Patterns of Reproductive Isolation in Nolana (Chilean Bellflower)

Abstract

We examined reproductive isolating barriers at four postmating stages among 11 species from the morphologically diverse genus Nolana (Solanaceae). At least one stage was positively correlated with both genetic and geographic distance between species.Postzygotic isolation was generally stronger and faster evolving than postmating prezygotic isolation. In addition, there was no evidence for mechanical isolation, or for reproductive character displacement in floral traits that can influence pollinator isolation. For sexually reproducing plants and animals, the origin of new species involves the evolution of reproductive isolating barriers between diverging lineages. Studying these isolating barriers therefore provides insight into the process of speciation (Coyne and Orr 2004). Several approaches have been used to examine the evolution of reproductive isolation within the same closely related group of species, including examining the relative strength of isolating barriers operating at different reproductive or developmental stages among different species pairs, across different degrees of evolutionary divergence, and/or among reciprocal crosses (Coyne and Orr 2004, and see below). These approaches aim to evaluate how rapidly barriers accumulate and which isolating barriers contribute most during initial divergence between lineages, among other questions. In combination, they can provide insight into the evolutionary forces and the genetic mechanisms responsible for the evolution of new, reproductively independent, lineages.First, examining the reproductive compatibility of a single species pair at multiple developmental stages (e.g., premating, postmating prezygotic, and postzygotic) can be used to infer which stages are most effective at reducing current gene flow between these species, and therefore which stages might have been more or less influential during their initial divergence (e.g., Ramsey et al. 2003;Kay 2006;Martin and Willis 2007;Mendelson et al. 2007;Maroja et al. 2009;Dopman et al. 2010). For example, based on the estimated contributions of multiple preand postzygotic reproductive barriers between two monkeyflower sister species, Mimulus lewisii and M. cardinalis (Ramsey et al. 2003), factors acting prior to hybridization (specifically ecogeographic isolation and pollinator isolation) were inferred to be the primary isolating barriers in this system. Data such as these can also suggest the evolutionary forces that are most likely responsible for reducing gene flow between species. For example, estimates of pre-and postzygotic barriers between M. guttatus and M. nasutus (Martin and Willis 2007) revealed that prezygotic barriers contributed most to total isolation, likely as a result of adaptive divergence in mating systems (i.e., shift to self-pollination) and edaphic ecology (i.e., drought avoidance via phenological acceleration).

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

“…This is consistent with the original study in the sense that only geographical distance was significant using the Mantel test (Jewell et al., 2012). The analysis using the genetic distance matrix also indicated that geographic distance was the only significant predictor of reproductive isolation (Table 4), and the coefficients were similar to the previous analysis.…”

“…The topology on the right was generated using Ward's hierarchical clustering method for pairwise genetic distances reported in Jewell et al. (2012)…”

“…These comparisons cannot be made using PICs or Mantel tests, so in many classical studies, the effects of these factors are instead indirectly compared (Coyne & Orr, 1989, 1997; Jewell et al., 2012; Moyle et al., 2004). Using a phylogenetic mixed model framework, and classic datasets on species ability to cross with one another, I formally tested the interactions among these effects on the evolution of reproductive isolation.…”

Merging Ecology and Genomics to Dissect Diversity in Wild Tomatoes and Their Relatives

“…Reproductive traits and reproductive isolation are both ecologically important and genetically tractable in our study groups, and in other close relatives (Smith et al 2008;Jewell et al 2012).…”

Multiple strong postmating and intrinsic postzygotic reproductive barriers isolate florally diverse species of Jaltomata (Solanaceae)

“…To address these potential collateral effects of rapid mating trait divergence, we require studies that examine multiple reproductive barriers among closely related taxa that differ in such trait(s), and are therefore able to simultaneously assess the relative importance of this divergence versus other axes of differentiation that might drive isolation between lineages (e.g. Jewell et al 2012; Martin and Mendelson 2016). …”