Evolutionary Significance of Polyploidy in the Pteridophyta

Klekowski, Edward J.; Baker, Herbert G.

doi:10.1126/science.153.3733.305

Cited by 169 publications

(101 citation statements)

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“…Ferns have especially high chromosome numbers, with Ophioglossum reticulatum possessing the highest known chromosome count among extant eukaryotes (n > 600; Khandelwal, 1990). However, a distinction should be made between heterosporous ferns that have chromosome counts similar to angiosperms (n = 15.99 on average) and homosporous ferns where the chromosome count is more than 3 times higher (n = 57.05 on average; Klekowski and Baker, 1966). This early work investigated a limited number of homosporous ferns and concluded that selfing was a common mechanism of reproduction (Klekowski and Baker, 1966).…”

Section: Introductionmentioning

confidence: 99%

“…However, a distinction should be made between heterosporous ferns that have chromosome counts similar to angiosperms (n = 15.99 on average) and homosporous ferns where the chromosome count is more than 3 times higher (n = 57.05 on average; Klekowski and Baker, 1966). This early work investigated a limited number of homosporous ferns and concluded that selfing was a common mechanism of reproduction (Klekowski and Baker, 1966). Combined with later observations of unusually high numbers of chromosomes in homosporous ferns (and the resultant assumption that this was due to polyploidy), the hypothesis emerged that polyploidy would present a plausible mechanism allowing the generation of novel genetic material to avoid homozygous inbreeding depression by pairing of homoeologous rather than homologous chromosomes during meiosis (Klekowski, 1973).…”

Section: Introductionmentioning

confidence: 99%

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Horsetails Are Ancient Polyploids: Evidence from Equisetum giganteum

et al. 2015

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Horsetails represent an enigmatic clade within the land plants. Despite consisting only of one genus (Equisetum) that contains 15 species, they are thought to represent the oldest extant genus within the vascular plants dating back possibly as far as the Triassic. Horsetails have retained several ancient features and are also characterized by a particularly high chromosome count (n = 108). Whole-genome duplications (WGDs) have been uncovered in many angiosperm clades and have been associated with the success of angiosperms, both in terms of species richness and biomass dominance, but remain understudied in nonangiosperm clades. Here, we report unambiguous evidence of an ancient WGD in the fern linage, based on sequencing and de novo assembly of an expressed gene catalog (transcriptome) from the giant horsetail (Equisetum giganteum). We demonstrate that horsetails underwent an independent paleopolyploidy during the Late Cretaceous prior to the diversification of the genus but did not experience any recent polyploidizations that could account for their high chromosome number. We also discuss the specific retention of genes following the WGD and how this may be linked to their long-term survival.

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Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Horsetails Are Ancient Polyploids: Evidence from Equisetum giganteum

et al. 2015

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“…The average chromosome number in homosporous ferns was estimated to be n ¼ 57.05, as opposed to n ¼ 15.99 in angiosperms (Klekowski and Baker 1966). Angiosperm species with chromosome numbers .n ¼ 14 are generally considered to be polyploid (Grant 1981).…”

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Genetic Map-Based Analysis of Genome Structure in the Homosporous Fern Ceratopteris richardii

Nakazato¹,

Jung

Housworth

et al. 2006

Genetics

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Homosporous ferns have extremely high chromosome numbers relative to flowering plants, but the species with the lowest chromosome numbers show gene expression patterns typical of diploid organisms, suggesting that they may be diploidized ancient polyploids. To investigate the role of polyploidy in fern genome evolution, and to provide permanent genetic resources for this neglected group, we constructed a high-resolution genetic linkage map of the homosporous fern model species, Ceratopteris richardii (n ¼ 39). Linkage map construction employed 488 doubled haploid lines (DHLs) that were genotyped for 368 RFLP, 358 AFLP, and 3 isozyme markers. Forty-one linkage groups were recovered, with average spacing between markers of 3.18 cM. Most loci ($76%) are duplicated and most duplicates occur on different linkage groups, indicating that as in other eukaryotic genomes, gene duplication plays a prominent role in shaping the architecture of fern genomes. Although past polyploidization is a potential mechanism for the observed abundance of gene duplicates, a wide range in the number of gene duplicates as well as the absence of large syntenic regions consisting of duplicated gene copies implies that small-scale duplications may be the primary mode of gene duplication in C. richardii. Alternatively, evidence of past polyploidization(s) may be masked by extensive chromosomal rearrangements as well as smaller-scale duplications and deletions following polyploidization(s).

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“…The average haploid chromosome number for homosporous ferns, 57.05 (1), is so high that up to 95% of all fern species have been considered polyploid (2). However, since the essence of polyploidy is the number of component genomes and not the number ofchromosomes, the genetic significance of such high polyploidy estimates may be questioned.…”

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confidence: 99%

Genetic evidence suggests that homosporous ferns with high chromosome numbers are diploid

Haufler

Soltis

1986

Proc. Natl. Acad. Sci. U.S.A.

136

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Homosporous ferns have usually been considered highly polyploid because they have high chromosome numbers (average n = 57.05). In angiosperms, species with chromosome numbers higher than n = 14 generally have more isozymes than those with lower numbers, consistent with their polyploidy. By extrapolation, homosporous ferns would be expected to have many isozymes. However, ongoing surveys indicate that within fern genera, species having the lowest chromosome numbers have the number of isozymes considered typical of diploid seed plants. Only species above these lowest numbers have additional isozymes. Therefore, homosporous ferns either have gone through repeated cycles of polyploidy and gene silencing or were initiated with relatively high chromosome numbers. The latter possibility represents a radical departure from currently advocated hypotheses of fern evolution and suggests that there may be fundamental differences between the genomes of homosporous ferns and those of higher plants. These hypotheses can be tested by genetic, karyological, and molecular techniques.

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Evolutionary Significance of Polyploidy in the Pteridophyta

Cited by 169 publications

References 4 publications

Horsetails Are Ancient Polyploids: Evidence from Equisetum giganteum

Horsetails Are Ancient Polyploids: Evidence from Equisetum giganteum

Genetic Map-Based Analysis of Genome Structure in the Homosporous Fern Ceratopteris richardii

Genetic evidence suggests that homosporous ferns with high chromosome numbers are diploid

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