Dean M. Castillo

Specialist pollinating seed predator exhibits oviposition strategy consistent with optimal oviposition theory

The outcome of mutualistic interactions depends on the costs and benefits for each of the partners, which have been shown to be both context‐ and species‐dependent. This phenomenon is seen in the interactions between plants in the genus Silene and moths in the genus Hadena. In this study, the interaction between native North American species Silene stellata and Hadena ectypa is examined to understand the factors that influence female H. ectypa oviposition decisions, a behaviour that influences both herbivore and plant fitness. While most studies focus on oviposition preference between different host plant species, here it is shown that for a specialist pollinating seed predator, oviposition preference occurs within a host species (and even within a plant) based upon individual flower age and pollination status. Female H. ectypa preferentially visited and oviposited on young flowers and flowers that were unpollinated. Larvae also preferred to feed on young fruits. Female H. ectypa oviposition choice was consistent with optimal oviposition theory, as oviposition preference was correlated with larval feeding preference and not just adult visitation preference.

Intraspecific sperm competition genes enforce post-mating species barriers in Drosophila

Sexual selection and sexual conflict are considered important drivers of speciation, based on both theoretical models and empirical correlations between sexually selected traits and diversification. However, whether reproductive isolation between species evolves directly as a consequence of intrapopulation sexual dynamics remains empirically unresolved, in part because knowledge of the genetic mechanisms (if any) connecting these processes is limited. Here, we provide evidence of a direct mechanistic link between intraspecies sexual selection and reproductive isolation. We examined genes with known roles in intraspecific sperm competition (ISC) in D. melanogaster and assayed their impact on conspecific sperm precedence (CSP). We found that two such genes (Acp36DE and CG9997) contribute to both offensive sperm competition and CSP; null/knockdown lines both had lower competitive ability against D. melanogaster conspecifics and were no longer able to displace heterospecific D. simulans sperm in competitive matings. In comparison, Sex Peptide (Acp70A)—another locus essential for ISC—does not contribute to CSP. These data indicate that two loci important for sperm competitive interactions have an additional role in similar interactions that enforce post-mating reproductive isolation between species, and show that sexual selection and sexual isolation can act on the same molecular targets in a gene-specific manner.

Invasive Silene latifolia may benefit from a native pollinating seed predator, Hadena ectypa, in North America.

The acquisition of new mutualists and escape from enemies are often essential for the establishment of invasive species. With its introduction to North America, Silene latifolia successfully escaped a number of generalist and specialist enemies, including the seed predator/specialist pollinator Hadena bicruris, but information regarding the acquisition of new mutualists in a community context has not been examined. We used field observations of mixed species arrays and laboratory feeding trials and compared floral scent and plant/pollinator morphological match to explore the interaction in North America of the invasive S. latifolia with the native pollinating seed predator, Hadena ectypa, in order to understand mechanisms of enemy release and mutualist facilitation underlying the successful invasion of S. latifolia. In mixed arrays, H. ectypa visited S. latifolia at a low frequency similar to the combined visitation of other non-Hadena nocturnal moth species. Differences in the floral scent profiles of S. latifolia and Silene stellata, a native coflowering congener and natural host of H. ectypa, combined with the lack of morphological match between H. ectypa and S. latifolia, likely contribute to these results. In the field study, only one H. ectypa egg was oviposited on S. latifolia, and this did not result in a successful fruit attack. Larvae feeding trials in the lab showed no initial feeding preference for pistils of either Silene species. Therefore, our study suggests that S. latifolia has escaped the cost of seed predation typically associated with visitation and oviposition by Hadena pollinators, a potential natural enemy, while taking advantage of pollination services provided by both H. ectypa and other native North American nocturnal moth pollinators.

Evolutionary Implications of Mechanistic Models of TE-Mediated Hybrid Incompatibility

New models of TE repression in plants (specifically Arabidopsis) have suggested specific mechanisms by which TE misregulation in hybrids might result in the expression of hybrid inviability. If true, these models suggest as yet undescribed consequences for (1) mechanistic connections between hybrid problems expressed at different postzygotic stages (e.g., inviability versus sterility), (2) the predicted strength, stage, and direction of isolation between diverging lineages that differ in TE activity, and (3) the association between species attributes that influence TE dynamics (e.g., mode of reproduction, geographical structure) and the rate at which they could accumulate incompatibilities. In this paper, we explore these implications and outline future empirical directions for generating data necessary to evaluate them.

