Abstract
BackgroundElodea canadensis, Egeria densa and Lagarosiphon major are dioecious clonal species which are invasive in New Zealand and other regions. Unlike many other invasive species, the genetic variation in New Zealand is very limited. Clonal reproduction is often considered an evolutionary dead end, even though a certain amount of genetic divergence may arise due to somatic mutations. The successful growth and establishment of invasive clonal species may be explained not by adaptability but by pre-existing ecological traits that prove advantageous in the new environment. We studied the genetic diversity and population structure in the North Island of New Zealand using AFLPs and related the findings to the number of introductions and the evolution that has occurred in the introduced area.ResultsLow levels of genetic diversity were found in all three species and appeared to be due to highly homogeneous founding gene pools. Elodea canadensis was introduced in 1868, and its populations showed more genetic structure than those of the more recently introduced of E. densa (1946) and L. major (1950). Elodea canadensis and L. major, however, had similar phylogeographic patterns, in spite of the difference in time since introduction.ConclusionsThe presence of a certain level of geographically correlated genetic structure in the absence of sexual reproduction, and in spite of random human dispersal of vegetative propagules, can be reasonably attributed to post-dispersal somatic mutations. Direct evidence of such evolutionary events is, however, still insufficient.
Highlights
Elodea canadensis, Egeria densa and Lagarosiphon major are dioecious clonal species which are invasive in New Zealand and other regions
AFLPs The DNA quality, the reproducibility of AFLP chromatograms, the different patterns observed in E. canadensis compared to E. densa and L. major and the similar geographical separation of Otamangakau and Taupo South populations both in E. canadensis and in L. major, indicate that AFLPs are appropriate for the study of genetic variation in these clonal species
Number of introductions Low levels of genetic diversity were found in all three species, which suggest one introduction or multiple introductions of similar genotypes for each of the three species in the northern part of the North Island, New Zealand (NZ)
Summary
Egeria densa and Lagarosiphon major are dioecious clonal species which are invasive in New Zealand and other regions. Invasive plants are of evolutionary interest because they are successful in areas where they have been introduced, sometimes in spite of limited genetic variation compared to their native range. Many plant species become successful invaders in spite of very limited or non-existing genetic variation This is the case in Clonal growth is often considered an evolutionary dead end, as defined by Stebbins [4] for self-fertilized plants. Not frequently, and their role in the evolution of clonal species has been extensively modelled [5].According to the somatic mutation theory of clonality of Klekowski [6,7] plants accumulate mutations in meristematic cells with age Such mutations reduce the likelihood of sexual reproduction and after a certain time, evolutionary changes depend exclusively on somatic mutations with advantageous phenotypes. In contrast to the situation in obligately outbreeding sexually reproducing organisms, monoclonal "populations" are common [9] and even in the case of clonal diversity, Hardy-Weinberg equilibrium cannot be expected to occur, as sexual reproduction may occur sporadically, if at all
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