|Jarret, Robert - Bob|
|OSWALD, W - Emerson University|
|ARUMUGANATHAN, KATHIRAVETPILLA - Benaroya Institute|
|SHIELDS, JOHN - University Of Georgia|
Submitted to: Crop Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 10/3/2016
Publication Date: 3/3/2017
Citation: Jarret, R.L., Bauchan, G.R., Oswald, W.W., Arumuganathan, K., Shields, J.P. 2017. Notes on Citrullus spp.: Pollen morphology, C values, and interspecific hybridization. Crop Science. 57:856-864. https://doi.org/10.2135/cropsci2016.08.0717.
Interpretive Summary: Watermelon (genus Citrullus) is an important crop in the US and globally. There are 3 to 6 wild relatives of watermelon that have been described in the scientific literature. All are native to western or southern Africa. The relationship of some of these to the cultivated watermelon is unclear. This study examined pollen grains from watermelon and its wild relatives to determine if the physical appearance of pollen from the wild relative (and proposed wild relatives) was similar to that of the cultivate watermelon. The results indicated that the pollen grains of all wild relatives were similar in appearance to the cultivated form. Two plants had pollen grains with an unusual appearance. These were determined to be mutant types and not typical. Based on the similarity of the pollen grains from the wild relative several of these were hybridized (successfully) with the cultivated watermelon and all produced fruit with seed indicating that these crop wild relatives may be of use in a breeding program using traditional methodologies.
Technical Abstract: Scanning electron and light microscopy were utilized to examine pollen of the currently recognized species (and forms) within the genus Citrullus (Cucurbitaceae). Materials examined included: C. lanatus (Thunb.) Matsum. & Nakai including the citron (C. amarus Schrad.) and egusi (C. mucosospermus (Fursa) T.B. Fursa) forms, C. colocynthis (L.) Schrad., C. rehmii de Winter, C. ecirrhosus (Cogn.) and a member of a closely related genus - Acanthosicyos naudinianus (Sond.) C. Jeffrey (Citrullus naudinianus (Sond.) Hook. F., Cucumis naudinianus Sond.). Pollen of all species and forms were similar in shape differing slightly in their polar (P) and equatorial (E) axes and P/E. In general, all were characterized as prolate and tricolpate with a small polar area and reticulate ornamentation. A mutant affecting pollen ornamentation was identified in PI 482261, a citron type from Zimbabwe. Pollen ornamentation on PI 482261 was rugulate. An examination of 15 additional accessions from neighboring areas in Zimbabwe revealed a second rugulate type in PI 482312, while the remaining 14 were reticulate. Artificial hybridizations between C. colocynthis (PI 652554) x A. naudinianus (GRIF 14032) produced 84 seed from three fruits. The seed were subsequently germinated in vitro and the seedlings (21) acclimated to greenhouse conditions. The F1 hybrids exhibited an angular growth pattern of the stem and storage root formation - characteristics associated with the male parent. Other characteristics such as vine branching, tendril length and shape, leave size and shape were intermediate between the parents. These data suggest that the introgression of traits from A. naudinianus to C. lanatus may be possible using C. colocynthis as a bridge species.