Ecological differentiation and reproductive isolation of two closely related sympatric species of Oenanthe (Apiaceae)

Abstract

Oenanthe conioides is a lower Elbe endemic plant species growing in the freshwater tidal zone around Hamburg (Germany). Its closest relative Oenanthe aquatica is widely distributed in Eurasia and grows in calm and shallow freshwater. The two species differ in habitat requirements but are otherwise sympatrically distributed, suggesting that ecological divergence has to be maintained in the face of gene flow. In the present study, we investigated ecological differentiation and reproductive isolation in these two species. An amplified fragment length polymorphism analysis found clear genetic differentiation between the two species implying reproductive isolation. A reciprocal transplantation experiment including artificial F1 hybrids showed strong selection against immigrants. In the two parental habitats, the non-native species are less fit than the native species. Hybrids are less fit in the habitat of Oe. aquatica but perform as well as the native species in the habitat of Oe. conioides. We hypothesize that selection against immigrants is the most important component of reproductive isolation between the two species, and that selection against immigrants is the result of exposure to cold and wind in the tidal habitat of Oe. conioides and to herbivory in the habitat of Oe. aquatica. These results are congruent with a role for differential ecological selection in the formation and maintenance of these two species.

No evidence of intrinsic reproductive isolation between two reciprocally non-monophyletic, ecologically differentiated mountain plants at an early stage of speciation

“…In congruence with recent studies revealing prezygotic reproductive isolation based solely on extrinsic, ecological barriers (Westberg et al 2010;Melo et al 2014), our example of H. pusillum and H. veselskyi clearly shows that spatial separation of populations and adaptive phenotypic differentiation may confer a strong reduction in gene flow, which is especially significant at the onset of speciation, when intrinsic barriers are not yet in place. However, reproductive isolation solely based on environmentally, extrinsic barriers may be sensitive to environmental changes and subsequent vegetation shifts; only completion of reproductive isolation by endogenous mechanisms may on the long term ensure the stability of the diverging taxa.…”

Ditch network sustains functional connectivity and influences patterns of gene flow in an intensive agricultural landscape

“…Such species may be particularly vulnerable to habitat fragmentation (Aavik et al 2014). Besides, O. aquatica is a colonizer species experiencing localized extinction/recolonization events (Westberg et al, 2010;Westberg and Kadereit, 2014; L. Favre-Bac, pers. obs.…”

Differentiation in morphology and flowering phenology between two Campanula thyrsoides L. subspecies

“…This study showed that compensation in C. thyrsoides was generally strong and that the response to the herbivory simulation did not differ between the subspecies for most traits (Table 2), suggesting either (a) that any contrasting herbivory pressure between the habitats of the two subspecies did not lead to genetic differentiation in these traits or (b) that the two habitats did not differ in herbivory pressure. Suzuki (2008) similarly found no differences in plasticity in response to clipping in a common garden experiment among three populations of the annual Persicaria longiseta with different long-term deer grazing histories, and Rotundo and Aguiar (2008) likewise observed similar tolerance to defoliation among three populations of Poa ligularis differing in recent sheep grazing histories, although other studies do report differences among varieties or closely related species in response to herbivory simulation (Welter and Steggal 1993;Westberg et al 2010). It may be that herbivory thresholds, above which plant performance is differentially affected in the subspecies, have not been reached in our experiment (Strauss and Agrawal 1999).…”