The cytogenetic characterization of Arachis species is useful for assessing the genomes present in this genus, for establishing the relationship among their representatives and for understanding the variability in the available germplasm. In this study, we used fluorescence in situ hybridization (FISH) to examine the distribution patterns of heterochromatin and rDNA genes in 12 Brazilian accessions of five species of the taxonomic section Arachis. The heterochromatic pattern varied considerably among the species: complements with centromeric bands in all of the chromosomes (A. hoehnei) and complements completely devoid of heterochromatin (A. gregoryi, A. magna) were observed. The number of 45S rDNA loci ranged from two (A. gregoryi) to eight (A. glandulifera), while the number of 5S rDNA loci was more conserved and varied from two (in most species) to four (A. hoehnei). In some species one pair of 5S rDNA loci was observed adjacent to 45S rDNA loci. The chromosomal markers revealed polymorphism in the three species with more than one accession (A. gregoryi, A. magna and A. valida) that were tested. The previous genome assignment for each of the species studied was confirmed, except for A. hoehnei. The intraspecific variability observed here suggests that an exhaustive cytogenetic and taxonomic analysis is still needed for some Arachis species.

Unreduced (2n) gametes are the driving force that leads to polyploidization of plants in nature and an important tool for ploidy breeding. Peanut (Arachis hypogaea L., Leguminosae) seems most likely to have originated by sexual polyploidization through interspecific hybridization between the wild diploid relatives—A. duranensis Krapov. & W.C. Greg. and A. ipaënsis Krapov. & W.C. Greg—and subsequent binding of the unreduced gametes in the hybrid. Therefore, aiming to contribute to understanding the event of polyploid origin of this important crop, we made diploid hybrids between A. duranensis and A. ipaënsis and then investigated the production of unreduced gametes in the hybrids by analyzing the size range of pollen, the constitution of the sporads, and the microsporogenesis process. The meiotic abnormalities found, such as persistent bridges, tripolar spindles, and cytomixis processes, would lead to the formation of restitution nuclei in both the first and second meiotic division. These failures in the regularity of meiosis lead to the formation of dyads and triads, and consequently to 2n pollen grains. These findings constitute the first evidence of the formation of unreduced gametes in intergenomic hybrids obtained between the wild progenitors of A. hypogaea and enable us to propose a model of origin of the crop through sexual polyploidization.

ABSTRACT. The peanut (Arachis hypogaea) is an important food crop in much of the tropical and semi-tropical parts of the world. The peanut is an allotetraploid with an AABB genome formula derived from diploids A. duranensis (A genome) and A. ipaënsis (B genome). The success of an introgression program that aims to improve cultivated varieties of the peanut depends on whether the chosen B genome species is homologous with the B genome of the peanut. While not directly involved in the origin of the peanut to the best of our knowledge, Arachis valida is a B genome species that could potentially be a bridge species or a source of new and different alleles, because of its resistance to diseases and pests. In this study, we investigated the crossability of A. valida with five other B genome species of section Arachis. Eight cross-combinations were made with A. valida and A. gregoryi, A. ipaënsis, A. magna, A. valida, and A. williamsii. Two hundred and forty pollinations were made yielding 61 fruit segments, 61 seeds, one abortion, and 24 hybrid plants. An analysis of the morphological characteristics and pollen viability confirmed that the plants 17574-17586 (2015) were hybrids. Our results indicated that higher pollen viability of hybrid plants corresponded with higher affinity between parent plants used in crossings. This conclusion corroborates much of previous research carried out by many other authors in the past.

Se describe una nueva especie de Arachis, con hojas trifolioladas, A. sesquijuga. Esta se diferencia de las otras especies trifolioladas, ambas con folíolos ternados, por la presencia de un raquis evidente, lo que caracteriza a la hoja como pinnada. Datos históricos sobre la acumulación de conocimiento referente a las raras especies trifolioladas son comentados en secuencia cronológica.