Intra- and Inter-Specific Crosses among Centaurea aspera L. (Asteraceae) Polyploid Relatives—Influences on Distribution and Polyploid Establishment

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.

Cytogenetic diversity in the polyploid complex Linum suffruticosum s.l. (Linaceae)

Polyploidy plays a significant role in the evolution and diversification of flowering plants. In several polyploid complexes, high morphological variability and plasticity coupled with cytogenetic diversity make it difficult to disentangle their evolutionary history. The main goal of this study was to gain insights into the role of whole genome duplications as one of the factors shaping the evolution of flowering plants. Linum suffruticosum s.l. has been described as a polyploid complex, with high morphological variability, but nothing is known about current cytogeographical patterns. We investigated cytotype diversity and distribution patterns in 151 populations covering most of the distribution range, in the Iberian Peninsula, south-eastern France, north-western Italy and Morocco, using flow cytometric analyses complemented with chromosome counts. A high cytogenetic diversity was found with five major cytotypes being detected (diploids, tetraploids, hexaploids, octoploids and decaploids) and with new ploidy levels being reported for the first time. The different ploidies were distributed parapatrically, with geographical structure and several contact zones. Most of the populations comprised one cytotype, but a few mixed-ploidy populations were observed. Our results suggest that whole genome duplications are one of the key mechanisms, alone or together with hybridization, governing the diversification of L. suffruticosum s.l. Genome size and/or chromosome counts might be useful tools for identifying specimens of L. suffruticosum s.l. Also, geographical overlap and high cytogenetic diversity suggest multiple origins of the polyploids. The diversity observed here has been mostly neglected to date and should be accounted when studying the biosystematics of this complex.

Speciation by triparental hybridization in genus Sorbus (Rosaceae)

Hybridization associated with polyploidization and apomixis is a frequent mechanism of speciation. Sorbus is a genus with ongoing hybridization resulting in a polyploid complex with different parental lineage. Triparens is the smallest hybridogenous subgenus of Sorbus so far known to comprise only two taxa, S. intermedia and S. × liljeforsii that combine the genomes of three taxa (S. aria agg., S. aucuparia and S. torminalis). To elucidate the origins of S. dacica, S. paxiana and S. tauricola, three new trigenomic candidates formerly believed to be of biparental origin with either S. aria agg. × S. aucuparia or S. aria agg. × S. torminalis lineage we combined data from HPLC and chloroplast DNA analysing additional 33 related taxa as well. We concluded that the ‘torminalis-type’ flavonoid profile and the ‘aucuparia-type’ plastid indicate the participation of both S. torminalis and S. aucuparia resulting in the formation of S. dacica, S. paxiana and S. tauricola. Sorbus aria agg. as the third ancestor and as a necessary link to meet genes of S. torminalis and S. aucuparia in one genome is obvious from morphological features (densely tomentose undersides of leaves). The tetraploid cytotypes and obligate pseudogamy of S. dacica and S. paxiana were determined by flow cytometry and are published here for the first time. The most probable evolutionary scenario for Triparens species is: 1. a diploid sexual S. aucuparia as pollen acceptor hybridized with a tetraploid apomictic taxon from the S. aria agg. producing a triploid apomictic taxon with ‘aucuparia-type’ plastid inherited maternally; 2. during a second crossing event this subgenus Soraria hybrid as maternal progenitor hybridized with the sexual diploid S. torminalis (providing gene(s) of apigenin O-glucuronide synthesis) forming a tetraploid Triparens hybrid with ‘aucuparia-type’ plastid and ‘torminalis-type’ flavonoids.

Genome Status of Lippia alba Polyploid Complex Long-term in vitro Cultivated

This study is the first report of a genetic stability analysis of a polyploid complex maintained in vitro for a long-time. Twenty-two accessions of Lippia alba, a medicinal species of economic importance, had been maintained under in vitro culture conditions for 7 years through sprouting of axillary buds. Four clones of each accession were analyzed, being three plants from in vitro bank and one cultivated in the field. We investigated the genetic stability of diploid, aneuploid, triploid, tetraploid, and hexaploid accessions. The investigation was carried out using flow cytometry, inter simple sequence repeat (ISSR) and simple sequence repeat (SSR) markers. No significant variation in nuclear DNA content was observed between the in vitro conserved plants and their respective field plant. Out of 23 ISSR primers screened, 8 primers were found to produce clear reproducible bands resulting in a total of 5456 bands. 86.36% of the analyzed plantlets (19 accessions) showed at least one polymorphic band. The polymorphic rate ranged from 1.61 to 33.87%. The SSR markers were used to confirm the absence or low occurrence of variation in accessions that showed no polymorphism or polymorphism for only one ISSR primer. The genetic instability detected in our study at the molecular level may be attributed to the natural instability of L. alba genome combined with the long-time in vitro maintenance.