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4
Enhanced Weediness: A Major
Environmental Issue
GENE1lAI PRINCIPLES
Perhaps the single most commonly voiced concern about the
introduction of genetically modified plants ~ that it might have
the potential to inadvertently produce a new weed or increase the
aggressiveness of existing weeds (R. K. Colwell et al., 1985; Sieve et
al., 1989~.
This chapter discusses three aspects of the concern: whether the
experience with the introduction of exotic plants into new environ-
ments (sometunes with the result that a weed problem is created) is a
valid analogy for the introduction of genetically modified plants; the
potential for domesticated crops to revert to a wild or weedy state;
and the potential for hybridization between domesticated crops and
wild relatives that might create or enhance weediness.
Evaluation of these issues first requires a careful definition of
terms. The term "weed" has been variously defined, depending on
the different perspectives of ecologists, agronomists, and the public.
in this report we define a weed as an unwanted or undesirable plant
in some human environments, that is, a plant that persists In hu-
man environments but is neither a crop (used for food, fiber, fuel,
pharmaceuticals, or turf) nor an ornamental plant.
37
OCR for page 38
38
A characteristic of human environments and consequently a
strong agent of selection among weeds is frequent disturbance, as
occurs in arable fields, roadsides, foot paths, and the margins of
reservoirs. Consequently, many plants that have become persistent
weeds are species that arose earlier because their phenotypes per-
mitted them to colonize special natural environments that exist in
frequently disturbed sites. Such plants often display rapid growth, a
short life cycle, high seed production, and long-distance dispersal of
seeds (Baker, 1974~. Not all colonizers are weeds, however, nor are
ad weeds colonizers.
Some weed species have also apparently required additional char-
acters ~ order to thrive in close association with humans. These ad-
vantages include escape from biotic control agents such as predators,
pathogens, and competitors (Harper, 1965~. Such an escape is e~ec-
tive if a plant is suddenly transported far beyond its native range and
therefore the range of one or more of its enern~es. It is not surprising,
then, that in most parts of the world, including the United States,
the bulk of the weed flora are exotic plants (Holm et al., 1977; Smith,
1985; Mack, 1986), members of a species that enters a range in which
that species has not occurred before (Mack, 19853. Perhaps most
successful (most widespread, persistent, and abundant) are those
weeds that have not only immigrated, but also have a long history of
close association with human settlement (Baker, 1974~.
Whether a plant becomes a weed depends on the relationship
of the plant to its environment, especially with respect to control
mechanisms that hold the organism in balance with that environ-
ment. A plant can become a weed if it escapes control by migrating
to a new environment that lacks the factors that controlled the plant
in its original habitat. In addition, a plant reman In its original
habitat may effectively escape a particular control factor, such as
predation by a specific insect pest, by gaining a trait that imparts to
it the ability to overcome the control factor. Any added trait that
enhances performance (such as frost resistance or drought tolerance)
would also be analogous to providing the plant with an advantage
sometimes gained by plants in a new environmental range. Although
this description is theoretically valid, it is necessary to keep in mind
that there is extensive experience in these kinds of modifications in
classical breeding. So far, weediness has not resulted from the addi-
tion of the traits of pest or herbicide resistance, nor frost or drought
tolerance.
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39
THE 1lElATIONSHIP BETWEEN THE INTRODUCTION OF
E:XO TIC PLANTS AND
GENETICALLY MODIFIED PLANTS
The term exotic species, as used here, refers not only to en-
t~rely novel species in a new habitat, such as the Asian weed ku~zu
(Pueraria lobata) in the United States, but also to any species with
an expanded geographic range, even when closely related plants are
already present. In addition, exotic species usually refer to plants
whose ranges were extended as a result of human intervention.
Ecological :[rnplications of ~troducmg Plants
with Many New Waits
Exotic species may not be strictly analogous to genetically modi-
fied organisms because many exotic species differ by many traits Tom
any of their neighbors in the new environment. Consequently, the
immigrants (such as Agropyron repens, Eicchornia crassipes, Schinus
terebinthifolius) will owe their success In spread and eventual nat-
ural~zation to a suite of characters (Hohn et al., 1977; Barrett and
Richardson, 1986; Morton, 19783. Genetically modified plants that
are likely to be introduced In the near future (say, over the next 10
years) will diner by only one or a few traits from cultivated forms
already in the same environment (the introduction of glyphosate-
tolerant tomatoes).
