There are four broadly distributed populations along the Atlantic and Gulf coasts of the United States (New England, the Mid-Atlantic, the South Atlantic and the Gulf coasts). After assess the genetic variability in tall form S. alterniflora collected the two regions using randomly amplified polymorphic DNA (RAPD), O'Brien et al. (1999) classified those populations into three geographic areas (New England, South Atlantic, and Gulf coast), closely approximating the three types defined by Seneca (1974).
The results of a comparative study on genetic variation of population collected from Gulf Coast (introduced populations)) and Eastern Coast of the USA (native populations) respectively indicated that S. alterniflora populations had a high genetic diversity, and intrapopulation genetic diversity was higher than that of interpopulations collected from different region (Perkins et al., 2002). There was a positive relationship between genetic variation and spatial distance of populations, and the spatial distribution of genetic structure was a continuum rather than a discrete regime (O'Brien et al., 1999). Furthermore, moderate disturbance could increase genetic diversity of populations (Edwards et al., 2005). For introduced populations with a long time history, its genetic diversity exhibited a lower level (Blum et al., 2004); however, the genetic differentiation maybe happened during the early stage of invasion (Daehler, 1998). It is an exemplum that a dwarf ecotype has evolved and spread since the introduction of S. alterniflora to San Francisco Bay from Maryland in the 1970s (Daehler et al., 1999). S. alterniflora as a notorious invasive species, the genetic diversity of the species was lower in variously spatial scales, whereas the genetic differentiation of intrapopulations was 22% (Deng et al., 2007). Thus, genetic differentiation have happened in the process of population expansion, and the ability of rapidly adaptive evolution resulted from the differentiation may be one of the reasons that the species widely expanded in a high rate.
Hybridization and introgression also are the basics of outbreak of invasive species. Hybridization between species or between disparate source populations may serve as a stimulus for the evolution of invasiveness (Ellstrand & Schierenbeck, 2000). After introduced into England, S. alterniflora hybridized with native Spartina maritima and resulted in a sterile hybrid S. x townsendii. Chromosome doubling in this hybrid gave rise to a new fertile allopolyploid species, Spartina anglica (2n = 122-124), a vigorous and aggressive perennial plant that has been actively colonising British salt marshes since its formation (Raybould et al., 1991). S. anglica displays wider ecological amplitude than its parents across the successional sequence of salt marsh zones (Thompson, 1991) and rapidly spread along the West-European coast(Baumel et al., 2001). Contrast to native S. foliosa, S. alterniflora had greater mall fitness, and the hybrids appeared vigorous and were recruiting more rapidly than their parents (Daehler & Strong, 1997; Anttila et al., 1998). Furthermore, hybrids could colonize the habitats where their parents were absent (Ayres et al., 1999). Introgression led to genetic pollution of native Spartina species and enhanced the invader (Ayres et al., 2008).
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