Genetic analyses can also reveal information about the number of introductions of individual alien species. For many aliens, multiple introductions are the rule. The numerous opportunities for transport of both aquatic and terrestrial plants and animals make multiple introductions increasingly frequent. In Australia, for example, the discovery of three new populations of the invasive marine alga Caulerpa taxifolia in New South Wales, Australia, led to a series of analyses of ribosomal DNA to try to determine their sources (Schaffelke et al. 2002). One was confirmed as originating from populations along the coast of Queensland.The remaining two populations, which are different from each other, apparently came from other sources, as yet unknown.
One of the best examples of multiple introductions of an alien plant comes from ongoing studies of cheatgrass (Bromus tectorum). As noted in chapter 3, cheatgrass has a wide native distribution in the Old World. Novak and Mack (2001) examined samples from 164 localities throughout the species' native and introduced ranges, using allozyme elec-trophoresis.These studies led to the identification of a number of genetic markers that are specific for locations in the species' native range. Using these markers, they are able to infer the source areas of populations in regions to which the species has been introduced.
In North America, at least seven or eight independent introductions of cheatgrass have occurred (Novak and Mack 2001; Bartlett et al. 2002). Most of these seem likely to have been direct introductions to western North America, since the genetic markers do not occur in the East.Two may have been to eastern North America, with subsequent spread westward to the Pacific Northwest. In other areas of the world, at least two introductions, from the Iberian peninsula and Morocco, appear to have 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 pattern 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 usually 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 populations 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 frequency 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 structure 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 chromosomal 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 possess only a single haplotype, but the specific haplotype is different in Cen tral America,Venezuela-Brazil, and the region from Colombia to Peru. In Argentina, a still different haplotype, unique to this world region, is com-mon.These patterns indicate that multiple introductions have occurred to the New World from the Mediterranean region or Africa. Single haplo-types occur in fruit fly populations in Hawaii and Australia. Australian populations might have been founded by flies from the Mediterranean region or the Andean region of South America. Hawaiian populations probably came from the Venezuela-Brazil area of eastern South America.
Mediterranean fruit flies have appeared occasionally in Florida, Texas, and California (Gasparich et al. 1997). These small outbreaks have been attacked vigorously with baited traps, release of sterile males, and insectici-dal sprays, the result usually being that eradication was claimed. Each outbreak has usually been considered as a new invasion, rather than an upsurge of a population reduced to an undetectable level. In California, for example, six outbreaks were recorded between 1975 and 1994 (Carey 1996).The locations of several of the outbreaks are similar and suggest that an established population exists. Haplotypes of most of the fruit flies in the six outbreaks match that common in Central America (Gasparich et al. 1997), so one or more introductions from this region may account for most outbreaks. In 1992, however, a different haplotype appeared in both northern and southern California, indicating that an independent introduction from a region other than Central America had occurred.Thus, at least two introductions, and possible more, have led to the California outbreaks.
In Florida, five outbreaks of Mediterranean fruit fly occurred between 1962 and 1994 (Gasparich et al. 1997). None of the haplotypes appearing in Florida can be traced to Central America. Furthermore, the haplotypes appearing in 1962-63, 1984 and 1994, and 1990 were different, indicating that four independent introductions probably occurred. Thus, multiple introductions have played a major role in the invasion of various world regions by this insect pest, with resurgence of populations from very low levels very likely playing a role in sudden outbreaks of the species in California. In any case, a very complex evolutionary potential is now present because of the existence of Mediterranean fruit fly populations of differing genetic makeup in different world regions.
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