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Peanut is an allotetraploid (2n = 2x = 40, AABB) of recent origin. Arachis duranensis and A. ipaënsis, the most probable diploid ancestors of the cultigen, and several other wild diploid species with different genomes (A, B, D, F and K) are used in peanut breeding programs. However, the genomic relationships and the evolutionary pathways of genome differentiation of these species are poorly understood. We performed a sequence-based phylogenetic analysis of the L1 reverse transcriptase and estimated its representation and chromosome distribution in species of five genomes and three karyotype groups with the aim of contributing to the knowledge of the genomic structure and evolution of peanut and wild diploid relatives. All the isolated rt fragments were found to belong to plant L1 lineage and were named ALI. The best supported phylogenetic groups were not concordant with the genomes or karyotype groups. The copy number of ALI sequences was higher than the expected one for plants and directly related to genome size. FISH experiments revealed that ALI is mainly located on the euchromatin of interstitial and distal regions of most chromosome arms. Divergence of ALI sequences would have occurred before the differentiation of the genomes and karyotype groups of Arachis. The representation and chromosome distribution of ALI in peanut was almost additive of those of the parental species suggesting that the spontaneous hybridization of the two parental species of peanut followed by chromosome doubling would not have induced a significant burst of ALI transposition.

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Knowledge of the chromosome variation in wild populations is essential to understand the pathways and restrictions of karyotype evolution in plants. The aim of this study is to conduct an intraspecific analysis of the karyotypes by fluorochrome banding and ribosomal DNA (rDNA) loci detection by fluorescent in situ hybridization (FISH) and of the meiotic behaviour in natural populations of Lathyrus nervosus, sect. Notolathyrus. Chromosome banding showed that, despite the high constancy in the karyotype formula and in the rDNA loci among populations, there is intraspecific variation in the amount and distribution pattern of 4',6-diamidino-2-phenylindole (DAPI ? ) heterochromatin. However, those changes were not related to the total chromosome length of the haploid complements. This fact demonstrates that structural chromosome changes may be one of the most important mechanisms for karyotype variation among natural populations of L. nervosus. The chromosome number surveyed at the population level revealed the first case of polyploidy in South American species and the first case of uneven polyploidy of the genus. All the chromosome markers analysed indicated that the polyploids found originated by autopolyploidy. The meiotic analysis showed different chromosome abnormalities that may be generating numerical and structural changes in the sporads. The finding of unreduced gametes that are alive at anthesis suggests sexual polyploidization as the most probable mechanism involved in the origin of these 3x and 4x autopolyploid cytotypes in L. nervosus.

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The genome size was surveyed in 13 Notolathyrus species endemic to South America by flow cytometry and analyzed in an evolutionary and biogeographic context. A DNA content variation of 1.7-fold was registered, and four groups of species with different DNA content were determined. Although, the 2C values were correlated with the total chromosome length and intrachromosomal asymmetry index (A1), the karyotype formula remained almost constant. The conservation of the karyotype formula is in agreement with proportional changes of DNA in the chromosome arms. Species with annual life cycle and shorter generation time had the lowest DNA content and the data suggest that changes in DNA content involved reductions of genome size in the perennial to annual transitions. The variation of 2C values was correlated with precipitation of the coldest quarter and, to some extent, with altitude. Additional correlations with other variables were observed when the species were analyzed separately according to the biogeographic regions. In general, the species with higher DNA content were found in more stable environments. The bulk of evidence suggests that changes on genome size would have been one of the most important mechanisms that drove or accompanied the diversification of Notolathyrus species.

Allopolyploidy is a significant evolutionary process involved in the origin of many crops, including peanut (Arachis hypogaea L.). The process usually results in a series of chromosome, genomic and epigenetic rearrangements in the derived polyploids. Here, we examined the chromosomal consequences undergone by AABB tetraploids of Arachis after the genome merger. For that objective, different chromosome markers and DNA contents were compared among peanut, its wild tetraploid ancestor, and the diploid genome donors A. duranensis Krapov. & W.C. Gregory (AA, female) and A. ipaënsis Krapov. & W.C. Gregory (BB, male). The analysis also included an artificially synthesized allotetraploid using A. ipaënsis as a female [(A. ipaënsis × A. duranensis)4×]. The karyotypes in the natural (originated ~10,000 yr ago) and newly synthesized allopolyploids have largely maintained the patterns of heterochromatin and ribosomal RNA loci detected in the diploid progenitors. Intergenomic translocations were not evident using genome in situ hybridization, and the DNA contents of the allotetraploids corresponded to the expected sum of those observed in their parental species. The analysis of ribosomal DNA loci and their association with nucleolar organizing regions revealed a rapid establishment of nucleolar dominance in favor of the A genome. The large macrostructural stability of karyotype observed here after polyploidization has not been frequently cited for polyploid crop plants. This stability is significant for peanut breeding, since it suggests that effective introgression of wild useful alleles into cultivated peanut may potentially occur in most of the extension of the A and B chromosome complements.

Notolathyrus is a section of South American endemic species of the genus Lathyrus. The origin, phylogenetic relationship and delimitation of some species are still controversial. The present study provides an exhaustive analysis of the karyotypes of approximately half (10) of the species recognized for section Notolathyrus and four outgroups (sections Lathyrus and Orobus) by cytogenetic mapping of heterochromatic bands and 45S and 5S rDNA loci. The bulk of the parameters analyzed here generated markers to identify most of the chromosomes in the complements of the analyzed species. Chromosome banding showed interspecific variation in the amount and distribution of heterochromatin, and together with the distribution of rDNA loci, allowed the characterization of all the species studied here. Additionally, some of the chromosome parameters described (st chromosomes and the 45S rDNA loci) constitute the first diagnostic characters for the Notolathyrus section. Evolutionary, chromosome data revealed that the South American species are a homogeneous group supporting the monophyly of the section. Variation in the amount of heterochromatin was not directly related to the variation in DNA content of the Notolathyrus species. However, the correlation observed between the amount of heterochromatin and some geographical and bioclimatic variables suggest that the variation in the heterochromatic fraction should have an adaptive value.