Interactions between a pollinating seed predator and its host plant: the role of environmental context within a population

Plant–insect interactions often are important for plant reproduction, but the outcome of these interactions may vary with environmental context. Pollinating seed predators have positive and negative effects on host plant reproduction, and the interaction outcome is predicted to vary with density or abundance of the partners. We studied the interaction between Silene stellata, an herbaceous perennial, and Hadena ectypa, its specialized pollinating seed predator. Silene stellata is only facultatively dependent upon H. ectypa for pollination because other nocturnal moth co-pollinators are equally effective at pollen transfer. We hypothesized that for plants without conspecific neighbors, H. ectypa would have higher visitation rates compared to co-pollinators, and the plants would experience lower levels of H. ectypa pollen deposition. We predicted similar oviposition throughout the study site but greater H. ectypa predation in the area without conspecific neighbors compared to plants embedded in a naturally high density area. We found that H. ectypa had consistently higher visitation than moth co-pollinators in all host plant contexts. However, H. ectypa pollinator importance declined in areas with low conspecific density because of reduced pollen deposition, resulting in lower seed set. Conversely, oviposition was similar across the study site independent of host plant density. Greater likelihood of very high fruit predation combined with lower pollination by H. ectypa resulted in reduced S. stellata female reproductive success in areas with low conspecific density. Our results demonstrate local context dependency of the outcomes of pollinating seed predator interactions with conspecific host plant density within a population.

Experimental evolution: Assortative mating and sexual selection, independent of local adaptation, lead to reproductive isolation in the nematode Caenorhabditis remanei

Using experimental evolution, we investigated the contributions of ecological divergence, sexual selection, and genetic drift to the evolution of reproductive isolation in Caenorhabditis remanei. The nematodes were reared on two different environments for 100 generations. They were assayed for fitness on both environments after 30, 64, and 100 generations, and hybrid fitness were analyzed after 64 and 100 generations. Mating propensity within and between populations was also analyzed. The design allowed us to determine whether local adaptation was synchronous with pre‐ and postzygotic reproductive isolation. Prezygotic isolation evolved quickly but was unconnected with adaptation to the divergent environments. Instead, prezygotic isolation was driven by mate preferences favoring individuals from the same replicate population. A bottleneck treatment, meant to enhance the opportunity for genetic drift, had no effect on prezygotic isolation. Postzygotic isolation occurred in crosses where at least one population had a large fitness advantage in its “home” environment. Taken together, our results suggest that prezygotic isolation did not depend on drift or adaptation to divergent environments, but instead resulted from differences in sexual interactions within individual replicates. Furthermore, our results suggest that postzygotic isolation can occur between populations even when only one population has greater fitness in its home environment.

Remating responses are consistent with male postcopulatory manipulation but not reinforcement inD. pseudoobscura

Abstract Reinforcement occurs when hybridization between closely related lineages produces low‐fitness offspring, prompting selection for elevated reproductive isolation specifically in areas of sympatry. Both premating and postmating prezygotic behaviors have been shown to be the target of reinforcing selection, but it remains unclear whether remating behaviors experience reinforcement, although they can also influence offspring identity and limit formation of hybrids. Here, we evaluated evidence for reinforcing selection on remating behaviors in Drosophila pseudoobscura, by comparing remating traits in females from populations historically allopatric and sympatric with Drosophila persimilis. We found that the propensity to remate was not higher in sympatric females, compared to allopatric females, regardless of whether the first mated male was heterospecific or conspecific. Moreover, remating behavior did not contribute to interspecific reproductive isolation among any population; that is, females showed no higher propensity to remate following a heterospecific first mating than following a conspecific first mating. Instead, we found that females are less likely to remate after initial matings with unfamiliar males, regardless of species identity. This is consistent with one scenario of postmating sexual conflict in which females are poorly defended against postcopulatory manipulation by males with whom they have not coevolved. Our results are generally inconsistent with reinforcement on remating traits and suggest that this behavior might be more strongly shaped by the consequences of local antagonistic male–female interactions than interactions with heterospecifics.

