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

Karl Hülber,Peter Schönswetter,Clara Bertel,Božo Frajman

doi:10.1007/s10682-016-9867-y

Karl Hülber, Peter Schönswetter + Show 2 more

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https://doi.org/10.1007/s10682-016-9867-y

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Journal: Evolutionary Ecology	Publication Date: Oct 18, 2016
Citations: 13	License type: CC BY 4.0

Affiliation: University of Vienna, Universität Innsbruck

Abstract

Adaptation to dissimilar habitats can trigger phenotypic and genetic differences between populations, which may, in the absence of gene flow, ultimately lead to ecological speciation. Reproductive isolation of diverging populations is a critical step at the onset of speciation. An excellent example for exploring the extent of reproductive isolation at early stages of speciation is provided by Heliosperma pusillum and H. veselskyi (Caryophyllaceae), two reciprocally non-monophyletic, ecologically differentiated species from the Alps. Interspecific gene flow—as revealed by recent genetic studies—is rare even between geographically close populations. Cross pollinations and fitness experiments revealed no evidence of intrinsic reproductive barriers, since fitness parameters measured under uniform conditions were not lower in inter- than in intraspecific crosses. Further, morphometric analyses of the offspring clearly showed that the differentiation of parental species is heritable. As parental phenotypes are likely adaptive, the intermediate morphology of hybrids may lead to reduced hybrid fitness in parental habitats. Altogether, H. pusillum and H. veselskyi provide an increasingly well characterised model system offering exciting insights into early stages of ecological speciation.

Highlights

Adaptation to different environmental conditions via divergent natural selection can generate phenotypic and genetic differences between populations, which may in the absence of gene flow over time lead to the formation of new species (Nosil 2012)
(2) Second, by applying morphometric analyses to offspring of intra- and interspecific crosses we investigated whether phenotypic differentiation between H. pusillum and H. veselskyi is heritable—indicating the onset of speciation—and whether hybrids exhibit morphologically intermediate phenotypes
Fitness parameters did not differ between intraspecific crosses (V–V, pollinations conducted within H. pusillum (P–P)) except for V–V showing higher mean seed weight (Table 2; Fig. 1)

Summary

Introduction

Adaptation to different environmental conditions via divergent natural selection can generate phenotypic and genetic differences between populations, which may in the absence of gene flow over time lead to the formation of new species (Nosil 2012). Intermediate stages in ecological speciation involve ecotypes (Lowry 2012), i.e. groups of populations within one species differentiated due to adaptation to local environmental conditions, which are frequently still interfertile (Hufford and Mazer 2003) and result from a combination of heritable and non-heritable traits (Pfennig et al 2010; Bonduriansky et al 2012). Differences in phenotypic traits associated with increased fitness in a specific habitat can result from environmental conditions experienced during ontogeny (phenotypic plasticity) but can be mediated by heritable (epi-)genetic differences (local adaptation via phenotypic differentiation, Flatscher et al 2012). In the latter case, non-adapted immigrants and hybrids exhibiting intermediate phenotypic traits may show reduced fitness (Rice and Hostert 1993; Rundle and Whitlock 2001)

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