Chromosome numbers and genome size variation in Indian species of Curcuma (Zingiberaceae)

Abstract

Background and aims: Genome size and chromosome numbers are important cytological characters that significantly influence various organismal traits. However, geographical representation of these data is seriously unbalanced, with tropical and subtropical regions being largely neglected. In the present study, an investigation was made of chromosomal and genome size variation in the majority of Curcuma species from the Indian subcontinent, and an assessment was made of the value of these data for taxonomic purposes.

Methods: Genome size of 161 homogeneously cultivated plant samples classified into 51 taxonomic entities was determined by propidium iodide flow cytometry. Chromosome numbers were counted in actively growing root tips using conventional rapid squash techniques.

Key results: Six different chromosome counts (2n = 22, 42, 63, >70, 77 and 105) were found, the last two representing new generic records. The 2C-values varied from 1.66 pg in C. vamana to 4.76 pg in C. oligantha, representing a 2.87-fold range. Three groups of taxa with significantly different homoploid genome sizes (Cx-values) and distinct geographical distribution were identified. Five species exhibited intraspecific variation in nuclear DNA content, reaching up to 15.1 % in cultivated C. longa. Chromosome counts and genome sizes of three Curcuma-like species (Hitchenia caulina, Kaempferia scaposa and Paracautleya bhatii) corresponded well with typical hexaploid (2n = 6x = 42) Curcuma spp.

Conclusions: The basic chromosome number in the majority of Indian taxa (belonging to subgenus Curcuma) is x = 7; published counts correspond to 6x, 9x, 11x, 12x and 15x ploidy levels. Only a few species-specific C-values were found, but karyological and/or flow cytometric data may support taxonomic decisions in some species alliances with morphological similarities. Close evolutionary relationships among some cytotypes are suggested based on the similarity in homoploid genome sizes and geographical grouping. A new species combination, Curcuma scaposa (Nimmo) Skornick. & M. Sabu, comb. nov., is proposed.

Fig. 1.

Geographical origin of the plant samples analysed. (A) Species from genome group I (1Cx = 0·30–0·33 pg) and x = 7; (B) species from genome group II (1Cx = 0·36–0·39 pg) and x = 7; (C) species from genome group III (1Cx = 0·41–0·43 pg) and x = 7; (D) species with 1Cx > 0·83 pg and x = 11 (circle, Curcuma vamana; diamond, Stahlianthus involucratus). Symbol explanation (unless otherwise indicated): closed circles, hexaploids; open circles, nonaploids; open diamonds, Curcuma-like species often placed into separate genera; open squares, high polyploids designated by a corresponding ploidy level.

Fig. 2.

Chromosome complements of (A) C. vamana (species with the smallest number of chromosomes) and (B) C. raktakanta (species with the highest number of chromosomes) showing 22 and 105 somatic chromosomes, respectively. Scale bars = 10 µm. In (A), two micrographs were taken at different focal planes and computer-merged in order to achieve sufficient image sharpness.

Fig. 3.

(A) Representative flow cytometric histogram documenting genome size determination in Curcuma oligantha (accession number 73325) using Bellis perennis as internal reference standard. (B) Flow cytometric evidence for genome size variation in Curcuma longa [simultaneous analysis of accessions 71436 (2C = 2·60 pg) and 86221-II (2C = 2·85 pg)]. Plant nuclei were isolated, stained with propidium iodide and measured simultaneously.

Fig. 4.

2C-value distribution (DNA pg means) for 51 taxa investigated. Symbol explanation: squares, diploids (2n = 2x = 22); diamonds, hexaploids (2n = 6x = 42); triangles, nonaploids (2n = 9x = 63); circles, higher polyploids designated by a corresponding ploidy level. Closed symbols, Curcuma species; open symbols, closely related taxa often placed into separate genera.

Fig. 5.

Phenotypic diversity of Curcuma species included in the study. (A) Curcuma vamana (accession number 84156); (B) C. cannanorensis (no. 84144); (C) C. raktakanta (no. 84120). (D) C. roscoeana (no. 73309); (E) C. oligantha (no. 73325); (F) C. ( = Paracautleya) bhatii (no. 73446); (G) C. ( = Hitchenia) caulina (no. 84178); (H) C. kudagensis (no. 84152); (I) C. ( = Kaempferia) scaposa (no. 77029). Individual plates not to scale.