Abstract
Reticulate evolution is characterized by occasional hybridization between two species, creating a network of closely related taxa below and at the species level. In the present research, we aimed to verify the hypothesis of the allopolyploid origin of hexaploid C. album s. str., identify its putative parents and estimate the frequency of allopolyploidization events. We sampled 122 individuals of the C. album aggregate, covering most of its distribution range in Eurasia. Our samples included putative progenitors of C. album s. str. of both ploidy levels, i.e. diploids (C. ficifolium, C. suecicum) and tetraploids (C. striatiforme, C. strictum). To fulfil these objectives, we analysed sequence variation in the nrDNA ITS region and the rpl32-trnL intergenic spacer of cpDNA and performed genomic in-situ hybridization (GISH). Our study confirms the allohexaploid origin of C. album s. str. Analysis of cpDNA revealed tetraploids as the maternal species. In most accessions of hexaploid C. album s. str., ITS sequences were completely or nearly completely homogenized towards the tetraploid maternal ribotype; a tetraploid species therefore served as one genome donor. GISH revealed a strong hybridization signal on the same eighteen chromosomes of C. album s. str. with both diploid species C. ficifolium and C. suecicum. The second genome donor was therefore a diploid species. Moreover, some individuals with completely unhomogenized ITS sequences were found. Thus, hexaploid individuals of C. album s. str. with ITS sequences homogenized to different degrees may represent hybrids of different ages. This proves the existence of at least two different allopolyploid lineages, indicating a polyphyletic origin of C. album s. str.
Highlights
How new forms arise in nature is a key question in evolutionary biology
One haplotype was found in C. ficifolium (5 accessions analysed), two in C. suecicum (9 accessions), two in C. strictum (8 accessions) and one in C. striatiforme (2 accessions)
The same haplotype was found in hexaploid C. album s. str
Summary
How new forms arise in nature is a key question in evolutionary biology. It is believed that a significant part of speciation and diversification in plants ( in angiosperms and ferns) involved reticulate evolution characterized by occasional hybridization creating a PLOS ONE | DOI:10.1371/journal.pone.0161063 August 11, 2016Allopolyploid Origin of Chenopodium album s. str.network of closely related taxa below and at the species level [1]. A new hybridogenous species may have the same number of chromosomes as its parents (homoploid hybridization), or be a descendant of either (i) cross-fertile progenitors that underwent doubling of structurally homologous genomes, i.e. autopolyploidy, or (ii) cross-sterile individuals escaping from sterility by chromosome doubling, i.e. allopolyploidy [2,3,4,5,6,7]. Hybridization can lead to rapid genomic changes, including chromosomal rearrangements, genome expansion, differential gene expression and gene silencing [8, 9]. These genomic changes may lead to beneficial new phenotypes, and selection for ecological traits may in turn alter genome structure [10]. Due to the contribution of previously isolated (and diverse) genotypes, hybrids are supposed to have higher genetic diversity compared to their parents [11,12,13,14]
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