Tomato (Lycopersicon esculentum Mill.) ranks as the leading
fresh and processed vegetable crop in the U.S. with approximately
200 thousand hectares (495,000 acres) planted for commercial production
(1). World production, which exceeded 77.5 million metric tons
in 1994, occupied approximately 2.85 million hectares (2).
World volume has increased approximately 10% since 1985, reflecting
a substantial increase in dietary use of the tomato. Nutritionally,
tomato is a significant dietary source of vitamin A and C. Furthermore,
recent studies have shown the importance of lycopene, a major
component of red tomatoes, to have important antioxidant properties
which reduce several cancers.
II. PRESENT GERMPLASM ACTIVITIES
Public tomato germplasm collections in the U.S. are currently
held at two locations. The P.I. collection is maintained at Geneva,
N.Y. This collection contains 5,318 accessions with 91.8% being
L. esculentum and the rest wild species. Presently
7.3% of the collection is unavailable for distribution. In 1995
342 accessions were requested. Since 1990 an average of 643 accessions
were requested per year which amounts to 12% of the collection.
The Tomato Genetics Resource Center (TGRC) at Davis, California
presently maintains 3209 accessions which includes 1059 wild tomato
species, 963 monogenic mutants and 1187 miscellaneous genetic
or cytogenetic stocks. In 1994 3501 stocks were sent in response
to 256 requests from 196 investigators. The relatively greater
use of accessions of the TGRC over the PI collection probably
reflects the preponderance of wild species in the former which
are used for disease resistance evaluation and the better characterization
of the mutant and genetic stocks which can then be utilized for
specific studies. The National Seed Storage Laboratory (NSSL)
maintains back up stocks of approx. 95% of the TGRC stocks but
only 39% of the P.I. Collection.
III. STATUS OF CROP VULNERABILITY
The present germplasm collections have been extensively used as
genetic resources for tomato improvement. Resistance to 42 major
diseases have been reported in present world collections and 23
of these resistances have been incorporated into adapted cultivars.
New diseases or new strains of some previously known diseases
appear on a regular basis and frequently the germplasm collections
serve as the only source of resistance to these new pathogens.
Use of the vast genetic resources in Lycopersicon species has been limited by a number of factors including; reliance on chemical controls, a reduction in the number of public tomato breeding
programs over the last 15 years, the lack of a core collection, the lack of evaluation information, the
difficulty in introgression of traits from some wild species -
most notably L. peruvianum and L. chilense, and
a lack of reliable screening criteria for several traits of interest
such as insect resistance and fruit flavor.
A survey of the tomato crop germplasm committee identified a series
of tomato problems that could be resolved (or minimized) by genetic
methods (Table 1).
(1) Vegetables, 1995 Summary; USDA, Agriculture Statistics Board; Publication Vg1-2.
(2) FAO Agriculture Yearbook, 1995.
Table 1. Tomato problems where genetic improvements would benefit
Type Priority Description CommentIV. GERMPLASM NEEDS
High Verticillium wilt race 2 Bacterial canker Late blight TYLCV & other geminiviruses
Medium Powdery mildew Fruit rots Corky root rot Bacterial spot
Low Bacterial speck race 2 Spotted wilt CMV PVY Target spot Phytophthora root rot
Insects Protocols important
Medium Silverleaf whitefly Aphids Nematodes
High Cold tolerance Salinity tolerance Protocols important
Medium Heat tolerance
High Soluble solids Sugar type Flavor Need to define components
Low Color Peelability/dicing Pectin chemistry Nutritional content Blossom-end smoothness
At present the major collections of wild germplasm (USDA at Geneva,
NY and TGRC at Davis, CA) contain more than 1,200 accessions of
the 9 species of Lycopersicon and 4 related species of Solanum.
Each Lycopersicon species is represented by large numbers of accessions,
which are generally well distributed over the respective ranges.
