Recurrent origins and adaptive advantage of polyploid Parnassia palustris

Abstract

Polyploidi er en sentral drivkraft for evolusjon hos dekkfrøede planter, og polyploide linjer er spesielt tallrike i arktiske strøk. I denne studien undersøkte jeg di- og tetraploide populasjoner av arten Jåblom, Parnassia palustris, som har divergerende distribusjon i Fennoskandia. Ved å bruke AFLP som genetiske markører fant jeg ut at arten har kolonisert Fennoskandia fra minst to refugier etter at isen trakk seg tilbake etter siste istid. Den høye genetiske diversiteten jeg fant i Fennoskandiske populasjoner kan være resultatet av slik sekundær kontakt mellom populasjoner som har vært isolerte fra hverandre under sist istid. Genetiske analyser viste at det er sannsynlig at tetraploidene i Europa har oppstått flere ganger, og/eller at genflyt har skjedd mellom di- og tetraploider. De molekylære analysene gir ikke grunnlag for å separere di- og tetraploidene i to linjer. Et veksteksperiment viste at forskjell mellom individer i overlevelse og vekst ikke kunne knyttes til polyploidi per se.

Background and aims: Polyploidy is a major driving force behind angiosperm evolution. The distribution of polyploid taxa has lead previous authors to suggest a connection between such genome duplication and major climatic events, like glaciations. In this study, European di- and tetraploid Parnassia palustris were used to explore how the genetic structure and distribution of the cytotypes relates to the glacial history of the study area, to evaluate the taxonomic treatment of the cytotypes, to search for evidence for interploidal gene flow and/or recurrent formation of the tetraploid, and to explore differences in ecological flexibility between cytotypes. Material and methods: Flow cytometry and AFLP were used to find ploidal levels and genetic structure. Growth experiments were performed in search for effects of ploidy on growth and survival during various day lengths and temperature treatments, and analysed employing Generalised Linear Models and Survival analyses. Results: Tetraploid populations were recorded from the Alps for the first time, and triploids were detected in areas where diploid and tetraploid populations are sympatric. Ploidal levels explained little of the genetic variation, and di- and tetraploid sympatric populations were genetically similar. High levels of genetic diversity were found in Fennoscandian populations. According to GLMs and survival analyses, survival was the only out of several fitness measures that was significantly affected by ploidy. High temperatures resulted in lower tetraploid survival. Conclusions: Tetraploids are mainly distributed in previously glaciated areas, and taxonomic rank for the tetraploid cytotype could not be supported. Immigration from at least two refugia probably met in a contact zone in Fennoscandia, resulting in high levels of genetic diversity. Gene flow between the cytotypes and/or recurrent origins of the tetraploid are probable processes. No differences in ecological flexibility could be attributed to polyploidy per se.