Abstract
A cytogenetic analysis of sixteen taxa of the genus Aeschynomene Linnaeus, 1753, which includes species belonging to both subgenera Aeschynomene (Léonard, 1954) and Ochopodium (Vogel, 1838) J. Léonard, 1954, was performed. All studied species had the same chromosome number (2n = 20) but exhibited karyotype diversity originating in different combinations of metacentric, submetacentric and subtelocentric chromosomes, chromosome size and number of SAT chromosomes. The plasticity of the genomes included the observation in a taxon belonging to the subgenus Aeschynomene of an isolated spherical structure similar in appearance to the extra chromosomal circular DNA observed in other plant genera. By superimposing the karyotypes in a recent phylogenetic tree, a correspondence between morphology, phylogeny and cytogenetic characteristics of the taxa included in the subgenus Aeschynomene is observed. Unlike subgenus Aeschynomene, the species of Ochopodium exhibit notable karyotype heterogeneity. However the limited cytogenetic information recorded prevents us from supporting the proposal of their taxonomic separation and raise it to the genus category. It is shown that karyotype information is useful in the taxonomic delimitation of Aeschynomene and that the diversity in the diploid level preceded the hybridization/polyploidization demonstrated in the genus. The systematic implications of our results and their value can be extended to other Dalbergieae genera as knowledge about the chromosomal structure and its evolution increases.
Highlights
All the complements contained chromosomes with secondary constrictions in the short arms associated with microsatellites or with macrosatellites, that can be located in metacentric chromosomes (e.g., Aeschynomene sp. prope americana, A. villosa var. villosa Poiret, 1816 and Aeschynomene sp. prope villosa), submetacentric (e.g., A. paniculata Willdenow ex Vogel, 1838) or subtelocentric (e.g., A. americana and A. rudis), and with a maximum number of six in A. villosa var. villosa
Our results show that these five species, together with A. evenia, besides being associated by other cytogenetic parameters (Figs 1, 2; Table 2), share the characteristic of exhibiting macrosatellites in the short arms of generally small chromosomes, which clearly differentiates them from series Americanae (Nod-dependent American clade recovered by Chaintreuil et al 2013) that exclusively exhibits microsatellites
Polyploidy has played an important role in the evolution of the genus, our results indicate that speciation in Aeschynomene has been accompanied by chromosomal remodeling events, as well as subtle changes in the number and position of secondary constrictions and associated satellites, and that these changes preceded duplications and aneuploidies previously recorded in species distributed in the New and Old World
Summary
The current infrageneric classification of New World Aeschynomene largely follows the taxonomic groups proposed by Rudd (1955, 1981) who recognized 67 taxa, recent estimates suggest the existence of 86 species (Fernandes 1996, Klitgaard and Lavin 2005). Subgenus Aeschynomene Léonard, 1954 includes hydrophytes herbs and shrubs with peltate stipules, fruits with articles separated by septa, and bilabiate calyx, growing in swamps, wet meadows, river channels and streams. 1954 are terrestrial herbs, shrubs and trees with basifixed stipules, fruits with articles separated by an isthmus, and campanulate calyx with five subequal teeth, occur in savannahs, pine and oak groves, rocky slopes, sandy beaches and dry places (Rudd 1955, Fernandes 1996)
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