|Chen, Jin feng|
Submitted to: Genome
Publication Type: Peer reviewed journal
Publication Acceptance Date: 6/20/2006
Publication Date: 7/20/2006
Citation: Chung, S., Staub, J.E., Chen, J. 2006. Molecular phylogeny of cucumis species as revealed by ccSSR marker length and sequence variation. Genome. 49:219-229. Interpretive Summary: The origin of plant species related to cucumber is Africa. African plant species related to cucumber have many economically important traits such as disease resistance and drought tolerance that are not seen in cucumber. These African species, however, do not cross-fertilize (mate) with cucumber to produce offspring. Recently, a wild species has been discovered in southern China that does cross-fertilize with cucumber. This species has genes for resistance to nematodes (soil borne worms), several important disease resistances (gummy stem blight, specific virus) and the ability to be grown at relatively low temperatures. These characteristics are not found in commercial cucumber. The genetic basis of why this Chinese wild species cross-fertilizes with cucumber, while African species do not is not known. If the genetic basis of this crossing behavior were known it would help scientists understand and devise ways for breaking down crossing barriers with African species to allow for incorporation of disease resistance and other economically important traits into cucumber. Thus a study was designed to define the genetic relationships among commercial cucumber and the Chinese and African wild species. The study used biochemical analyses to study the DNA of these species for comparative analysis. Results from DNA analysis indicate that African species are distantly related to cucumber, while the Chinese species is closely related to cucumber. If was found that the Chinese species is more closely related to cucumber than melon which is has be previously considered an ancestral form (closely related to) of cucumber. The data suggest that it will be very difficult to cross-fertilize African and Chinese wild cucumber-like (related to) species, and thus will be very difficult to introduce genes from wild African species into commercial cucumber for its improvement. However, genes from the Chinese species can be used to improve commercial cucumber. The plant breeding that now uses the Chinese species in the USDA is being currently used for cucumber improvement. The introduction of ecomonically important genes from the Chinese species will lead to improved pest resistance, and thus improve the competitiveness of the U.S. grower in the global market place.
Technical Abstract: The genus Cucumis has been partitioned into two subgenera Cucumis (x = 7) and Melo (x = 12). Although the genetic relationships among species within this two subgenera have been well studied using nuclear genetic markers, recent development of chloroplast markers consensus chloroplast simple sequence repeats primer pairs (ccSSR) obtained from tobacco chloroplast DNA provides opportunities to study their phylogeny in greater detail. Therefore, to investigate phylogenetic relationships in this genus, nine ccSSRs (ccSSR3, 9, 11, 13, 14, 17, 20, 21, and 23) were employed for DNA fragment length variation and five amplified fragments of consensus chloroplast simple sequence repeat markers (ccSSR4, 12, 13, 19, and 20) were sequenced using total DNA of thirteen African Cucumis species (x = 12) accessions, three Cucumis melo L. (x = 12) accessions, two Cucumis sativus L. (x = 7) accessions, and one Cucumis hystrix Chakr. (x = 12) accession. A Citrullus lanatus (Thunb.) Matsum. & Nakai (x = 11) accession was used as a outgroup. DNA fragment length and sequence substitution variations (150 sites) were detected, and used in UPGMA cluster analyses. Subsequently, 80 informative sequence substitution variants were detected and employed in parsimony analysis to determine the evolutionary relationships. While in some cases ccSSR marker analyses indicates comparatively close species relationships, crossing and meiotic behavior in F1 hybrids between such species does not. While fragment length analysis revealed the existence of three major species clusters (i.e., a group of African Cucumis species, a group composed of C. melo accessions, and a group containing C. sativus, and C. hystrix species), sequence variation analysis identified (i.e., a group of African Cucumis species and a group composed of C. melo, C. sativus, and C. hystrix species). In these analyses, C. ficifolius (2x) and C. aculeatus (4x) of the African Cucumis species clustered together. This result does not agree with reported isozyme analyses, but does agree with previously characterized chromosome homologies between these two species. Although African Cucumis species and C. hystrix do not share a close relationship, genetic affinities between C. sativus and C. hystrix are considerable. Combined evidence from previously published studies and data presented herein, lend support to the hypothesis that C. hystrix is a progenitor species of C. sativus, or that they at least share a common ancestral lineage.