Cytogenetics of Diphysa americana (Mill.) M.Sousa (Leguminosae-Papilionoideae-dalbergioid clade), a rare species from the coast of Oaxaca, Mexico

Abstract

Diphysa Jacq. is an essentially Mexican and Central American genus that includes 21 species and only one cytogenetic report. In this work, a surface spread and air-drying method was used to obtain the karyotype of Diphysa americana (Mill) M.Sousa, a rare native tree that grows in a coastal town in the State of Oaxaca, Mexico. Metaphase cells showed a 2n = 20, consistent with the predominant diploid number in the dalbergioid clade. This number contrasts with a previously reported 2n = 16. The karyotypic formula 5m + 5sm, first proposed for a species of the genus, denotes a slightly asymmetric karyotype. The presence of secondary constrictions associated with satellites on the short arms of a pair of sm chromosomes and other cytogenetic parameters require studies in other species of the genus to verify their taxonomic utility. In addition, cells in prometaphase exhibited a circular fragment of unknown origin like that observed in a species of the genus Aeschynomene, also dalbergioid. This fragment could be related to extrachromosomal circular DNA (eccDNA) observed in other plants. Diphysa is a small, cytogenetically favorable genus, and further studies will exhibit the karyotypic diversity that underlies its diversification.