Print PageThe fundamental questions in evolutionary biology seek to understand the mechanisms that facilitate and maintain biodiversity. The role of organismal interactions across different trophic levels in fostering diversification is important to understanding species formation and maintenance. Diversity can beget diversity when speciation by one organism drives diversification of associated organisms in adjacent trophic levels, a term coined sequential or cascading speciation. If organisms diverge and create new niches to exploit, catalyzing a “chain reaction” of speciation events, what are the (1) frequency of such events and (2) barriers to gene flow in diverging populations that reinforce diversification? The apple maggot fly, Rhagoletis pomonella (Diptera: Tephritidae), is a textbook example of sympatric speciation in action. Races of hawthorn and apple flies exist today that differ in eclosion timing, host odor preference and allele frequencies. Recently the R. pomonella–attacking parasitoid, Diachasma alloeum (Hymenoptera: Braconidae), has undergone sequential speciation. Paralleling flies, wasps exhibit differences in allele frequencies, eclosion timing, and odor preference. We tested the sequential speciation hypothesis in Diachasmimorpha mellea, another Rhagoletis–attacking parasitoid, by examining eclosion timing, host odor preference and microsatellite allele frequencies across sympatric populations attacking flies in the R. pomonella species group (apple-, hawthorn-, blueberry- and dogwood-infesting).
Results/Conclusions
When overwintered under standard lab conditions, adult D. mellea mean eclosion times differ between host associations (pooled across populations: apple = 86 days, hawthorn = 97, dogwood = 137). These eclosion times parallel the fruiting phenology of each host plant species in nature and the eclosion timing of each Rhagoletis species or host race reared in the same standard lab conditions. When given a choice (odor vs. no odor) in a y-tube, apple, hawthorn, blueberry and dogwood derived D. mellea differentially orient to the arm of the y-tube housing their natal odor and avoid non-natal odors (P < 0.05 in all four cases). In two sympatric populations 9/21 and 10/21 microsatellite loci analyzed displayed significant allele frequency differences between comparisons of blueberry/hawthorn and apple/hawthorn derived parasitoids respectively. Initial ecological and genetic evidence supports the sequential speciation hypothesis in D. mellea: allochronic isolation via eclosion timing and behavioral isolation via host odor discrimination contributes to host-associated genetic structure in sympatry.
