Biology Reference
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origin of these flies.” The 1997 southern California infestation is “well separated
from either of the two previous groupings
…
these results suggest it is not appro-
priate to group or depict all of them as homogeneous” (
He and Haymer 1999
).
They concluded “the extreme separation of the Hawaiian populations from these
California (and Florida) infestations also suggests that Hawaii can be considered
a very unlikely source.” Furthermore, “Samples from the California 1997 infes-
tation are also well separated from all other populations, suggesting that none
of the worldwide populations sampled here can be considered likely sources.”
Thus, “the multiple infestations detected within California in recent years are not
likely to represent a single, homogenous population that is similar to the “estab-
lished” populations seen in Guatemala or Argentina” (
He and Haymer 1999
). The
authors noted that additional surveys of these markers in populations from other
regions of the world are desirable to improve resolution of Medfly population
relationships. They concluded that analysis of these alleles in ancestral African
populations, where considerably more genetic variability occurs, is desirable.
Gomulski et al. (1998)
evaluated variability in the size of the first intron in the
alcohol dehydrogenase
gene to assess 16 populations from five geographical
regions: Africa, the Mediterranean basin, Latin America, Hawaii, and Australia.
PCR primers were developed that spanned the intron between exons 1 and 2.
PCR product sizes varied from 1400bp to 3450bp and were grouped into four
categories: short, medium, long, and very long. Most variants were found only
in the African populations and only a few migrated from Africa with the colo-
nizing populations. The results obtained were congruent with those obtained
by analyzing allozyme variation and showed a gradual and large reduction in
intron variability.
Gomulski et al. (1998)
concluded that drift, bottleneck effects,
and migration were important in explaining the observed intron-size variability.
Villablanca et al (1998)
analyzed multiple nuclear-gene introns in an effort
to provide sufficient information to resolve the origins of the Medfly popula-
tions in California and Hawaii despite the reduction in variability expected due
to founder effects and genetic bottlenecks. They chose to work with multiple
introns because they concluded that mitochondrial DNA is poorly suited to stud-
ies of invasions unless the invading population is large or grows rapidly; mtDNA
is subject to strong genetic drift due to its maternal and haploid mode of inheri-
tance. They also argued that RAPD-PCR data are difficult to interpret and may
lack repeatability; microsatellites are effective, but require a long period of
development for each new taxon. Their review of previous work of molecu-
lar analyses of Medflies indicated, “little genetic variation has been uncovered
within invading populations with both allozymes
…
and mtDNA.”
