Sequential divergence and the multiplicative origin of community diversity

Phenotypic and genetic variation in one species can influence the composition of interacting organisms within communities and across ecosystems. As a result, the divergence of one species may not be an isolated process, as the origin of one taxon could create new niche opportunities for other species to exploit, leading to the genesis of many new taxa in a process termed “sequential divergence.” Here, we test for such a multiplicative effect of sequential divergence in a community of host-specific parasitoid wasps, Diachasma alloeum, Utetes canaliculatus, and Diachasmimorpha mellea (Hymenoptera: Braconidae), that attack Rhagoletis pomonella fruit flies (Diptera: Tephritidae). Flies in the R. pomonella species complex radiated by sympatrically shifting and ecologically adapting to new host plants, the most recent example being the apple-infesting host race of R. pomonella formed via a host plant shift from hawthorn-infesting flies within the last 160 y. Using population genetics, field-based behavioral observations, host fruit odor discrimination assays, and analyses of life history timing, we show that the same host-related ecological selection pressures that differentially adapt and reproductively isolate Rhagoletis to their respective host plants (host-associated differences in the timing of adult eclosion, host fruit odor preference and avoidance behaviors, and mating site fidelity) cascade through the ecosystem and induce host-associated genetic divergence for each of the three members of the parasitoid community. Thus, divergent selection at lower trophic levels can potentially multiplicatively and rapidly amplify biodiversity at higher levels on an ecological time scale, which may sequentially contribute to the rich diversity of life.Population divergence is a fundamental evolutionary process contributing to the diversity of life (1). Studies of how new life forms originate typically focus on how barriers to gene flow evolve in specific lineages, resulting in their divergence into descendent daughter taxa. As a result, evolutionary biologists now have a good understanding of how variation within a population is transformed by selection into differences between taxa (1–3). What is less well understood is whether the divergence of one population has consequences that ripple through the trophic levels of an ecosystem and affect entire communities of interacting organisms. Studies in paleontology (4–6), community ecology (7, 8), systematics (8, 9), and ecosystem genetics (10, 11) suggest that evolutionary change in one lineage can influence entire communities of organisms. For example, when the genotype/phenotype of a “foundation” species influences the relative fitness of other species, evolutionary change(s) in this genotype/phenotype may affect organisms in adjacent trophic levels (10, 11). If these evolutionary changes are linked to ecological adaptation and reproductive isolation (RI), associated organisms may diverge in parallel, potentially creating entire coevolved communities distinct from one another (12–15). Therefore, population divergence may not always be an isolated process, as the differentiation of one taxon could beget the divergence of many others.Such “sequential” or “cascading” divergence events may be particularly relevant to understanding why some groups of organisms, like plants, the insects that feed on them, and the parasitoids that attack the insects, are more diverse and species-rich than other groups (8, 9, 12–15). Specifically, when phytophagous insects diversify by adapting to new host plants, they create a new habitat for their parasitoids to exploit (Fig. 1). If a parasitoid shifts to the new habitat, it can encounter the same divergent ecological selection pressures as its insect host, which could result in the parallel divergence of insect host and parasitoid (12–15) (Fig. 1A). Moreover, sequential divergence may have multiplicative effects in generating biodiversity, as the shift of an insect to a new plant may open a new niche opportunity for not just one but the entire community of parasitoids attacking the insect host (12, 13) (Fig. 1B). However, few convincing examples of sequential divergence exist (12–15), and in no study is there both genetic and ecological evidence for sequential divergence multiplicatively amplifying biodiversity.Open in a separate windowFig. 1.Three scenarios of codivergence in a host−parasitoid system. (A) A single sequential divergence event, (B) sequential divergence with multiplicative amplification of biodiversity, and (C) cospeciation in allopatry. In A, codivergence is driven by the cascade of divergent ecological selection pressures across trophic levels in sympatry. Here, a degree of divergent ecological adaptation must accompany the host shift such that parasitoids are not merely moving between geographically separated hosts. In B, the multiplicative effects of sequential divergence can be seen as several members of the parasitoid community diverge in parallel with their host. In C, codivergence (cospeciation) occurs after host plant, fly, and parasitoid populations become jointly geographically isolated (black bar), resulting in parallel allopatric speciation. Here, little differentiation need accompany the initial host shift of fly or parasitoid. Cospeciation is not necessarily driven by the creation and adaptation to new niches but by the concordant geographic and reproductive separation of hosts and parasitoids.Here, we test for the multiplicative effects of sequential divergence in the community of parasitoid wasps (Hymenoptera: Braconidae) that attack fruit flies in the Rhagoletis pomonella sibling species complex (Diptera: Tephritidae) (Fig. 2). Several features of the biology and biogeography of the Rhagoletis−parasitoid system make it ideal for investigating the multiplicative divergence hypothesis and allow us to directly test multiple criteria supporting sympatric host race formation (16) and sequential divergence (12, 13), summarized in 1–3, 17–19), the hypothesized initial stage of ecological speciation (16, 17). The short time frame and sympatric spatial context of R. pomonella’s shift to apple exclude passive codivergence or speciation (Fig. 1C) as an explanation for differentiation. Specifically, fly and wasp populations could not have diverged in concert, because they became jointly geographically separated in the past (Fig. 1C). Rather, if flies and wasps display concordant adaptations, it is likely due to the direct effects of divergent ecological selection resulting from host shifts cascading from host plants to flies to parasitoids (Fig. 1 A and B).Open in a separate windowFig. 2.The community of host-specific parasitoids that attack members of the (A) Rhagoletis pomonella sibling species complex: (B) D. alloeum, (C) D. mellea, and (D) U. canaliculatus that (E) emerge from the fly pupal case as adults following overwintering. (Scale bar, 1 mm.)