Ku~zu Is a familiar example of a deliberately mtrocluced exotic
organism that has proven to have undesirable features. It illustrates
the public's worst perceptions of errant organisms and s~multan~
ously exemplifies an exotic organism that ~ not analogous to any
hypothetical genetically modified organism. Originally introduced
into the United States from China and haps in the late nineteenth
century for ornamental purposes, ku~zu was eventually touted as
an excellent stabilizer of soil embankments and as a forage crop on
unproductive land. Cash incentives were even provided at one time
to encourage farmers to plant it on abandoned fields (Miller, 1983~.
By the 1950s, however, detrimental aspects of ku~zu were recog-
nized, as the vine often grows far beyond the site of its local mtro-
duction. It now commonly grows over forest canopies and telephone
lines ~ the southeastern United States. Ku~zu's success is based on
a combination of features: it readily propagates vegetatively, it can
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40
grow on infertile soil and low amounts of soil water (Forseth and Ter-
amura, 1987), and it has few (if any) serious parasites or herbivores
in its new environmental range.
Ku~zu exemplifies how the combined action of many traits mtro-
duced into a new environment results in a weediness problem. Our
knowledge of invasions and particularly the characteristics that spell
success or failure for immigrants is limited (Harper, 1982; S~mberIoff,
1985), despite the attempts to identify the putative characteristics
of successful weeds (Baker, ?986; Bazzaz, 1986~.
Ecologically l~portant Changes that Result
from Smog Genetic Alteratiom
Even though exotic species such as ku~zu are not strongly analo-
gous to genetically modified plants, circumstantial evidence suggests
that a change in only a few characters can sometimes make a plant a
successful invader. Within the large grass genus Bromus are several
annual species that have become successfully naturalized In different
temperate regions. Bromus tectorum spread rapidly In the interior
Pacific Northwest in the early part of this century, whereas other
members of the genus such as B. motlis and B. iTizGeformis are
much less common even though they were introduced earlier (Mack,
1981~. In contrast, B. mollis is much more prominent than B. tec-
torum in the Central Valley of California, and B. secalinus can be
a serious weed of cereals in northern Europe (Salisbury, 1961~. The
differences among these closely related species that explain their var-
ious success in new environmental ranges may be related to different
tolerances to frost (B. rigidus and B. rubens are less tolerant than B.
tectorum) and different flowering times (B. japonicus flowers before
the onset of drought) (Hulbert, 1955~. These species are morpholog-
ically similar and also share many ecologically important traits, yet
they differ in their degree of success in their new ranges.
The exotic woody genus Casuarina provides another example in
Florida. The two species, C. equisetifolia and C. glauca were de-
liberately introduced into southern Florida. The first has become a
serious pest, while the second persists only locally. The most appar-
ent difference between these closely related species Is the inability of
C. glauca to produce seed in the new range (Morton, 1980), which
thereby limits its dispersal.
Other examples of environmentally important single-trait
changes are demonstrated by the spread of Chondritia juncea ire
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41
Australia and the role of insect herbivory in influencing the compet-
itive ability of barley. Chondrilia juncea (skeletonweed) is a serious
weed In the wheat-grow~g regions of southeast Australia. It has
three morphological forms In Australia termed A, B. and C that
differ most obviously in leaf shape, flower morphology, and fruit char-
acter~stics. Before a biological control program wan initiated ~ the
early 1970s, form A was much more widespread than the other two.
But form A has proven to be much more susceptible than forms B
and C to the deliberately released rust fungus, Puccinia chondrillina.
As populations of form A have become infected, they have become
less competitive than they hack been, and their range has declined.
Much of the range vacated by form A has been filled concorn~tantly
with forms B and C (Burdon et al., 1981~.
