Abstract
Peanut, Arachis hypogaea (Linnaeus, 1753) is an allotetraploid cultivated plant with two subgenomes derived from the hybridization between two diploid wild species, A. duranensis (Krapovickas & W. C. Gregory, 1994) and A. ipaensis (Krapovickas & W. C. Gregory, 1994), followed by spontaneous chromosomal duplication. To understand genome changes following polyploidy, the chromosomes of A. hypogaea, IpaDur1, an induced allotetraploid (A. ipaensis × A. duranensis)4x and the diploid progenitor species were cytogenetically compared. The karyotypes of the allotetraploids share the number and general morphology of chromosomes; DAPI+ bands pattern and number of 5S rDNA loci. However, one 5S rDNA locus presents a heteromorphic FISH signal in both allotetraploids, relative to corresponding progenitor. Whilst for A. hypogaea the number of 45S rDNA loci was equivalent to the sum of those present in the diploid species, in IpaDur1, two loci have not been detected. Overall distribution of repetitive DNA sequences was similar in both allotetraploids, although A. hypogaea had additional CMA3+ bands and few slight differences in the LTR-retrotransposons distribution compared to IpaDur1. GISH showed that the chromosomes of both allotetraploids had preferential hybridization to their corresponding diploid genomes. Nevertheless, at least one pair of IpaDur1 chromosomes had a clear mosaic hybridization pattern indicating recombination between the subgenomes, clear evidence that the genome of IpaDur1 shows some instability comparing to the genome of A. hypogaea that shows no mosaic of subgenomes, although both allotetraploids derive from the same progenitor species. For some reasons, the chromosome structure of A. hypogaea is inherently more stable, or, it has been at least, partially stabilized through genetic changes and selection.
Highlights
The genus Arachis (Linnaeus, 1753) is native to South America, with Arachis as the largest botanical section
Earlier evidence from cytogenetics, genetic mapping and analysis of progeny derived from crosses of A. hypogaea with an induced allotetraploid [(A. ipaensis K30076 × A. duranensis V14167)4x] (Fávero et al 2006) showed that their genomes had not undergone large-scale rearrangements since polyploidization (Fávero et al 2015, Ramos et al 2006, Seijo et al 2007, Foncèka et al 2009, Shirasawa et al 2013)
Meiotic pairing in A. hypogaea is described as presenting the bivalents, with rare univalents, trivalents, and quadrivalent exceptions (Husted 1936), there is an indication of limited homeologous pairing between A and B subgenomes, as the recent genetic studies suggested that cultivated peanut may be better classified as a segmental allotetraploid with predominantly disomic, but partially tetrasomic genetics (Leal-Bertioli et al 2015, Bertioli et al 2016, Clevenger et al 2017)
Summary
The genus Arachis (Linnaeus, 1753) is native to South America, with Arachis as the largest botanical section. Most species are diploids (2n = 2x = 20), but there are a few aneuploids and two tetraploids: A. hypogaea (Linnaeus, 1753), the cultivated peanut (groundnut) and A. monticola (Krapovickas & Rigoni, 1958) (2n = 4x = 40) (Krapovickas and Gregory 1994, Valls and Simpson 2005). A. hypogaea has its origin estimated between 3,500 and 9,400 years ago (Bonavia 1982, Simpson et al 2001, Bertioli et al 2016), from one or few events of hybridization between two wild diploid species, followed by spontaneous polyploidization (Singh 1986, Kochert et al 1996, Grabiele et al 2012). In A. hypogaea, the secondary constrictions observed on the B subgenome chromosomes have been silenced (Seijo et al 2004), a common event in polyploids called nucleolar dominance (Navashin 1934, Preuss and Pikaard 2007)
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