Abstract
Numerous studies have examined the genetic diversity and genetic structure of invading species, with contrasting results concerning the relative roles of genetic diversity and phenotypic plasticity in the success of introduced populations. Increasing evidence shows that asexual lineages of aphids are able to occupy a wide geographical and ecological range of habitats despite low genetic diversity. The anholocyclic aphid Melanaphis sacchari is a pest of sugarcane and sorghum which originated in the old world, was introduced into the Americas, and is now distributed worldwide. Our purpose was to assess the genetic diversity and structuring of populations of this species according to host and locality. We used 10 microsatellite markers to genotype 1333 individuals (57 samples, 42 localities, 15 countries) collected mainly on sugarcane or sorghum. Five multilocus lineages (MLL) were defined, grouping multilocus genotypes (MLG) differing by only a few mutations or scoring errors. Analysis of a 658 bp sequence of mitochondrial COI gene on 96 individuals revealed five haplotypes, with a mean divergence of only 0.19 %. The distribution of MLL appeared to be strongly influenced by geography but not by host plant. Each of the five MLL grouped individuals from (A) Africa, (B) Australia, (C) South America, the Caribbean and the Indian Ocean including East Africa, (D) USA, and (E) China. The MLL A and C, with a wide geographic distribution, matched the definition of superclone. Among aphids, M. sacchari has one of the lowest known rates of genetic diversity for such a wide geographical distribution.
Highlights
Range expansion of exotic species can result from either evolutionary adaptation or generalism and plasticity often associated with a change in niche [1]
We considered that the five groups defined five multilocus lineages (MLLs) which grouped slightly distinct multilocus genotypes (MLG) resulting from step mutations or scoring errors (Table 1, Figure 1)
Molecular analysis revealed a very low genetic diversity among 57 samples collected in 15 countries on two main hosts, with 36 MLGs structured in five MLLs
Summary
Range expansion of exotic species can result from either evolutionary adaptation or generalism and plasticity often associated with a change in niche [1]. The concept of ‘‘superclones’’ emerged [7] when a few asexual genotypes of the same species were able to colonize a wide geographical or ecological range of habitats [8,9,10,11] The capacity of these populations to adapt to different conditions could be the result of a preadaptation capacity for phenotypic plasticity rather than local selection acting on genetic diversity [12,13]. What is more, this capacity may be enhanced by their high rate of reproduction and population expansion [14]. Clonal aphids are good models to assess the ability of asexual lineages to show rapid and widespread adaptive changes to ecological conditions [15]
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