A similar reversal of competitive roles has also been documented
between two cultivars of barley (HOT]eUm vulgare) that display a
difference in their resistance to the aphid Schizaphis gTaminum. Un-
der greenhouse conditions the aphid-res~st ant cultivar competes less
weD in mixtures of the cultivars. If the aphid is introduced into the
mixtures, the competitive advantage of the susceptible cultivar ~ lost
(Windle and Franz, 1979a; Winnie and Franz, 1979b).
These examples illustrate that small genetic differences be-
t~veen closely related plants con produce phenotypes with different
ecological properties that can increase or alter a plant's geographic
range or enhance its aggressiveness in its normal range. How likely
is this phenomenon for genetically mollified crops or other plants
berg considered for field testing? Although most ecologically im-
portant traits remain unchanged, the interaction among these traits
determines whether a species will become naturalized in a region.
For example, a species could spread because it tolerates herbivores
and parasites and tolerates some aspects of the physics environ-
ment (such as salinity) in the new range. Gottlieb (1984) compiled
a list of diverse traits in plants that can be governed by one or a few
genes. Whether the plant is erect or prostrate, branched or not, an
annual or a biennial, or bears its leaves basally or higher on the stem
can all be governed by a few genes. The suggestion from this list
of traits is that major changes in plant architecture and subsequent
performance could be achieved through rather small gene changes or
insertions by recombinant techniques. Such changes in architecture
would be readily detectable ~ greenhouse and field tests. The likeli-
hood that these changes would occur randomly (and be retained) is
very small.
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42
~ ~ ~e
We do not know to what extent successful naturalization of
exotic organisms hinges on their possession of one or a few traits
rather than a group of characters. Multiple genes inducing multiple
crates Snouiu Increase the probability of assembling an organism that
can cause ecological changes if grown on a large scale.
Several exotic species (for example, Cytisus scoparius, Ales eu-
ropaea, Leucaena [eucocephala) owe much of their successful natural-
ization to their ability to fix nitrogen in a new environment that is
chronically low in nitrogen (Vitousek, 1986~. Nitrogen fixers, such as
Ain?`s spp., characteristically are the first invaders on newly formed
volcanic soils. Myrica faya, a smut exotic tree, is rapidly altering the
nitrogen balance on volcanic sites in the Hawaiian islands. As the
nitrogen content of the volcanic soil has increased, new species have
become established on these sites (Vitousek, 1986~.
Relatively minor genetic changes can produce plants with altered
ecological properties, a phenomenon plant breeders have capitalized
on for decades; for example, introducing a single gene in wheat
can impart resistance to a specific race of stem rust. Similarly,
herbicide-res~stant canola and soybean plants have been produced by
minor genetic changes. Such changes have not resulted in increased
weediness of these widely used crops.
TElE ABII`ITY OF CROPS TO REVERT TO A WILD OR
WEEDY CONDITION
Crops that have been subjected to long-term breeding (for
example, beans, maize, and wheat) are less likely to revert to a
wild state than crops that retain many wild characters (artichokes,
forage grasses, and grain amaranths). Highly domesticated crops
have lost their ability to compete effectively with the wild species in
natural environments. Domesticity arises because many characters
that would enhance weediness (seed shattering, thorns, seed dor-
mancy, and bitterness) have been deliberately elimunated from the
crop plant through intensive breeding efforts. The reassuring history
for cultivated crops does not completely preclude a genetically modi-
fied crop from becoming weedy, but it suggests that the likelihood of
that event is small. As new traits are inserted into cultivated crops,
they might possibly change the crop in an ecologically significant
way, but past experience with classical breeding h" shown this to be
a manageable problem. Field trials should identify such possibilities.
The descendants of crops may become weeds ~ agricultural
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43
fields, and in some circumstances they may move beyond the bound-
aries of the field and become weeds ~ sern~natural or even natural
communities. More than a decade ago, Harlan (1975) compiled an
often-cited list of the wild races of crops that included many row
crops such as beets, cabbages, and watermelons. The relevance of
these examples depends, in part, on the level of domestication in the
crop.
Some crops such as artichoke, sugar beets, and some citrus
(Gade, 1976; Pickersgill, 1981; Thomsen et al., 1986), seem prone to
become weedy. The ability to revert to a weedy condition has never
been attributable to traits deliberately retained in the domesticated
crops-that is, traits that have been the object of an active breeding
program.
