Desiccation resistance in interspecific Drosophila crosses: Genetic interactions and trait correlations

We used crosses between two closely related Drosophila species, Drosophila serrata and D. birchii, to examine the genetic basis of desiccation resista nce and correlations between resistance, physiological traits, and life-his tory traits. D. serrata is more resistant to desiccation than D. birchii, a nd this may help to explain the broader geographical range of die former sp ecies. A comparison of F-2's from reciprocal crosses indicated higher resis tance levels when Fels originated from D. birchii mothers compared to D. se rrata mothers. However, backcrosses had a resistance level similar to that of the parental species, suggesting an interaction between X-linked effects in D. serrata that reduce resistance and autosomal effects that increase r esistance. Reciprocal differences persisted in hybrid lines set up from the different reciprocal crosses and tested at later generations. Increased de siccation resistance was associated with an increased body size in two sets of hybrid lines and in half-sib groups set up from the F-4's after crossin g the two species, but size associations were inconsistent in the F-2's. No ne of the crosses provided evidence for a positive association between desi ccation resistance and glycogen levels, or evidence for a tradeoff between desiccation resistance and early fecundity. However, fecundity; was positiv ely con-elated with body size at both the genetic and phenotypic levels. Th is study illustrates how interspecific crosses may provide information on g enetic interactions between traits following adaptive divergence, as well a s on the genetic basis of the traits.