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Title: Cytomolecular characterization of rRNA gene sequences among Citrullus species and subspecies using fluorescent in situ hybridization (FISH) technology

Author
item REDDY, U - West Virginia State University
item ARYAL, N - West Virginia State University
item ISLAM-FARIDI, N - West Virginia State University
item TOMASON, Y - West Virginia State University
item Levi, Amnon
item NIMMAKAYALA, P - West Virginia State University

Submitted to: Genetica
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/16/2013
Publication Date: 3/1/2013
Citation: Reddy, U.K., Aryal, N., Islam-Faridi, N., Tomason, Y., Levi, A., Nimmakayala, P. 2013. Cytomolecular characterization of rRNA gene sequences among Citrullus species and subspecies using fluorescent in situ hybridization (FISH) technology. Genetica. 60:427-440.

Interpretive Summary: Watermelon is an important vegetable crop that is grown in 44 of the 50 United States. Due to many disease and pest problems in watermelon, there is a continuous need to enhance disease and pest resistance in watermelon varieties. However, there is limited knowledge on genetics of watermelon and genes that could be useful to improve resistance in cultivated varieties. In this study, USDA scientists have collaborated with scientists at West Virginia State University on using a technology called “fluorescent in-situ hybridization” (FISH) to study the structure of chromosomes in wild type watermelons collected in Africa compared to those of common American varieties. The researchers identified significant differences in the structure of chromosomes in the wild versus the cultivated watermelons. The knowledge gained in this study should be useful for researchers and plant breeders interested in developing breeding program strategies to improve disease resistance in new watermelon varieties.

Technical Abstract: In previous studies, many DNA markers showed strong preferential (non-Mendelian) segregation in F2 and BC1 genetic populations derived from crosses between wild type watermelon [C. lanatus (Thunb.) Matsum. et Nakai subsp. lanatus var. citroides (Bailey) Mansf. ex Greb.] (CLC) and watermelon cultivars [(C. lanatus subsp. vulgaris Schrad. ex Eckl. et Zeyh.) Fursa,; also known as C. lanatus var. lanatus] (CLL), indicating that wide differences may exist in genome structure of these Citrullus subspecies. To further characterize these differences, we performed fluorescent in-situ hybridization (FISH) analysis using rRNA gene probes, known to be useful chromosome markers in genome evolution and organization studies. The FISH analysis identified significant differences in distribution and configuration of the 18S-28S and 5S rRNA gene sequences among Citrullus species and subspecies. The CLL and C. colocynthis (L.) Schrader (CC) genomes have two 18S-28S rRNA gene sequences, each localized on a different chromosome, while the 5S rRNA gene sequence is interstitially co-localized with one of these 18S-28S rRNA gene sequences. In contrast, the C. rehmii De Winter (CR) genome has one 18S-28S rRNA and one 5S rRNA gene sequences, while the CLC genome has one 18S-28S rRNA and two 5S rRNA gene sequences, each localized on a different chromosome. We hypothesize that these differences in rRNA gene loci configuration reflect major events in the Citrullus genome evolution that led to further speciation and cultivation of watermelon.