, lychee, and Chinese longli (D. confinis) showed morphologically distinct trichomes, while Borneo longli (D. longan subsp. malesianus) showed similar trichomes to D. longan. Flow cytometry revealed a 34% variation in genome sizes among Chinese longli cultivars, with genome sizes ranging from smaller to larger than both longan and lychee. Borneo longli had a genome 9% smaller than that of longan and 27% smaller than that of lychee. Distinct leaf hair morphology and genome size variation suggest that Chinese longli is not an intergeneric hybrid. It should be classified in a separate genus, and the tested cultivars should be classified as separate species within this genus. On the other hand, these results suggest that Borneo longan does not constitutes a subspecies within D. longan, but a different species that likely evolved independently from longan in northern Borneo Island. The study of different molecular markers supports Chinese longli and Burneo longli as separate groups from lychee or longan, verifying the findings from trichome morphology and genome size analyses. A reclassification of longli within the Sapindaceae family would be necessary.