Similar but not identical: cryptic speciation of Saxifraga rosacea
Numerous models have been proposed to explain how new species arise. Climate change is one of the mechanism known to apply strong selection pressures and increase the chance of speciation. During the Quaternary period, glacial advances and retreats redrafted landscapes by modifying the geographical...
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| Format: | Dissertation |
| Sprache: | Englisch |
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Philipps-Universität Marburg
2021
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| Zusammenfassung: | Numerous models have been proposed to explain how new species arise. Climate change is one of the mechanism known to apply strong selection pressures and increase the chance of speciation. During the Quaternary period, glacial advances and retreats redrafted landscapes by modifying the geographical ranges of plants and animals. Their distribution area were subjected to dramatic envionmental changes that could have accelerate the speciation rate, due to divergences selection, genetic drift or population isolation. In plant species, inability to quickly migrate with drastic climate space movements may have favor genome modifications like polyploidization, a well-known abrupt mode of speciation in angiosperm species by genome duplication. A higher number of genome copies thus a more divers set of alleles, may provide advantages to cope with harsher climatic conditions, contributing to species differenciation of plants with different cytotypes. This thesis investigates the ecological, demographical and taxonomical differences between two glacial plants relicts of Saxifraga rosacea with distinct distribution areas and different ploidy levels, the hexaploid Saxifraga rosacea subsp. sponhemica and the octoploid Saxifraga rosacea subsp. rosacea.
The investigation of 22 populations of the two closely related Saxifraga cytotypes showed that plants of the octoploid Saxifraga occured in sites with lower mean annual temperature and were more cold tolerant than plants of the hexaploid Saxifraga. Also, plants performed better in their region of origin in a two transplant sites experiment with contasting conditions indicating local adaptation. But among populations of the octoploid, genetic differences in survival were larger than among populations of the hexaploids suggesting a greater genetic variability that may enable the Saxifraga taxa with a higher ploidy level to occupy a larger distribution range, as revealed by probabilities of occurence according to our Maxent niche modelling. Our results suggest that cytotypes differences may lead to ditribution segregation and a higher ploidy level may enable to occupy broader ecological niches. Differences in geographical distribution and habitat fragmentation may have consequences on population dynamics, as growth rate or population sturcture, and extinction risks of the two cytotypes thus require different conservation plans.
Monitoring, for several years, the variability and demography dynamics of Saxifraga rosacea subsp. sponhemica populations and Saxifraga rosacea subsp. rosacea populations enlightened strong decline of population growth rates of both Saxifraga and this trend was stronger in sponhemica ssp. populations than in rosacea ssp. populations. Despite decade longevity of both Saxifraga plants and comparable population dynamics with few young and few old individuals, the two cytotypes may encounter different fates. Analyses of populations size-class structures showed that with current climatic conditions, populations of octoploids could remain stable but populations of hexaploids will decline due to low fecundities. Still, extinction risks models project negative futures for both Saxifraga and need management and conservation programs. This finding support the hypothesis that with similar habitat and demography characteristic, two closely related cytotypes may still have different fates with ongoing climate changes. Demographic studies are important to establishing suitable conservation actions for fragmented populations.
Management of isolated populations of endangered species often includes genetic diversity and gene flow studies however taxonomy clarification should also be investigated, especially with closely related taxa. We studied the effects of within and between population crosses, hybridization and backcrosses of the two Saxifraga cytotypes on reproductive fitness, performances and taxonomical traits of two generations of offspring. Evidences of outbreeding depression were found at F1 generation on hybrids between the two cytotypes and drastic loss of fitness at F2 generation confirmed a reproductive isolation between the hexaploid Saxifraga rosacea subsp. sponhemica and the octoploid Saxifraga rosacea subsp. rosacea. Taxonomy of the two Saxifraga as subspecies should be revised, the two cytotypes should be established as two different species and their respective area of distribution should be reviewed. Taxonomical traits should also be reevaluated as two of three leaves morphological traits used in previous literature showed too important variations to remain reliable taxonomical criterion. It would be essential to reexamine the legal protection status of the two Saxifraga and to set up appropriate management plans. Artificial gene flow among the isolated populations of each cytotype could be considered to counter signs of inbreeding depression revealed by the hybrid vigor of between population crosses compared to within population crosses. Also the low performances of hybrids in the F2 generation could explain the absence of known mixed population thus mixing populations of the two cytotypes could subvert the conservation of wild populations of the two Saxifraga.
Overall, the results of this thesis explore the performances of plants with different ploidy levels and provide new insights on the role of polyploidization in increasing the evolutionary potential of plant species to adapt to changing environmental conditions. It highlights the importance of using integrative approaches by combining ecological, demographical and taxonomical studies to establish appropriate management plans for the conservation of rare and endangered closely related plant species. |
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| Umfang: | 141 Seiten |
| DOI: | 10.17192/z2022.0054 |