Species differences in reproductive timing and egg load in two fly species adapted to different host plants

Selection acts on reproductive timing and egg number to maximize
reproductive success based on available resources. Species often vary in
these life history traits due to differences in trait variation and local
selection pressures, especially when species are adapted to different
environments. Yet, species may share similar life history traits given
common correlations, such as a positive relationship between body size and
fecundity. In this study, we tested whether species adapted to different
environments differ in their reproductive timing and egg load using two
species of Tephritid fruit flies: Rhagoletis pomonella adapted to downy
hawthorn and Rhagoletis suavis adapted to black walnut. In both species,
older females were more likely to produce eggs and have more eggs.
However, reproductive timing differed between species; hawthorn flies
produced eggs earlier than walnut flies. In contrast to the common
association of larger body size with earlier reproduction and higher egg
counts, we did not find that adult female body size predicted reproductive
timing in either species. In hawthorn flies with earlier reproductive
maturity, body size did not affect egg number. For walnut flies with later
reproductive maturity, larger females produced more eggs. Our findings
suggest that while time to accumulate resources and develop eggs is a
common constraint, reproductive strategies are likely shaped by adaptation
to specific host plants. Our study highlights the complex interaction
between selection pressures and trait correlations in shaping life history
traits across species.