Table 1.

Summary of conditions (criteria) conducive to and supporting hypotheses of sympatric host race formation (modified from ref. 16) and sequential divergence (modified from refs. 12 and 13)

Criteria supporting hypotheses of sympatric host race formation and sequential divergence	Rp	Da	Dm	Uc
Criterion 1. Shift to new host resource and multiple host-associations occur in sympatry or close geographic proximity	yes	yes	yes	yes
Criterion 2. Host-associated populations form distinct genetic clusters (spatially replicable), but experience gene flow at appreciable rates	yes	yes	yes	yes
Criterion 3. Females, but also potentially males, display host preferences and discriminate among alternate hosts	yes	yes	yes	yes
Criterion 4. Host choice is linked to mate choice facilitating assortative mating and resulting in prezygotic habitat isolation	yes	yes	yes	yes
Criterion 5. Host selection and fidelity are under some degree of genetic control and not due solely to maternal, learning, or environmental effects	yes	yes, tested in one direction	not tested	not tested
Criterion 6. Differences in insect phenologies tracks differences in the host phenologies resulting in temporal (allochronic) isolation	yes	yes	yes	yes
Criterion 7. Insect phenology under some degree of genetic control and not due solely to maternal or environmental effects	yes	yes	yes	yes
Criterion 8. Fitness tradeoffs exist between host-associated populations resulting in migrants and hybrids having reduced fitness	yes, sometimes	not tested	not tested	not tested