HYBRIDIZATION BETWEEN CROPS AND TH1:IR
WILD RELATIVES
Two closely related ecological questions that may be important
to the introduction of genetically modified plants are (1) Does hy-
bridization between crops and their wild relatives result in transfer
of traits from the cultivated form to the wild relative? and (2) Does
such gene flow increase the weediness of wild relatives? If the oh
portunity exists for the transfer of genetic traits from a genetically
modified organism to a wild (and potentially weedy) relative, a po-
tential problem exists. The problem poses three relevant questions:
(1) Does the genetically modified crop have extant relatives? (2)
What is the extent of hybridization between crop and relatives in
nature? and (3) What is the current ecological role of the relative in
natural ecosystems?
Practically all crops have wild relatives at some taxonomy level.
The more important question Is whether wild relatives occur in the
range in which the genetically modified crop is grown or wiD be
grown. The answer varies, as no one region of the world includes the
home range of most crops, although arid central Asia and Asia Minor
are the centers of origin for many crops (Table t13. Southeast Asia
includes the home range of many weeds. Temperate North America,
especially the United States, includes the home ranges for very few
crops, as U.S. agriculture is based largely on crops of foreign origin.
This paucity of crops derived from North American sources means
there will be relatively few opportunities for hybridization between
crops and ward relatives in the United States, except where both
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44
TABLE 4-1 Crops and Their Probable Regions of Origin. tNote That
Comparatively Few Crops Are Native to North America)
Crop Scientific Name
Common Name
EUROPE AND TEMPER-ATE ASLA
Cereals Avena sating L. Oats
A. stri~osa Schreb. Fodder oats
Hordeum ~ruI~are L. Barley
Secale cereale L. Rye
Triticum aestivum L. Bread wheat
Pulses Cicer arietinum L. Chick-pea
Lens esculenta Moench Lentil
Pisum sativum L. Garden pea
Vicia faba L. Broadbean
Root and Beta vul~aris L.
tuber crops Brassica raPa L.
Daucus carota L.
Raphanus sativus L.
Beet, manger, chard
Turnip
Carrot
Radish
Oil crops Brassica camPestris L. Rapeseed
Carthamus tinctorius L. Safflower
Linum usitatissimum L. Flax, linseed
Olea eurouea L. Oli
Fruit and Ficus carica L. Fig
nuts Ju~lans retrial. English walnut
Phoenix dactvlifera L. Date palm
Prunus amY~dalus Stokes Almond
P. armeniaca L. Apricot
P. atrium L. Cherry
P. domestics L. Plum
Pvrus communis L. Pear
Vegetables Cucumis melo L. Melon
and spices Allium cePa L. Onion
A. sativum L. Garlic
Brassica oleracea L. Cabbage, cauliflower,
Brussels sprouts, kale,
kohlrabi, broccoli
Cucumis sativus L. Cucumber
Lactuca sativa L. Lettuce
Forage crops Bromus Dermis Leyss. Smooth bromegrass
DactYlis ~lomerata L. Orchardgrass, cocksfoot
F`estuca arundinacea Schreb. Tall fescue
Medicazo sativa L. Alfalfa
Phleum pretense L. Timothy
Trifolium spp. The true clockers
Drug crops Digitalis Purourea L. Digitalis
Papaver somniferum L. Codeine, morphine, opium
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45
Table 4-1 (continued)
Crop Scientific Name Co~runon Name
AFRICA
Cereals Orvza ~laberrima Steud. African rice
Pennisetum americanum (L.) Pearl millet
K. Schum.
Sorghum bicolor (L.) Moench Sorghum
Pulses Vienna un£uiculata (L.) Walp.
Root and Dioscorea cavenensis Lam.
tuber crops
Cowpea
Yam
Oil Crops Elaeis zuineensis Jacq. Oil palm
Ricinus communis L. Castor oil
Fruits and nuts Coloc~rnthis citrullus (L.) Watermelon
Fiber plants Goss~rPium herbaceum L. Old world cotton
o
Forage crops C`rnodon dactvlon (L.) Pers.