The collections are, however, deficient in certain restricted
areas -- mostly in territory that is of difficult access. Collecting
expeditions were sponsored by IBPGR in 1980, 1984, 1986, 1987,
and 1988. Additional wild/primitive germplasm was collected in
1985 and 1995 on trips sponsored by other agencies. These forays
have covered areas that were poorly represented in existing collections.
Drs. Cuartero Nuez and Diaz of the "La Mayora" Experiment
Station near Malaga, Spain also collected valuable populations
in Peru in 1983. The TGRC assumed responsibility of increasing
and maintaining the stocks collected by these expeditions.
In view of these circumstances, the status of wild tomato germplasm
is considered good and vastly better than that of many other crop
plants. Certain remote areas in the watershed of R o Mara on are
known to harbor populations of L. peruvianum and possibly
other species. Without evaluation of these populations, it is
impossible to assess the need to mount expeditions into the difficult
and rather inaccessible terrain of their habitats.
Another aspect of collection is the acquisition of germplasm for
research projects that have terminated or are anticipated to terminate
in the near future. A great wealth of tomato germplasm exists
in such holdings, consisting of cultivars, breeding lines, genic
and chromosomal variants, and other stocks. In the recent past
it has been possible to acquire the valuable items in certain
collections; unfortunately, in others, collections were discarded
before useful items could be salvaged. It is suggested that the
Tomato CGC survey collections that could be in jeopardy now or
in the near future and request that caretakers assist the NPGS
in acquiring the most valuable items. The amount of funds to be
budgeted for such activities to cover costs of correspondence,
publicity, and acquisition should be minor.
Of particular interest from both terminated and active programs
are enhanced stocks from wild species. This is especially true
for stocks from the peruvianum complex since it is so difficult
to obtain hybrids and the first backcross. Breeders who introgress
genes select for those of interest, but the early generation lines
may contain valuable genes which were not selected for. Having
such lines available for other researchers could save them years
of time and a lot of work. The collection and maintenance of these
stocks could be a major input to the Geneva center and may require
B. Maintenance and Preservation
1. PI Collections:
The PI collection contains nearly 5000 accessions. About 500 of
these need to be regrown because of poor germination and/or (rarely)
quantity on hand. The seeds of both collections (Davis and Geneva)
are maintained at 4-5oC and 30-35% relative humidity.
Field increases with no pollination control seem to be suitable
for L. esculentum accessions, all L. parviflorum and L.
cheesmanii, and about half of the L. pimpinellifolium
accessions. The other species must be protected against outcrossing
by isolation or hand-pollinations in the greenhouse.
Twenty-four plants are used for regrows of the self-pollinating
accessions, up to 100 for the outcrossing lines.
2. Tomato Genetics Stock Center
As mentioned, the Tomato Genetics Stock Center, Davis, CA, has
3,145 accessions fairly equally divided into: wild tomato species,
monogenic mutants, and miscellaneous genetic/cytogenetic stocks.
a. Wild species. It is of no avail to acquire new, valuable
accessions of tomato species unless they are maintained properly
and the seeds stored under optimal conditions. Experience gained
in managing various collections served to instruct how these operations
can be best conducted. In respect to maintenance, an understanding
of the genetic composition and natural mating system of a given
accession is vitally important to proper management of increasing
seed stocks. The wild species most closely related to the tomato
are classified as follows in respect to their mating systems:
Autogamous: L. cheesmanii, L. esculentum, L. parviflorum
Self-compatible: L. chmielewskii, L. pimpinellifolium
Self-compatible/incompatible: L. hirsutum, L. pennellii, L. peruvianum
Allogamous (entirely self-incompatible): L. chilense, S. juglandifolium, S. lycopersicoides, S. ochranthum, S. sitians
A further breakdown is necessary within the facultative species
because their subspecific groups can differ radically in their
maintenance requirements. Thus, in L. pimpinellifolium the situation
varies from the extremes of nearly complete autogamy at the margins
of the distribution to high levels of outcrossing in the central
region, intermediate situations being found in parts of the intervening
territory. The extent of outcrossing very closely parallels the
extent of genetic variation within accessions. L. hirsutum
is an example of a facultative species in which populations of
the central area are self-incompatible, hence obligatorily allogamous
and highly polymorphic, whilst the northern and southern peripheries
are populated by self-compatible, highly uniform colonies.
