Biology Reference
In-Depth Information
colonies are derived from a few individuals, however, and the flowering
times of the different patches vary considerably. Although inbreeding
depression is high in many self-fertilizing plants, many highly self-fertile
clones exist and carry out most of the seed production in the overall pop-
ulation. The relatively unoccupied mudflat habitat also favors successful
germination and growth of seeds produced by selfing plants that might
have failed in their native range on the Atlantic coast. In this case, selec-
tion is apparently favoring self-fertilization, at least during the early stages
of the invasion process (Daehler 1999).
Selfing mating systems, in general, have been postulated to evolve in
species that follow a strategy of colonization of new or newly created
habitats (see, e.g., Holsinger 1988). Genetic bottlenecking that results
from small founder populations may eliminate the advantage of genetic
recombination and thus favor increased certainty of reproduction through
selfing.
On the other hand, shifts toward increased outcrossing have been doc-
umented in a number of introduced plants (Brown and Marshall 1981).
In Australia, soft brome (
Bromus mollis
) and subterranean clover (
Tr ifolium
subterraneum
), both primarily selfing plants, have shown increases in out-
crossing rates.This shift presumably increases the level of genetic recom-
bination, which may increase adaptability of these species to local envi-
ronmental conditions.
Rapid adaptation of seed production and germination characteristics
of several plants have been documented. Common groundsel (
Senecio vul-
garis
) exhibits a variety
vulgaris
that occurs in both ruderal habitats and
agricultural habitats, such as cropland, orchards, and plant nurseries. Pop-
ulations in these two habitats are genetically differentiated ecotypes (Leiss
and Müller-Schärer 2001). The ecotype from agricultural habitats pro-
duced more flowering heads and, when fertilized, showed a greater
increase in seed production than the ruderal ecotype. Similar results were
obtained for common chickweed (
Stellaria media
) by Sobey (1987).
The Chinese tallow tree (
Sapium sebiferum
) was introduced to Georgia
in the late 1700s. Subsequently, in the early twentieth century, trees were
introduced to Louisiana and Texas from an undocumented source. The
latter introductions have proven invasive, in contrast to the Georgia plants
(Siemann and Rogers 2001). In a common garden experiment in which
trees were grown from seedlings for 14 yr, 60-70% of those from
Louisiana and Texas began to set seed, whereas less than 20% of those
from Georgia or from Taiwan, where the species is native, became repro-
