Objective To optimize the chromosome preparation procedure and investigate the karyotype of malabar spinach (Basella alba) and to provide a theoretical basis for studying genetic and phylogenetic relationships of different types of malabar spinach.

Method The cultivar 'large leaf malabar spinach' was used as plant material. Factors that affected chromosome preparation including organ selection, pretreatment and dissociation durations were investigated. With the optimized protocol, the karyotypic characteristics were analyzed.

Result The proportions of mitotic cells and metaphase cells of the main root tip samples were the highest compared to other sampled organs. Six-hour pretreatment using 0.002 mol·L^-1 8-hydroxyquinoline resulted in the highest degree of chromosome contraction, the best shape and evenly spreading of chromosomes. Eight-minute dissociation at 60 ℃ using 1 mol·L^-1 HCl resulted in the highest contrast with clear chromosome staining and transparent cytoplasm. The karyotype formula of malabar spinach is 2n=2x=44=38 m(2SAT)+6 sm. The constitution of relative length is 20 M2+22 M1+2 S, and the ratio of chromosome length to width is 1.93. The relative length ranges from 3.13% to 6.06%, the centromeric index ranges from 35.19% to 47.86%, and the arm ratio ranges from 1.09 to 1.84. The 10^th, 16^th and 17^th pairs of chromosomes belong to submetacentric chromosomes and the rest are metacentric chromosomes. Two satellites were observed at the 13^th pair of chromosomes. The karyotype asymmetry index is 57.89%, and malabar spinach has the 1A karyotype according to Stebbins' classification criteria.

Conclusion This study established an optimal chromosome preparation procedure for malabar spinach, and revealed its karyotypic characteristics from the cytogenetic aspects.