Biology Reference
In-Depth Information
occurred to the Canary Islands. At least two introductions have also
occurred to Argentina, both from localities in eastern Europe.
One result of the multiple introductions of cheatgrass is a mosaic pat-
tern of population genetics (Novak et al. 1993). For example, cheatgrass
populations in southern British Columbia possess high frequencies of a
genetic marker derived from a source area in eastern Europe. In eastern
Washington, populations possess high frequencies of a genetic marker of
unknown origin. In spite of the reduction in genetic variability that usu-
ally characterizes individual founder populations, genetic variants from
different parts of the native range also may come together as populations
grow and spread. Thus, in north-central Washington, cheatgrass popula-
tions possess both of the above markers.
Multiple introductions are also implicated for a cladoceran known as
the long-spined water flea (
Bythotrephes longimanus
). Changes in the fre-
quency of various gene loci between 1989 and 1996 demonstrated that
there has been a continuing introduction to North America of water fleas
from Lake Ladoga in western Russia. The present genetic structure of
North American
B. longimanus
is more similar to that of the organisms in
Lake Ladoga than it was originally (Berg et al. 2002). The genetic struc-
ture of long-spined water flea populations in 1989 showed strong founder
effects, including an excess of heterozygote genotypes at one gene locus.
By 1996, the genetic structure of the North American populations closely
matched that of populations in Lake Ladoga, and the frequency of
homozygote and heterozygote genotypes matched the Hardy-Weinberg
expectation for stable populations.
One of the most interesting and complex patterns of introduction of
an alien species to much of the world involves the Mediterranean fruit fly
(
Ceratitis capitata
). Gasparich et al. (1997) examined nucleotide variation
in portions of the mitochondrial gene for NADH dehydrogenase for flies
from more than 100 populations in Europe, Africa, North and South
America, and Australia.The greatest diversity of haplotypes was found in
sub-Saharan Africa, where the number of haplotypes, or individual chro-
mosomal arrangements, was greater than in all other areas of the world
combined. Mediterranean fruit flies became established around the
Mediterranean Sea in the early 1800s, and their populations now show
only two common haplotypes of those represented in sub-Saharan Africa.
By the early 1900s, Mediterranean fruit flies had invaded the New World.
The pattern of haplotypes in Central and South America is complex.
Most populations in Central America and northern South America pos-
sess only a single haplotype, but the specific haplotype is different in Cen-
