Abstract
How polyploids become established is a long-debated question, especially for autopolyploids that seem to have no evolutionary advantage over their progenitors. The Centaurea aspera polyploid complex includes diploid C. aspera and two related tetraploids C. seridis and C. gentilii. Our purpose was to study the mating system among these three taxa and to analyze its influence on polyploid establishment. The distribution and ploidy level of the Moroccan populations, and forced intra- and inter-specific crosses were assessed. Allotetraploid C. seridis produced more cypselae per capitulum in the intra-specific crosses. It is a bigger plant and autogamous, and previous studies indicated that selfing forces the asymmetric formation of sterile hybrids. All these characteristics help C. seridis to avoid the minority-cytotype-exclusion effect and become established. Inter-specific hybridization was possible between C. aspera and C. gentilii, and with the symmetric formation of hybrids. However, 49% of the hybrid cypselae were empty, which probably reveals postzygotic barriers. Autotetraploid C. gentilii produced the same number of cypselae per capitulum as those of the diploid parental, has an indistinguishable field phenotype, is allogamous, and symmetrically produces hybrids. Therefore, C. gentilii does not seem to have the same competitive advantages as those of C. seridis.
Highlights
Polyploidy is one of the most important and ubiquitous driving forces in plant evolution [1,2,3].In a climate-change context, Levin (2019) proposed that polyploidization will be one of the most frequent speciation modes in the 500 years, with an increase in the percentage of polyploid plants from 35% today to more than 50%
As all the Moroccan sampled individuals were tetraploid (4×), and in accordance with the collected data, C. gentilii is represented by tetraploid populations on the southern Atlantic coast of Morocco, while C. seridis is represented by tetraploid populations on the northern Atlantic and Mediterranean
Our results showed that the allotetraploid C. seridis produces almost twice the number of cypselae in the intra-specific crosses compared to C. aspera and C. gentilii
Summary
In a climate-change context, Levin (2019) proposed that polyploidization will be one of the most frequent speciation modes in the 500 years, with an increase in the percentage of polyploid plants from 35% today to more than 50%. This hypothesis is related to the fact that polyploidy can occur in sympatry, and neopolyploids can be established in only a few generations; they might be reinforced in rapidly changing scenarios [4]. Autopolyploids are considered to form by genome multiplication, while allopolyploids derive from hybridization between species with differentiated genomes, either by chromosome doubling. The term “segmental allopolyploid” is used to describe polyploids that do not exhibit strict bivalent formation across all chromosomes or disomic inheritance at all loci [5]
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