Open in a separate windowAlso shown is whether these criteria have been empirically tested and confirmed in R. pomonella (Rp) species complex flies and three members of the parasitoid wasp community attacking the flies: D. alloeum (Da), D. mellea (Dm), and U. canaliculatus (Uc). Data for Da are from Forbes et al. (14) and the current study (criterion 5).Second, the apple and hawthorn host races of R. pomonella belong to a closely related group of sibling species, including Rhagoletis mendax (host: blueberry, Vaccinium spp.), Rhagoletis zephyria (host: snowberry, Symphoricarpos spp.), and the undescribed flowering dogwood fly (host: Cornus florida). All of these taxa purportedly radiated via sympatric host shifts (17–23). In addition, other species in the genus, such as the eastern cherry fly, Rhagoletis cingulata (host: black cherry, Prunus serotina) are sympatric with R. pomonella group flies (23). Thus, the potential for sequential divergence in the Rhagoletis parasitoid community extends beyond the host races, with multiple cooccurring fly resources existing for wasps to attack, satisfying criterion 1.Third, Rhagoletis in the eastern United States are attacked by a community of host-specific endoparasitoid wasps that include the species Diachasma alloeum, Utetes canaliculatus, and Diachasmimorpha mellea (24, 25) (Fig. 2). All three species have a free-living, sexually reproducing adult life stage. This life cycle eliminates vertical transmission as a factor facilitating codivergence. U. canaliculatus oviposits into Rhagoletis eggs laid beneath the skin of ripe fruit, while D. alloeum and D. mellea oviposit into late instar larvae feeding within fruit (24). A degree of niche partitioning for oviposition sites therefore exists among species (25), potentially facilitating coexistence on the same fly host. As a result, multiple host associations of wasps exist in close geographic proximity, fulfilling the requirements of criterion 1 for sympatric race formation and sequential divergence for the wasp community as well.In addition, a previous study documented that one parasitoid attacking Rhagoletis, D. alloeum, is undergoing sequential divergence (14). Population genetic surveys, field observations, behavioral assays of host choice, and studies of life history timing support the existence of an ecologically derived population of D. alloeum attacking the recently formed apple-infesting host race of R. pomonella, meeting criteria 2, 3, 4, 6, and 7. We hypothesize that if U. canaliculatus and D. mellea are undergoing sequential divergence, they will show similar patterns of host-associated ecological and genetic divergence.Two dimensions of divergent ecological selection generate RI among host-associated populations of Rhagoletis and D. alloeum: host-specific mating (habitat isolation) and differences in eclosion phenology (temporal isolation). With respect to habitat isolation, Rhagoletis (26) and D. alloeum (14) court and mate on or near the fruit of their respective host plants. The most important long- to intermediate-range cues that flies use to find and discriminate among plants are the volatile compounds emitted from the surface of ripening fruit (27–30). Flies display genetically based behavioral preference for natal fruit surface volatiles and avoid the volatiles of alternative fruit (29). Similarly, D. alloeum prefer natal and avoid nonnatal host fruit volatiles in behavioral assays (14), supporting criterion 3. Consequently, differences in host choice translate directly to mate choice, generating prezygotic habitat-related RI for both flies and wasps, fulfilling criterion 4. Additionally, host odor discrimination may also act as a postzygotic barrier to gene flow in R. pomonella, as they suffer behavioral host choice sterility mediated by a reduced chemosensory ability to find suitable host fruit for mating and oviposition (30). Whether or not hybrid D. alloeum display a similar behavior is unknown. Lastly, criteria 5 is partially met for Rhagoletis, as the host fruit environment has no effect on host odor discrimination behaviors for hawthorn-origin R. pomonella reared in apple fruit, indicating that host selection and fidelity are under (partial) genetic control (27, 30). Similar experiments have not yet been conducted in D. alloeum (but see Reciprocal Rearing of Diachasma).With respect to temporal isolation (criterion 6), the timing of overwinter diapause is an important host-related ecological adaptation for Rhagoletis. The host plants of Rhagoletis fruit at different times of the year (19, 31, 32). For example, apple varieties favored by R. pomonella ripen 3–4 wk before native hawthorns in sympatry. Thus, flies must eclose to coincide with the availability of ripe fruit to find mates and oviposition sites. Rhagoletis are univoltine, and their lifespan is short (1 mo). Differences in eclosion timing between races therefore results in partial allochronic mating isolation (19, 29–32). The differences in eclosion timing also confer a degree of postzygotic isolation because hybrids will possess eclosion patterns asynchronous with fruit ripening (29, 31, 32). Rhagoletis attacking blueberries and flowering dogwoods display similar differences in eclosion time related to variation in host fruiting phenology (31).The life cycle of D. alloeum mirrors that of Rhagoletis, generating the same divergent ecological selection pressures. As a result, populations of D. alloeum attacking different Rhagoletis eclose to match the phenology of fly larvae feeding within host fruit (14). In addition, longevity of D. alloeum (∼2 wk) is half that of Rhagoletis, generating even more pronounced allochronic mating isolation compared to the fly (14), supporting criterion 6. Significant allele frequency differences between sympatric populations of D. alloeum attacking different fly hosts (criterion 2) were associated with differences in eclosion time (14), confirming criterion 7. The same has also been found for Rhagoletis (19, 31, 32), connecting host-related life history adaptation and RI to patterns of genetic differentiation among flies and wasps.Here, we test for the multiplicative hypothesis of sequential divergence in the Rhagoletis−parasitoid system using the criteria in 33, 34), difficulty in reciprocally transplanting wasps precludes these experiments at this time but remain an area for future study.