Biology Reference
In-Depth Information
13.7.4.6 Some Lessons Learned
The Medfly case study illustrates several points: molecular tools vary in their sen-
sitivity, ease of use, cost, and time to develop. Despite differences in the methods
used, the various researchers agreed on some key findings: Medfly populations
in the Los Angeles area were unlikely to have come from Hawaii; the population
present in the Los Angeles area may be due either to the presence of an estab-
lished population or to multiple infestations from the same source, or both.
Molecular markers of six Medflies from the San Diego population were clearly
different from the markers found in Medflies collected in the Los Angeles area.
The San Diego Medflies sampled most closely resembled Medflies from Hawaii.
Analysis of the infestation area by entomologists correlated the San Diego infes-
tation with a family that had just returned from a trip to Hawaii. Thus, combin-
ing molecular and ecological data may provide more information than relying
on a single approach (A. Malacrida, personal communication).
It should be clear that insect population-genetic structure can be quite com-
plex; simply assuming that molecular markers will allow an unequivocal conclu-
sion as to the population's geographic origin may be unrealistic, especially if
primary, secondary, and tertiary invasions have occurred (
Figure 13.4
). Genetic
variability and structuring is dependent upon events in the population's his-
tory, including bottlenecks, drift, selection and hybridization. The Medfly case
study shows that increasingly refined molecular methods are available, and
the statistical and other analyses used to reach conclusions are becoming more
sophisticated.
13.7.5 Plant Defenses to Insect Herbivory
Plants face a variety of biotic (bacteria, fungi, insects, and other herbivores)
and abiotic (drought, heat, salinity, and UV damage) stresses in their environ-
ment (
Strauss and Agrawal 1999
). In response, plants evolved both constitutive
and inducible defenses that have a genetic basis (
Zheng and Dicke 2008
). Direct
defenses include synthesis of secondary plant compounds that affect attraction
and deterrence of insects, or inhibit insect growth and development. Induced
defenses include proteinase inhibitors and polyphenol oxidases that inhibit the
digestive enzymes or reduce the nutritive content of the plant. Indirect defenses
also include releases of volatiles that signal the location of pests on infested plants
to their natural enemies. There are many genes involved in plant-defense mecha-
nisms, making it difficult to analyze their role using traditional genetic methods.
DNA microarrays can be used to obtain significant advances in our under-
standing of plant defenses against insect herbivores (
Schenk et al. 2000
, Baldwin
