Ecological differentiation, lack of hybrids involving diploids, and asymmetric gene flow between polyploids in narrow contact zones of Senecio carniolicus (syn. Jacobaea carniolica, Asteraceae).

Karl Hülber,Peter Schönswetter,Manuela Winkler,Jana Krejčíková,Michaela Sonnleitner,Jan Suda,Gerald M Schneeweiss

doi:10.1002/ece3.1430

Abstract

Areas of immediate contact of different cytotypes offer a unique opportunity to study evolutionary dynamics within heteroploid species and to assess isolation mechanisms governing coexistence of cytotypes of different ploidy. The degree of reproductive isolation of cytotypes, that is, the frequency of heteroploid crosses and subsequent formation of viable and (partly) fertile hybrids, plays a crucial role for the long-term integrity of lineages in contact zones. Here, we assessed fine-scale distribution, spatial clustering, and ecological niches as well as patterns of gene flow in parental and hybrid cytotypes in zones of immediate contact of di-, tetra-, and hexaploid Senecio carniolicus (Asteraceae) in the Eastern Alps. Cytotypes were spatially separated also at the investigated microscale; the strongest spatial separation was observed for the fully interfertile tetra- and hexaploids. The three main cytotypes showed highly significant niche differences, which were, however, weaker than across their entire distribution ranges in the Eastern Alps. Individuals with intermediate ploidy levels were found neither in the diploid/tetraploid nor in the diploid/hexaploid contact zones indicating strong reproductive barriers. In contrast, pentaploid individuals were frequent in the tetraploid/hexaploid contact zone, albeit limited to a narrow strip in the immediate contact zone of their parental cytotypes. AFLP fingerprinting data revealed introgressive gene flow mediated by pentaploid hybrids from tetra- to hexaploid individuals, but not vice versa. The ecological niche of pentaploids differed significantly from that of tetraploids but not from hexaploids.

Highlights

Ecological differentiation is among the most important mechanisms of reproductive isolation among cytotypes of different ploidy in heteroploid plant species (Levin 1983)
Patterns of niche differentiation in areas of immediate contact allow inferring whether contact zones represent hybrid zones, that is, habitats suited for both cytotypes, or mosaic zones, that is, a microspatial mixture of habitats each suited for a single cytotype
Niche differences in contact zones are expected to be smaller compared to both adjacent pure populations and the entire distribution ranges of the cytotypes, whereas no such reduction in niche differences is expected in case of mosaic zones

Summary

Introduction

Ecological differentiation is among the most important mechanisms of reproductive isolation among cytotypes of different ploidy (for simplicity termed “cytotypes” from here on) in heteroploid plant species (Levin 1983) It may arise as a direct consequence of genome duplication (Otto and Whitton 2000) or result from subsequent disruptive selection (Petit et al 1999; Ramsey and Schemske 2002). Contact zones of cytotypes – we use the term in a strict sense to encompass areas of close spatial proximity of individuals of different ploidy – can be observed in many taxa. They provide a unique opportunity to assess isolation mechanisms governing coexistence of cytotypes, such as ecological differentiation (Petit et al 1999). Niche differences in contact zones are expected to be smaller compared to both adjacent pure populations and the entire distribution ranges of the cytotypes, whereas no such reduction in niche differences is expected in case of mosaic zones

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Conclusion