Biology Reference
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ported a genetic basis for host preferences (Feder et al. 1998).The rate of
gene flow between these two forms is now estimated to be about 6%
annually (Filchak et al. 2000).
Recent studies of apple and hawthorn biotypes of the apple maggot
fly in Michigan showed that the seasonal difference in fruiting time, com-
bined with the difference in temperature experienced by larvae in fruit of
these trees, forms the selective basis for the strong divergence of biotypes
(Feder and Filchak 1999; Filchak et al. 2000). Feeding by the apple bio-
type was found to begin 3-4 wk earlier than that by the hawthorn bio-
type. This was correlated with the flies' emergence as adults. Larvae and
pupae of the apple biotype also experience warmer temperatures. Filchak
et al. (2000) conducted laboratory experiments to determine the genetic
basis for biotype differences, using a set of four allozymes the frequencies
of which had been correlated with the two biotypes. These experiments
showed that allozymes correlated with the apple biotype were favored
under warm conditions and those correlated with the hawthorn biotype
under cooler conditions. Thus, partial isolation in time, together with
selection based on temperature differences existing in the microhabitats of
the different plant hosts, was responsible for the divergence of these bio-
types.
The apple maggot fly has also adopted sour cherry (
Prunus cerasus
) as
a new host (Shervis et al. 1970). Two close relatives of the apple maggot
fly,
Rhagoletis fausta
, native to pin cherry (
Prunus pennsylvanica
), and
Rhago-
letis cingulata
, native to black cherry (
Prunus serotina
), have also shifted to
sour cherry (Feder 1995). These forms also showed some evidence of
development of distinct host races on this new host species (Diehl and
Bush 1984).
Members of the genus
Rhagoletis
seem particularly prone to evolving
new host relationships. In Chile, another maggot fly,
Rhagoletis conversa
,
uses tomatillo (
Solanum tomatillo
) as its native host.A host race has evolved
on black nightshade (
S. nigrum
), an alien plant native to Europe (Diehl and
Bush 1984).
Evolutionary Shifts of Native Herbivorous Insects to
Introduced Noncrop Plants
Several well-documented examples of shifts of native herbivores to non-
crop plants are now available.The case involving Edith's checkerspot, dis-
cussed at the beginning of the chapter, is one. Many other native insects
have shown similar evolutionary shifts.
