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Villablanca et  al. (1998) “found a wealth of genetic variability within invad-
ing populations.” Introns evolve more quickly than the protein-coding regions
of a gene and are expected to retain variation due to their diploid and biparen-
tal inheritance. The intron-sequence variation can be subjected to “phylogenetic
analysis, cladistic analysis of gene flow, as well as standard population genetic
and coalescence analysis of alleles” ( Palumbi 1996, Roderick 1996 ). Medfly pop-
ulations in Africa, California, Hawaii, Brazil, and Greece were evaluated using
primers constructed based on Medfly sequence data from the literature; introns
from four single-copy nuclear genes were amplified that have conserved posi-
tions across species. Single-copy genes were used to avoid analysis of nonspe-
cific PCR products that could occur from multiple-gene copies or pseudogenes.
The four loci were: muscle-specific actin intron 1, chorion s36 intron 1, vitello-
genin 1 gamma intron 2, and Cu/Zn superoxide dismutase (SOD) intron 1. The
PCR products were cloned and sequenced and were found to be specific to the
targeted loci, with no evidence for pseudogenes. The sequence data were ana-
lyzed to eliminate sequences in which errors were incorporated by Taq poly-
merase, which has a misincorporation rate of approximately one per 1000 bases.
Villablanca et  al. (1998) eliminated these erroneous sequences by sequencing
between one and three clones per individual and then identifying and remov-
ing “singletons.” Singletons are variability that occurs in only one sequence of
an alignment and, although “not all singletons are PCR errors, but considering
them to be so results in a conservative measure of allelic diversity.” The remain-
ing sequences were analyzed phylogenetically using Templeton's network
method, which allows reconstruction of phylogenies from potentially recombin-
ing DNA fragments ( Villablanca et al. 1998 ).
Villablanca et  al. (1998) interpreted the phylogenetic analysis of four intron
sequences as follows: “The phylogeny of alleles shows that there is no phylo-
geographic structuring at the population level. Few alleles are shared between
African and invading [California, Hawaii, Brazil and Greece] populations . The
phylogenies of alleles, similarly, do not provide evidence that any invading pop-
ulation is monophyletic . Although the phylogeny of alleles is not useful for
phylogeographic analysis in this case, it is still essential in that it demonstrates
that alleles might be shared among populations simply because all populations
are ultimately derived from Africa and not because they share a common inva-
sion history.” The authors pointed out that the next step is to “sample popula-
tions more thoroughly and test for population subdivision.” Phylogeography is
the study of relationships among genotypes (phylogeny of alleles or haplotypes)
from one or more populations that are examined relative to their geographical
location ( Roderick 1996 ).
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