Dizitaria decumbenn Stent
Era~rostis lehmanniana
Panicum maximum Jacq.
Drug plants Scoffed arabica L.
Cereals and
Bermuda grass
Pangolagrass
Lovegrass
Guineagrass
Coffee
CHINA
Fazop~rum esculentum Moench Buckwheat
pseudocereals Organza sativa L. Rice
Panicummiliaceum L. Proso millet broomcorn
,
millet
,Setaria italica (L.) Beaux.
Pulses Glycine max (L.) Merr.
Root and Brassica raps L.
tuber crops Dioscorea esculenta (Lour.)
Italian millet,
foxtail millet
Soybean
Turnip
Chinese yam
Oil Crops Brassica camuestris L. Rapeseed
B. iuncea (L.) Czern. & Coss. Mustard seed oil
Vegetables Alium baker) Regel Chinese shallot
and spices ,Cinnamomum cassia Blume Spice
Cucumis satires L. Cucumber
Zin~iber officinale Roscoe Ginger
Drug plants Camellia sinensis (L.) Ktze.
Cinnamomum camphor (L.)
1
Tea
Camphor tree
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46
Table 4-1 (continued)
Crop Scientific Name
Common Name
SOUTHEAST ASIA AND PACIFIC ISLANDS
Cereals and Orvza sating L.
pseudocereals
Oil crops Cocos nucifera L.
Sesamum indicum L.
Fruits and nuts
Vegetablen Elettana cardamomum (L.)
and spices Maton
SvzY~ium aromaticum (L.)
Merr. & Perry
M~ristica fra~rans
Piker ni~rum L.
Solanum melonzena L.
Citrus aurantiifolia Swingle
C. aurantium L.
C. limon (L.) Burm. f.
C. nobilis Lour.
C. paradisi Macfad.
C. sinensis (L.) Osb.
Musa acuminta Colla
M. balbisiana Colla
Starch and
sugar plants
(not roots)
Cereals Zea maYs L.
Fruits and nuts Anacardium accidentals L.
Rice
Coconut
Sesame
Lime
Sour orange
Lemon
Tangerine
Grapefruit
Sweet orange
Banana (A genome)
Plantain (B genome)
Cardamom
Closure
Nutmeg
Black pepper
Eggplant
Saccharum officinarum L.
Sugarcane
MESOAMERICA AND SOUTH AMERICA
Corn
Cashew
Ananas comosus (L.) Merr. Pineapple
13ertholletia excelsa HBK. Brazil nut
Papaya
Gray papaya
Avocado
Guava
Carica papaya L.
Carica candicans A.
Persea americana Mill.
Psidium zuaiava L.
Vegetables
and spices
Capsicum annuum L.
Capsicum baccatum L.
Cucurbita maxima L.
Cucurbita pepo L
.
Phaseolus Paris L.
LYcopersicon esculentum Mill.
Solanum tuberosum L.
Vanilla planifolia Andr.
Pepper
Pepper
Squash
Squash, pumpkin
Bean
Tomato
Potato
Vanilla
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47
Table 4-1 (continued)
Crop Scientific Name
Common Name
Fiber plants Gossynium hirsutum L.
Drug plants Nicotiana tabacum L.
Theobroma cacao L.
NORTH AMERICA
Oil crops Helianthus annus L.
Upland cotton
Tobacco
Cacao, chocolate
Sunflower
Fruits and nuts Vitis labrusca L. Fox grape
V. rotundifolia Michaux. Muscadine grape
Vaccinium macrocarpon Aiton Cranberry
Vaccinium (several species) Blueberry
Fra~aria several species) Strawberry
Rubus idaeus Richardson Red raspberry
Rubus (several species) Blackberry
Rubus (several species) Dewberry
Vegetables Helianthus tuberosus L. Jerusalem artichoke
ADAPTED FROM: Harlan, 1975.
crop arid wall relatives have immigrated (Table 12~. The incidence
of hybridization between genetically modified crops and wild relatives
can be expected to be lower here than in Asia Minor, southeast Asia,
the Asian subcontinent, and South America, and greater care may
be needed in the introduction of genetically modified crops in those
regions.
