Watermelon Germplasm Resources and Breeding
全球西瓜遺傳資源與育種研究
Dr. Xingping Zhang
Syngenta Seeds, Inc., USA
Watermelon is indigenous to the dry plains of tropical and subtropical Africa, most likely in
the general area of present day Botswana. It was disseminated around the world from 800 to
1600 CE with continuous selection for desirable eating qualities. Because of many years of
cultivation, as well as continual selection for desirable fruit qualities, a narrow genetic base
exists among watermelon cultivars and they are highly susceptible to a large number of
diseases and pests. In contrast, wide genetic diversity exists among wild watermelon types,
including the central Citrullus species 'C. lanatus var. citroides' that thrive throughout the
African continent, or the desert watermelon C. colocynthis that grow in the deserts stretching
through North Africa, the Middle East, and South and Central Asia. These Citrullus species are
known to have viable sources of disease and pest resistance genes that can be useful in
enhancing watermelon cultivars.
Genetic diversity is the base of crop genetic improvement in meeting crop production needs
for yield, quality, resistance and tolerance. Future watermelon genetic improvement will heavily
rely on the genetic diversity of native traits offered by watermelon germplasm. The National
Plant Germplasm System of USDA-ARS (http://www.ars-grin.gov/npgs/index.html) has played
a significant role in acquiring, preserving, evaluating, documenting and distributing
watermelon germplasm.
There are 24accession of Citrullus colocynthis, 3accessions of
Citrullus ecirrhosus, 256 accessions of Citrullus lanatus , 136 accessions of Citrullus lanatus
var. citroides , 1445 accessions of Citrullus lanatus var. lanatus, and 5 accessions of Citrullus
rehmii in the National Plant Germplasm System. These watermelon germplasm were collected
in 67 countries. The evaluated watermelon accessions are documented in Germplasm Resources
Information Network (GRIN), http://www.ars-grin.gov/cgi-bin/npgs/html/desclist.pl?151, with
32 descriptors. 251accessions are selected as the core subset to represent the whole collection.
The European Central Cucurbits database is being developed at the COMAV, Spain
(http://www.comav.upv.es/eccudb.html) on the initiative of the European Cooperative Program
for Crop Genetic Resources Networks (ECP/GR). The database contains the passport
information on 6187 accessions belong to Citrullus genus. The database is being developed
following the IPGRI/FAO Multicrop Passport Descriptors List. The database will be extended
to include characterization and evaluation data. The characterization data are a set of ten to
fifteen descriptors, those included in the minimum descriptors list for melons, cucumbers,
6 全球西瓜遺傳資源與育種研究 06-09 (2009)
watermelons and Cucurbita species, agreed by the members of the Cucurbits Working Group
(CWG)(http://www.ecpgr.cgiar.org/Workgroups/Cucurbits/Cucurbits.htm). The database allows
on-line searches using twenty one of the twenty seven fields included in the database. All the
information included in the database can be downloaded. A list of the contributors, a database
description and information on cucurbits taxonomy are also provided. It is believed that high
duplication of watermelon accessions are existed in the European Central Cucurbits database.
Planned improvement by public and private groups began in the late 19th and early20 th
centuries. By the mid-20th century, the most successful open-pollinated cultivars in the U.S.
were, 'Charleston Gray' (C.F. Andrus, USDA), 'Jubilee' (J.M. Crall, University of Florida),
'Crimson Sweet' and 'Allsweet' (C.V. Hall, Kansas State University), ) and 'Sugar Baby' (M.
Hardin, Geary, Okla.). 'Crimson Sweet' is still popular and widely grown throughout the
world. 'Charleston Gray' and the variants are the major open-pollinated cultivar in Venezuela,
Africa and the Middle East and 'Sugar Baby' is still grown in Southeast Asia. Hybrids replaced
open-pollinated cultivars because they were more uniform, higher yielding and provided
exclusivity to commercial seed companies and breeders by the end of the 20th century.
