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Title: DNA markers and pollen morphology reveal that Praecitrullus fistulosus is more closely related to Benincasa hispida than to Citrullus spp.

item Levi, Amnon
item Harris-Shultz, Karen
item Wechter, William - Pat
item Kousik, Chandrasekar - Shaker
item Thies, Judy

Submitted to: Genetic Resources and Crop Evolution
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/29/2010
Publication Date: 7/13/2010
Citation: Levi, A., Harris-Shultz, K.R., Wechter, W.P., Kousik, C.S., Thies, J.A. 2010. DNA markers and pollen morphology reveal that Praecitrullus fistulosus is more closely related to Benincasa hispida than to Citrullus spp. Genetic Resources and Crop Evolution. 57:1191-1205.

Interpretive Summary: Watermelon and melon are important vegetable crops in the US. Following the ban on methyl bromide there is an increased interest by seed companies and growers in finding cucurbit species related to watermelon and melon that are resistant to soil borne diseases and can be used as rootstocks (the lower part of the plant stem with a healthy root system) for grafting watermelons and melons. The cucurbit family also includes important vegetables such as cucumber, squash and bottle gourd. Among these cucurbits, there is a species “Praecitrullus fistulosus” which has morphological features similar to watermelon. This species is grown mainly in South Asia (India and Pakistan) and is known for its tender fruits that are consumed as cooked vegetables. The ARS is maintaining over 30 accessions of this species at the USDA, ARS Plant Genetic Resources and Conservation Unit, Griffin, GA. P. fistulosus might be a valuable rootstock for grafting watermelon and melon because it thrives in hot dry regions. However, the taxonomic classification of P. fistulosus is still unclear and there is no sufficient information to determine if it is indeed a member of the watermelon genus (Citrullus spp.). In this study, we used DNA markers to determine the genetic relationships among watermelon, melon, cucumber, bottle gourd, the wax gourd (Benincasa hispida) and P. fistulosus. Also we examined pollen morphology. The pollen morphology and DNA data show that P. fistulosus is more closely related to the wax gourd which is also known to be native of South and Central Asia. Thus, P. fistulosus should not be considered a watermelon, but an independent species. The information should be useful for plant taxonomists, and for plant breeders and growers interested in developing vegetable varieties of P. fistulosus, and for researchers interested in grafting experiments of watermelon on P. fistulosus or Benincasa hispida.

Technical Abstract: The round melon Praecitrullus fistulosus (Stocks) Pangalo has been cultivated in Asia since ancient times and has been considered an underexploited crop in the western world. In the US, there is an increased interest in using P. fistulosus as a commercial vegetable, and possibly as a rootstock for grafting watermelon, melon, or cucumber. However, the taxonomic classification of P. fistulosus is incomplete and for many years it has been considered a close relative of watermelon [Citrullus lanatus subsp. vulgaris (Schrad. ex Eckl. et Zeyh.) Fursa] and was previously classified as Citrullus lanatus subsp. fistulosus (Stocks) Duthie & J.B. Fuller. Here, we used two sets of DNA markers to assess the genetic similarity of P. fistulosus in relation to Citrullus spp. {including Citrullus lanatus subsp. vulgaris, C. lanatus subsp. lanatus, Citroides group [also known as C. lanatus (Thunb.) Matsum. et Nakai subsp. lanatus var. citroides (Bailey) Mansf. ex Greb.], and C. colocynthis (L.) Schrad.}, Cucumis spp. (including C. melo, C. sativus, C. anguria, C. meeusei, C. zeyheri), Benincasa hispida (Thunb.) Cogn, Lagenaria siceraria (Mol.) Standl. and Cucurbita spp. [including C. moschata (Duchesne ex Poir.) and the winter squash C. maxima (Duchesne. ex Lam.)]. The first marker set comprised 501 markers that were produced by 38 primer pairs derived from watermelon expressed sequenced tags (ESTs) containing simple sequence repeat (SSR) motifs (designated as EST-SSR primers; produced 311 markers), and by 18 primer pairs derived from ESTs that do not contain SSR motives (designated here as EST-PCR primers; produced 190 markers). The second marker set comprised 628 markers that were produced by 18 sequence related amplified polymorphism (SRAP) primer pairs. The phylogenetic data indicated that among these cucurbit species, the wax gourd B. hispida is the closest to the P. fistulosus. Pollen observations, using light microscopy, indicated that each of the cucurbit genera examined here has unique pollen morphology. The Cucurbita spp. have globular pollen grains with a stigmatic surface. The L. siceraria has polygonal pollen grains with symmetrical boundaries, while the Citrullus spp. and Cucumis spp. have ovular (conical) and triangular shaped pollen grains (respectively). The B. hispida and P. fistulosus have spherical or semispherical pollen grains. These pollen features might be in agreement with the phylogenetic relationships of these two species based on DNA markers. Analysis with 12 SRAP primer pairs revealed low genetic diversity among 18 United States Plant Introductions (PIs) of P. fistulosus, indicating the need to expend the germplasm collection of this cucurbit crop.