Summary
Plant Invasion and Inter-Specific Hybridization
Summary
Forces affecting the rate of spread and increase of hybrids between newly introduced
commercially bred crop plants or introduced exotic species and their related species are
qualitatively similar. However, the precise effect of the forces on the likely
environmental impact of such hybrids, may depend strongly on the nature of the gene
or genes introduced into the native species via breeding or the specific characteristics
of the introduced exotic species.
The qualitative similarity between exotics and the products of conventional or
evolutionary modification suggests that a historical view of the environmental impact of
hybrids on traditionally produced crops or exotic species and their relatives would be of
use in estimating the probable fate of hybrids containing transgenes in the environment.
However, with certain classes of transgenes for which there are no existing analogues,
there will need to be greater care in assessing the possible risks associated with
release into the environment.
Plant Invasion and Inter-Specific Hybridization
Interspecific hybridization is one of the major mechanisms for generating new taxa in
the plant kingdom (Stebbins, 1959, Stace, 1975, Grant, 1981). There are many cases in
which hybridization between an indigenous and alien plant species has resulted in a
new, sexually fertile taxon. One well-documented example of this is the result of a
hybridization event between Senecio vulgaris, common groundsel, and S. squalidus,
the Oxford ragwort, (see Abbott, 1992, for a review). S. vulgaris is a native herb of the
British Isles. The common form, S. vulgaris var. vulgaris, produces a capitulum
containing only disc florets, which tend to be highly self-fertilizing and to produce
relatively non-dispersed seed. A more recent form, S. vulgaris var. hibernicus, differs
from the common form in that ray florets are produced in the capitulum. Seed obtained
from ray florets tends to be more outcrossed and to disperse further than that of disc
florets. The difference in capitulum type is due to variation at a single locus (Abbott et
al., 1992).
The first report of wild radiate groundsel was in 1832 from Oxford, England, following
the escape from the Oxford Botanic Garden in 1794 of the radiate species S. squalidus,
a native of Sicily. Since this time, S. squalidus has spread widely throughout the British
Isles, often following railway tracks, whose limestone ballast provides a well-drained
medium somewhat reminiscent of its native Sicily. At the present time, S. squalidus is
often found growing alongside S. vulgaris in open, disturbed habitats. Highly self-sterile
hybrids (S. x baxteri) are formed at low frequency with S. vulgaris as the female parent
but backcrosses to S. vulgaris are fertile. The hybrid radiate groundsel, S. vulgaris var.
hibernicus, is postulated to be a stabilized introgressant, resulting from repeated
backcrossing of S. x baxteri with S. vulgaris var. vulgaris, on the basis of three lines of
evidence: (1) the strong similarity of the pattern of spread of S. squalidus and S.
vulgaris var. hibernicus throughout the British Isles over the past 160 years; (2)
tetraploid plants, produced by backcrossing S. x baxteri to S. vulgaris var. vulgaris, bear
a close resemblance to S. vulgaris var. hibernicus; and (3) an allozyme marker, absent
from non-radiate groundsel populations but present at high frequency in S. squalidus, is
found at intermediate frequencies in radiate groundsel.
Another species appears to have resulted from the original self-sterile hybrid, S. x
baxteri, namely S. cambrensis. In this species, the original sterility barrier has been
overcome by a doubling in ploidy from the hybrid (2n=30) to S. cambrensis (2n=60).
Evidence from isozymes and cpDNA (Ashton & Abbott, 1992, Harris & Ingram, 1992)
has confirmed the allopolyploid nature of S. cambrensis and shown it to have originated
in at least two separate localities.
The example above is one of several indicating that self-sterility of a hybrid is not a
sufficient barrier, per se, to invasion by genes derived from an alien into an indigenous
species. Indeed, the production of fertile allopolyploids and stabilized introgressants are
well documented, and not uncommon, phenomena whereby genes from exotic species
have been combined with those of the native flora.
The possible influence of the hybrid on existing plant communities will be discussed in
several sections: a) a perspective on the origin and evolution of crop plants, b) plant
invasion and inter-specific hybridization, c) the effects of evolutionary forces on the fate
of hybrids and d) the effects of structure of the indigenous recipient plant population on
the rate of spread and increase of a hybrid.