If a crop has no relatives within the distance its pollen can travel,
no hybrids will develop. Spatial separation Is an obvious barrier to
hybridization, but only anecdotal knowledge exists on the actual
limits of pollen transport (Elistrand, 1988~. Furthermore, even if
relatives are nearby, there Is no assurance that viable hybrids wid
be produced, as there often are many formidable barriers to gene
flow, such as differences in ploidy level, flowering time, tends breeding
systems (Sirnmonds, 1979~. ~ fact, the deliberate introduction of
genes from wild relatives into certain crop species by classical breed-
ing techniques has been achieved only by manipulating the flowering
tune and by repeated hand pollination (as in potatoes). Even if
fertilization is accomplished naturally, there is no assurance that
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48
T~LE 4-2 Some C~ps Growing SympatricaHy in tbe Onited States witb
Congene" or Wild Races with Wbicb Natur~1 Hyb~dization Is Possible.
C=p
Prim~ Ceue Pool
Sor~um bicolor (so~bu=)
RsDh~us sst1vus (radisb]
SetaHa itallca (~xt~1 ~uet)
Br~sica r~s (tumip)
Br~slca c~mnestris (rape)
Amaranthus c~entus (am~rantb];
^. c~udatus; ^. bvoochondriscus
Bets vul~ds (beet)
D~ucus carota (~t]
"eliantbus annuns (sunDower)
Cucurblts oeno ~qu~b, pumpkin]
Bec~le cereale (~e)
L~ctucs sativ~ (~ttuce)
~ens satlva (oat)
Cvnara scol~mus (~hoke)
S. halenense (Jobnson ~-sj
R. r~ohanlst~m (wild rsdiab)
S. italics ~equently naturahzed
~ ~ weed, m~ not ~st in the
United States
B. c~mpestHs (~ ~)
B. camDest~s (~Hd ~,
Deld mustard}
8. hvbrldus; A. oowellIi;
A. retronexus
- h~e o~ w~dy ~e in
Europe
D. cs~ta soot c~ota
H. annuns (wild ~o~bs);
H. bolande~
C. tex~na (w~d ~)
S. cereale ~d S. montanum
L. se~ols
A. ~tus (w~d o~)
C. scolvmus (wild types)
~TED P ILY FRO~: N. W. Si~ond~ ed~ 1979, and ~~renc"
tbe~in.
~rtber plants w1D be produced. ~r a gene to p~s be~en rel~
tlve and crop ~nd be permanently lncorp grated luto eltber tbe crop
or tbe rel~lve] ~trogre~lon (~troducdon ~ a gene ~om one gene
co~lex luto ~otber) ~st occur regul~ly (^derson' 1949j. Ibls
occurs ~ exceedlugly 1~ hequency ln many crops ~d w~d relative
. . .
comDm~lons.
Evldence kr gene introgress10n by bybrldlzatlon be~en crops
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49
and wild relatives has often been only circumstantial. Because plant
breeders are usually concerned with the detection and elunination
of wild traits in a crop, the low incidence of documented transfer
and ~ntrogression that occurs from crops to wild relatives may be
an artifact. A complication in reliably identifying such Retrogression
continues to be the possibility of convergent evolution between crop
and wild relatives. The mechanism by which this could occur is ease
fly envisioned: Art agronorn~c practice such an seed sorting by size
imposes strong directional selection in a wild relative (or even an
unrelated weed) for those phenotypes with the same seed size as the
crop (Barrett, 1983~. Seed size, shape, and even color can be re~nark-
ably similar between the crop and the weed without hybridizations
occurring.
Forty years ago, plant breeders In India selected for increased
anthocyan~ production in cultivated rice in an attempt to Prove
the ability of paddy workers to d~scri~nate between otherwise in-
distinguishable seedlings of cultivated and wild rice (Oryza species).
