Abstract
Although polyploidy is considered a ubiquitous process in plants, the establishment of new polyploid species may be hindered by ecological competition with parental diploid taxa. In such cases, the adaptive processes that result in the ecological divergence of diploids and polyploids can lead to their co-existence. In contrast, non-adaptive processes can lead to the co-existence of diploids and polyploids or to differentiated distributions, particularly when the minority cytotype disadvantage effect comes into play. Although large-scale studies of cytotype distributions have been widely conducted, the segregation of sympatric cytotypes on fine scales has been poorly studied. We analysed the spatial distribution and ecological requirements of the tetraploid Centaurea seridis and the diploid Centaurea aspera in east Spain on a large scale, and also microspatially in contact zones where both species hybridise and give rise to sterile triploid hybrids. On the fine scale, the position of each Centaurea individual was recorded along with soil parameters, accompanying species cover and plant richness. On the east Spanish coast, a slight latitudinal gradient was found. Tetraploid C. seridis individuals were located northerly and diploid C. aspera individuals southerly. Tetraploids were found only in the habitats with strong anthropogenic disturbance. In disturbed locations with well-developed semi-fixed or fixed dunes, diploids and tetraploids could co-exist and hybridise. However, on a fine scale, although taxa were spatially segregated in contact zones, they were not ecologically differentiated. This finding suggests the existence of non-adaptive processes that have led to their co-existence. Triploid hybrids were closer to diploid allogamous mothers (C. aspera) than to tetraploid autogamous fathers (C. seridis). This may result in a better ability to compete for space in the tetraploid minor cytotype, which might facilitate its long-term persistence.
Highlights
Polyploidy is considered a ubiquitous process that has played a key role in plant evolution (Wendel et al, 2016)
We addressed the following questions: (i) what is the spatial structure and what are the ecological requirements of diploids and their allotetraploid derivatives across east Spain? (ii) What is the microspatial distribution pattern of individuals in mixed-ploidy plots? (iii) Does the distribution of triploids in contact areas correspond to patterns of crossability between the diploid C. aspera and the tetraploid C. seridis? (iv) Is there any correlation between taxa distribution on a fine scale and ecological microhabitat characteristics? In coastal dunes, there are strong gradients of various environmental factors that run perpendicularly to the shoreline
Distribution of the Centaurea taxa in east Spain and south France, and ecological preferences The occurrence of the diploid C. aspera and/or tetraploid C. seridis individuals in all the 39 studied sites is shown in Fig. 1 and Table 1
Summary
Polyploidy is considered a ubiquitous process that has played a key role in plant evolution (Wendel et al, 2016). The coexistence of diploids and polyploids can be fairly stable for different factors, of which ecological divergence following adaptive processes is among the most important (Mable, 2003; Hulber et al, 2009; Mraz et al, 2012) This divergence is driven by an environmentally-dependent selection along an abiotic or biotic gradient, which results in the differentiation of the ecophysiological requirements of different related cytotypes. Polyploids may display a better fitness advantage in novel environments due to both increased genetic diversity, on which selection can act, and novel biochemical pathways and transgressive characters (Leitch & Leitch, 2008) This new steady state may confer onto them a predisposition towards both the extension of their ecological amplitude and quick adaptation to changing environmental conditions (Ramsey, 2011; Hulber et al, 2015). Differentiation among related cytotypes can be reflected in shifts in the abundance of accompanying species (Johnson, Husband & Burton, 2003), spatial segregation based on distinct ecological preferences within the same habitat type (Raabova, Fischer & Munzbergova, 2008), or separation of cytotypes into plant communities that differ in structure and physiognomy (Lumaret et al, 1987)
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