Methods of maintenance must obviously be regulated according to
mating system and extent of genetic variation in order to maintain
integrity of the accessions. The highly autogamous accessions
(nearly always genetically uniform) need to be increased from
no more individuals than necessary to produce the desired seed
quantity. At the other extreme, the obligate outcrossing accessions
with very high levels of genetic variability need to be increased
from as many individual plants as possible.
Seed stocks are increased and placed under optimal storage conditions
as soon as possible after the original accession is received because
under travel/collecting conditions seeds may be exposed to conditions
unfavorable for viability. The planting for increase is made by
sampling seeds from each wild plant in order to obtain as much
of the original variation as possible. Our general practice with
this group is to crowd as many plants per container as possible
without interfering with individual seed producing capacity. These
accessions must be grown in the fall-winter greenhouse to prevent
cross-contamination and to have appropriate day lengths for flowering.
Pollen is collected by a mechanical vibrator after all plants
are in flower, aliquots taken from each plant in the population.
After the pollen mass is collected it is well shaken and used
for hybridizing flowers of all plants. Two such pollinations at
weekly intervals usually suffice to produce adequate seed supplies.
When ripe, fruits are harvested from each plant and seeds extracted
and thoroughly blended to provide as adequate as possible representation
of the original variation of the accession in question.
b. Genic and chromosomal stocks. Maintenance procedures
vary considerably between subcategories. Many items reproduce
well by automatic selfing; male steriles and other sterile types
must be propagated via heterozygotes; autotetraploids and many
genic variants require hand pollination; trisomics must be selected
from mixed populations and are mated with diploids; weak genotypes
must be maintained in greenhouses and may require special feeding,
grafting, or other care; and stocks of certain other accessions
that produce defective seeds must be replenished frequently. In
certain instances, even with optimal care, very low seed yields
are obtained. Otherwise, maintenance procedures do not differ
from the bulk of the tomato accessions.
A preliminary evaluation proposal for tomato germplasm was developed
by the Tomato CGC in 1979. The coordinated system of tomato germplasm
evaluation that was established listed 3 objectives: (1) Obtain
data on 29 high priority descriptors using a uniform evaluation
system; (2) Identify suitable sites and cooperators for the evaluation
of each descriptor; (3) Compile data, enter it in the database,
and publicize its availability. The plan was established with
the overall focus of providing information on important characters
to the user community and thus increase utilization of the collections.
Little has been done on evaluation due primarily to a lack of
funds. Many quantitative traits such as soluble solids, necessitate
evaluation in important tomato production areas, possibly more
than one, to lend credence to the results. This may be more expensive
and more cumbersome to handle but would be of more value to the
user community. It is also important that evaluation for disease
resistances that do not have a proven artificial screening technique
be carried out in regions where there is adequate disease pressure
so accurate data can be obtained. Simple metric traits such as
fruit color, plant habit, and pedicel type etc. could be collected
in Geneva or elsewhere.
Often data are obtained by researchers on traits of interest,
but reports are not given directly to the germplasm centers. Users
could often provide data on traits if they were aware such traits
were of interest. Efforts should be made to facilitate this by
correspondence from the germplasm centers. Possibly the data sheets
outlining desired characteristics could be distributed along with
The role of the public sector in tomato improvement has declined
substantially in the past two decades. At least 12 breeding positions
have been eliminated. In view of the long term nature of tomato
improvement efforts which entail use of germplasm collections,
it would appear likely that expanded use of these collections
would be encouraged by expanded funding of public germplasm enhancement
efforts. Certainly enhancement efforts are needed for difficult
areas such as insect resistance and salt tolerance where wild
species have been known to have tolerance for years but little
in the way of improved germplasm has been developed.
A. FUNDING PRIORITIES
B. SUGGESTED FUTURE TOMATO CGC ACTIVITIES
Updated June 24, 1996.