Although the cultivated rice seedlings were readily identified at first
by their purple leaves, within several plant generations the trait had
been transferred to the wild relative, thus rendering the trait use-
less from a cultivation standpoint (Parker and Dean, 1976~. Other
putative examples of gene flow from crop to wild relative have been
reported for crosses between corn and teos~nte, Eastern Carrot"
and wild carrots, ~kayseri" alfalfa and weedy relatives, and between
durum wheat and wild emmer wheat. The evidence Is mainly mor-
pholog~cal and therefore subject to alternative interpretations (for
example, convergence after mutation in the wild relative and subse-
quent directional selection) (Small, 1984~.
Other examples, also based largely on morphological evidence,
occur among the cultivated Amaranthine ca?~datus, A. cruentus, and
A. hypochondriacus. Each of these species forms hybrids with one or
more weedy amaranths ~ California and Mexico. Gene flow to the
weedy amaranths is probably more obvious and persistent because of
the strong selection by hand-cultivation against the preservation of
hybrids with the wild parent's trait of dark seed (Saner, 1967; Tucker
and Sauer, 1958~. ~ the Sacramento-San Josquin delta, Tucker and
Saner (1958) identified many amaranth hybrids that resulted from
crosses between crop add wild relatives. They maintained, without
direct evidence, that under cultivation in the light, highly fertile
organic soil in the region, hybrids could out-compete their weed
parents (A. hybridus, A. powellii, and A. retropexus) because they
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so
had acquired traits from their crop parents for more robust stature
and high fecundity.
Gene flow has apparently occurred from crop to wild relative in
rye (Secale spp.) in California, where a weedy rye probably derived
from a cross between S. cereale and S. montanum has become increas-
ingly corklike through introgression with the cultivated S. cereale.
This introgression has proceeded to such an extent that farmers have
abandoned efforts to grow cultivated rye for human consumption and
are deliberately sowing the hybrids for forage (Jain, 1977; Suneson
et al., 1969~. ~ each of these examples, the putative transfer of
a trait from the crop to the wild relative has resulted In the relax
fives' becoming more similar to the crop; ~ the above-cited example
with Asian rice, the ~ntrogression resulted only in an enhancement
of mimicry of the crop.
Evidence ~ restricted to morphological or cytological similari-
ties between the crop and the wild relative. However, much of the
evidence Is circumstantial rather than exper~rnental; clear demon-
stration of introgression depends on molecular analyses of isozymes
or other techniques. Recent work with molecular marker loci has
refuted several earlier claims of ~ntrogression in Helianthus (Riese-
berg et al., 1988) and some reports of introgression between maize
and teos~te (Doebley, 1984~. Even with ~sozyme studies there Is the
possibility for an alternative interpretation; the crop and the wild rel-
ative may share aDeles derived from a common ancestor rather than
through more recent introgression. Consequently, the best evidence
for recent gene transfer arises in cases in which a wild relative pos-
sesses alleles in common with a crop, but only in those populations
that have recently come into sexual contact with the crop.
Convincing evidence for a transfer of genes from a crop to a
wild relative does exist in several crowed complexes: African rice,
maize, and Cucurbita. Second (1982) hap shown that African rice,
Oryza brev:7igulata, contains more isozym~c variation than cultivated
rice, O. glaberrima. His data suggest that this variable weedy rice
arose through introgression between the wild form and cultivated
rice. Doebley (1984) found evidence for introgression of cultivated
maize into Zea diploperennis; one plant possessed two alleles that had
not been found previously ~ the wild species but that are common
in maize. Because the two loci are tightly linked, there is at least the
strong suggestion that the chromosome segment carrying these loci
was transferred through hybridization Alto Zea diploperennis. In the
southern United States the cultivated Cucurbita pepo (squash) occurs
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in the same area as the wall species, CUCUTbi~a texana (Texas gourd).
Decker and Wilson (1987) found that alleles typical of the cultivar
can occur in the wild species. This introgressiorr enhanced weediness
in the sense of making the hybrids more difficult to distinguish from
the crop (that is, their mimicry of the crop increased), but the hybrid
was no more aggressive, nor did it have an enhanced ecological range.
Consequently, the products of the inadvertent transfer of crop genes
to relatives have been confined to the field in which the plants were
grown. From the standpoint of eradicating the weeds, the result of
this introgression is at worst undesirable.