Reinforcement of conspecific sperm precedence weakens sexual selection in sympatric populations of Drosophila

It is typically assumed that sexual selection unilaterally drives the evolution of reproductive isolation, but selection for increased reproductive isolation could feed back on sexual selection when these processes share a genetic basis. Direct selection for isolation is most likely to occur in the context of ‘reinforcement’, where selection acts to increase pre-zygotic barriers to reduce the costs of heterospecific matings. Most studies of reinforcement focus on pre-mating barriers to reproduction, however post-mating traits, such as conspecific gamete precedence, are also ubiquitous barriers to reproduction that can potentially respond to reinforcing selection. Conspecific sperm precedence (CSP) also has a known shared genetic basis with intrapopulation sperm competition, allowing for the possibility that selection for increased CSP in sympatry could alter intrapopulation sperm competition specifically in these sympatric populations. We test this prediction with the sister species Drosophila pseudoobscura and D. persimilis, using two sympatric and two allopatric populations of D. pseudoobscura. Consistent with a pattern of reinforcement, the sympatric populations had higher mean CSP. Reinforcement, in turn, constrained sexual selection in sympatric populations by decreasing the average offensive sperm competitive ability within populations, allowing less opportunity for sexual selection to operate. These data demonstrate that strong reinforcing selection for reproductive isolation can have consequences for sexual selection and sexual interactions, in these important postmating sperm competition traits.

Factors contributing to the accumulation of reproductive isolation: A mixed model approach

Abstract The analysis of large datasets describing reproductive isolation between species has been extremely influential in the study of speciation. However, the statistical methods currently used for these data limit the ability to make direct inferences about the factors predicting the evolution of reproductive isolation. As a result, our understanding of iconic patterns and rules of speciation rely on indirect analyses that have clear statistical limitations. Phylogenetic mixed models are commonly used in ecology and evolution, but have not been applied to studies of reproductive isolation. Here I describe a flexible framework using phylogenetic mixed models to analyze data collected at different evolutionary scales, to test both categorical and continuous predictor variables, and to test the effect of multiple predictors on rates and patterns of reproductive isolation simultaneously. I demonstrate the utility of this framework by re‐analyzing four classic datasets, from both animals and plants, and evaluating several hypotheses that could not be tested in the original studies: In the Drosophila and Bufonidae datasets, I found support for more rapid accumulation of reproductive isolation in sympatric species pairs compared to allopatric species pairs. Using Silene and Nolana, I found no evidence supporting the hypothesis that floral differentiation elevates postzygotic reproductive isolation. The faster accumulation of postzygotic isolation in sympatry is likely the result of species coexistence determined by the level of postzygotic isolation between species. In addition, floral trait divergence does not appear to translate into pleiotropic effects on postzygotic reproductive isolation. Overall, these methods can allow researchers to test new hypotheses using a single statistical method, while remedying the statistical limitations of several previous methods.

Assortative mating and self-fertilization differ in their contributions to reinforcement, cascade speciation, and diversification

Abstract Cascade speciation and reinforcement can evolve rapidly when traits are pleiotropic and act as both signal/cue in nonrandom mating. Here, we examine the contribution of two key traits—assortative mating and self-fertilization—to reinforcement and (by extension) cascade speciation. First, using a population genetic model of reinforcement we find that both assortative mating and self-fertilization can make independent contributions to increased reproductive isolation, consistent with reinforcement. Self-fertilization primarily evolves due to its 2-fold transmission advantage when inbreeding depression (d) is lower (d < 0.45) but evolves as a function of the cost of hybridization under higher inbreeding depression (0.45 < d < 0.48). When both traits can evolve simultaneously, increased self-fertilization often prohibits the evolution of assortative mating. We infer that, under specific conditions, mating system transitions are more likely to lead to increased reproductive isolation and initiate cascade speciation, than assortative mating. Based on the results of our simulations, we hypothesized that transitions to self-fertilization could contribute to clade-wide diversification if reinforcement or cascade speciation is common. We tested this hypothesis with comparative data from two different groups. Consistent with our hypothesis, there was a trend towards uniparental reproduction being associated with increased diversification rate in the Nematode phylum. For the plant genus Mimulus, however, self-fertilization was associated with reduced diversification. Reinforcement driving speciation via transitions to self-fertilization might be short lived or unsustainable across macroevolutionary scales in some systems (some plants), but not others (such as nematodes), potentially due to differences in susceptibility to inbreeding depression and/or the ability to transition between reproductive modes.