Nearly all commercial watermelon production has used hybrid cultivars in Japan, Korea
and China since the 1990s. Hybrids predominate in the developed countries throughout the
world in the early 21st century. In the United States, “Royal Sweet' is the first major F1 hybrid
varieties. However, 'Sangria', the first hybrid Allsweet type cultivar, characterized by an
attractive rind pattern with deep red, high quality flesh, have been the fashion of seeded
watermelon since its development in 1985. This accomplishment led to development of an
array of such types in the United States. In Europe and South America the most significant
hybrids include main season variety Crimson Tide, early maturity variety Crisby and more
recently Crimson Sweet type hybrid TopGun. These and other similar varieties have lead to the
change of varieties from OP to F1 gradually although large amount of production in Europe,
Africa, Middle East and South America are still using OP varieties. 'Xincheng No.1', a hybrid
between a grey skin globe shape Asia watermelon and a selection of Charleston Gray, had lead
to the transformation of OP to F1 in commercial watermelon in late 1980's in China because
of its good productivity and good seed producibility. 'Zhengza No.5' replaced the early OP
varieties at the similar time. 'Xinhongbao' was very popular in 1990's in China because of its
productivity and uniformity. The main season production shifted to high yield and Fusarium
wilt resistant variety 'Xinong No.8', a hybrid between 'Sugarlee' and an elongated Asia
watermelon, in late
1990's. The early protected production has been using high quality
varieties 'Jingxin No.1' and '8424' since 1990's in China.
The most important change in modern watermelon variety was the development of the
seedless watermelon. O.J. Eigsti of Chicago State University recognized the impact of
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colchicine on induction of polyploidy in his famous 1938 paper. Hitashi Kihara (1980 –1894), a
world-renowned Japanese plant geneticist, then developed the first triploid watermelon. His
results first published in 1947 were based on research in polyploidization via colchicine, hybrid
breeding, and the studies on the physiology of parthenocarpy. To accomplish this feat, the
chromosome number of the female parent, 2x= 22, usually with a gray-rind, doubled (4x=44),
to produce a tetraploid by application of colchicine. (Some breeding programs now use
dinitroaniline to successfully induce tetraploidy in watermelon). A selected tetraploid is then
crossed with a diploid male parent, usually a striped-rind type, to produce a striped-rind triploid,
3x=33, which is sterile but when crossed with a diploid produces seedless fruit. Kihara's work
was unknown outside of Japan until 1951 because of World War II. It became known in the
western world through a report in the Proceedings of the American Society for Horticultural
Science that was recognized with the Vaughan Award for the outstanding paper in vegetable
crops. The concept was commercialized in the U.S by Eigsti with the introduction of 'Tri-X 313'
in 1962 and the formation of the American Seedless Watermelon Seed Corporation. 'Tri-X
313' is still a popular cultivar in many parts of the world and has many look alikes, using very
similar tetraploid parents, under various names. It took many years for triploids to become a
major factor in the watermelon industry because of several deficiencies in early cultivars such
as poor seed germination, presence of hard seed hull (coat) in the fruit, erratic performance,
and high seed costs. In addition, growers were slow to adopt new technologies necessary for
successful triploid watermelon production. However, use of seedless watermelon in
commercial production in mid 1990's has significantly increased watermelon yield in the USA.
More than 80% watermelons are seedless watermelon in the USA today. The seedless
watermelon varieties mostly used in commercial production in China include 'Heimi', 'Quality
No.1' and 'Xuefeng Seedless', and a lot more varieties are available for commercial production
now. A major advance occurred in the early 21st century with the introduction of the hybrid
personal size seedless watermelons by Syngenta Seeds. These important new watermelons are
sold under the PureHeart™ brand in the United States and Solinda™ brand in Europe. An
array of rind patterns is available and all are characterized by thin rinds and deep red seedless
flesh. Other companies quickly seized on this concept and began marketing seeds of their own
mini watermelon cultivars. These watermelons have become extremely popular based on a
combination of superb flavor, crisp flesh, seedlessness, and convenient size.
Staminate triploid watermelon flowers do not produce sufficient pollen to effect complete
pollination. So, diploids were planted in separate rows or within the row to provide additional
pollen. Each diploid plant requires a dedicated area within the field so the entire field may have
had only 50 to 80% triploids, depending on the planting configuration. In 2003, the seed
industry made available distinct pollenizers, e.g. the Super-pollenizer
TM
SP-1 offered by
Syngenta Seeds, that can be interplanted among the triploid plants allowing for 100% of the
field to be planted with triploids. This concept has further advanced triploid production in
8 全球西瓜遺傳資源與育種研究 06-09 (2009)
North America.
Discovering and charactering useful native traits and their genetic variation among the
existing crop germplasm is an important and challenging task. This is particularly true for the
under-studied crop watermelon. The recently developed molecular markers, especially the
powerful SNP markers being produced from watermelon genomics work conducted at public
institute and private company, will help the characterization of watermelon germplasm and
discovering useful native watermelon traits and their genetic diversity. The genomic derived
tools will also enable watermelon breeders to more effectively manipulate the native traits in
watermelon genetic improvement. Future watermelon breeding will be more than the
improvement of yield, resistance and soluble solids in the fruit. Traits of fresh cut quality,
environmental stress tolerance, and functional compounds like lycopenes in the flesh will be
important breeding targets.
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