The hybridization between cultivated sorghum and one or more
of its wild relatives is more serious. "Hybrid grain sorghums (SOT_
chum bicolor,} Is produced through the cytoplasmic male-sterility
method In which two inbred lines are hybridized. The seed is har-
vested from the male-sterile plant. If pollen of one or more weedy
sorghums is inadvertently allowed to fertilize the stigmas of the male-
sterile plants, the offspring are useless commercially and represent
a genetically diverse cluster of races and "off-types called shatter-
cane. These plants usually express many traits of the wild parent,
such as the perennial habit (inherited from Sorghum halepense),
height, or self-sow~g seed (Baker, 1972), a trait inherited from
Sorghum sudanense.
Hybrids bearing traits of S. halepense (Johnson grass) present
potentially serious weed problems because the vegetatively vigor-
ous S. halepense Is eradicated only with great difficulty and expense
(Holm et al., 1977; Warwick and Black, 1983~. The direct role of
introgression with the cultivated sorghums In the enhancement of
weediness in S. halepense ~ not clear, but the circumstantial evi-
dence at least suggests the production of more persistent plants. De
Wet (1966) maintains that S. halepense in its native range in the
01d World has never been an excessively weedy plant and that its
weediness was enhanced coincidentally with its introgression with
cultivated sorghum in the United States. Acquisition of these traits
is unusual in that their advantage to the weedy offspring ~ not con-
fined to enhancing the weed's mimicry of the crop. If these Johnson
grass populations extend their already major ecological role outside
agricultural fields, they will represent the most extreme category of
known risk associated with gene flow from crop to weedy relative.
Biotypes of S. halepense in the northeastern United States apparently
have acquired traits of ecological importance through introgression,
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52
including such crop-like features as earlier flowering, greater seed pro-
duction, larger individual seed weight, and subsequently more rapidly
emerging seedings than other biotypes (Warwick et al., 1984~.
The male sterility method produces a similar, although less ~e-
rious, weed problem In the cultivation of sugar beets for seeds in
northern Europe. If these mal~sterile plants are inadvertently polli-
nated by the pollen of Beta v?~Igaris subsp. maritime (wild sea beet),
some cultivar x wild Fit hybrid seed eventually wait be produced in
the crop field (Pickersgill, 1981~.
While hybridization between a crop and its wild relative may
not be prevented by morphological, cytological, and developmental
barriers, there is little likelihood that domesticated traits will be
retained In a wall relative. Much of the emphasis in plant breeding
has been toward traits that would reduce adaptation to the wall
(for example, enhanced of} content in the seed, or an enlarged fleshy
root), especially if enhanced production for these features came at
the expense of plant fitness. Important cornmercia] trmts, such as
pest resistance, that have the potential to alter the ecology of wall
relatives have not been a problem with the possible exception of gene
transfer from cultivated sorghum to Johnson grass.
SUMMARY POINTS
1. The analogy between the introduction of an exotic species
into a new environment and the introduction of a genetically modi-
fied crop plant is tenuous because introduced exotic plants that have
caused problems bring with them many traits that enhance weedi-
ness, whereas genetically modified plants are modified in only a few
characteristics.
2. Genetic modifications of only a few genes can produce a
modified plant with significant, ecologically important alterations.
However, genetically modified crops are not known to have become
weedy through the addition of traits such ~ herbicide and pest
resistance.
3. Domesticated crops, such ~ soybeans, corn, and wheat,
have been genetically modified to such an extent that they can no
longer compete effectively with wild species in the natural ecosystem.
These crops are unlikely to revert to a weedy condition upon further
genetic modification. Some forage grasses are more likely to revert
to a weedy condition.
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53
4. Most crop plants in the United States are not native, and,
unless weedy close relatives have been imported, no close relatives
with which the crop knight hybridize are present. However, where
cross-hybridiz~ng wild relatives do exist ~ close proximity (such as
the sunflower), precautions may be necessary to limit gene flow from
the crop to the wild relative. Gene introgression, when demonstrated,
has often caused the wild species to become more like the crop,
with consequences of enhanced weediness of the wild relative largely
confined to agricultural fields.
Representative terms from entire chapter:
wild relative
