|Van Buren, Robert|
Submitted to: Tropical Plant Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/3/2011
Publication Date: 11/17/2011
Publication URL: http://handle.nal.usda.gov/10113/56412
Citation: Van Buren, R., J. Li, F. Zee, J. Zhu, C. Liu, A.K Arumuganathan, R. Ming. 2011. Longli is not a hybrid of longan and lychee as revealed by genome size analysis and trichome morphology. Tropical Plant Biology. 4:228-236. Interpretive Summary: Fruit similarities between lychee, longan and longli led to the hypothesis that longli is a natural hybrid between longan and lychee. Whether longli is the result of a natural hybridization event or is simply another species, or group of species in the Dimocarpus genus is highly contested in the longan and lychee fruit industry. If longli is an intergeneric hybrid, it might easily be backcrossed with longan or lychee for crop improvement or quantitative trait loci (QTL) mapping. If longli is a distinct species, it may have genes encoding desirable traits found in neither longan nor lychee that could be useful for plant breeding. The genome sizes of lychee, longan, longli, ‘Malesianus’,and rambutan for planning genome sequencing and resequencing projects is unknown except for one record for lychee. Accessions of lychee, longan, ‘Malesianus’and rambutan from the USDA germplasm collection were analyzed by flow cytometry to estimate genome size. The average genome size estimates of lychee 554 mega base pairs (Mbp), longan 444 Mbp, rambutan 339 Mbp, ‘Malesianus’ 404 Mbp. Five varieties of longli belonging to D. confinis were collected from Guangdon and Guangxi provinces, China. The longchuan longli had the smallest estimated genome size of 450 Mbp, Guangzhou varieties 501 Mbp and Linshan varieties 551 Mbp. The ‘Lingchuan’ and ‘Yiongning’ longli had estimated genome sizes of 612 and 678 Mbp respectively. The ‘Malesianus’ genome was 9% smaller than longan and 27% smaller than the lychee, which is much greater than the 0.9 – 2.3% variation within lychee and longan. Therefore we conclude that ‘Malesianus’ is most likely not a hybrid of lychee and longan. The wide range of genome size variation among the five longli accessions is a line of evidence that they are not intergeneric hybrids, which would have a relatively uniform genome size. There is little genetic diversity within cultivated longan and lychee germplasm so that interspecific hybridization and backcrosses would likely be beneficial for crop improvement. The abundant genetic diversity of longli could serve as useful backcross material to improve fruit quality and flower induction, disease resistance, and robustness in lychee and longon. Genome size variation, trichome morphology and ISSR and RFLP molecular markers suggests that longli should be classified under a new, independent genus, with several separate species. ‘Malesianus’ has been classified as a subspecies of longan. Its smaller genome size than that of longan and unique trichome morphology indicates that is does not belong to the species Dimocapus longon sub spp. malesianus but could be an independent species in its own right classified as Dimocarpus malesianus.
Technical Abstract: Lychee, longan, longli, and rambutan are closely related, commercially important fruit trees in the Sapindaceae family. Longli fruits are morphologically similar to both lychee and longan, displaying a yellow-brown pericarp like longan, and small, sharp protuberances like lychee. These similarities have led to the hypothesis that longli is the result of intergeneric hybridization between longan and lychee. Scanning electron microscopy and flow cytometry were used to examine trichome morphology and genome size, respectively, to test this hypothesis. Longan, lychee, longli (D. confinis), and ‘Malesianus’ (D. longan sub spp malesianus) had morphologically distinct trichomes. The genome sizes for lychee (554 Mb), longan (444 Mb), ‘Malesianus’ (404 Mb), and rambutan (339 Mb) are distinctive and in a narrow range. ‘Malesianus’ has a genome 9% smaller than that of longan and 27% smaller than that of lychee. It is likely a species that evolved independently in northern Borneo island, and could be classified as a species, Dimocarpus malesianus, not a subspecies of longan as presently stated. Flow cytometry revealed a 50% variation in genome sizes among longli varieties, with genome sizes ranging from 450 to 678 Mb, beyond the range between longan and lychee. The genome size variation and distinct leaf hair morphology suggest that longli is not an intergeneric hybrid, and it is likely a separate genus evolved independently. The tested cultivars with distinctive genome sizes within D. confinis could be classified as separate species.