Variability of the 5S and 45S rDNA sites in Passiflora L. species with distinct base chromosome numbers

Abstract

Cytologically, the species of Passiflora with known chromosome number can be divided into four groups: (1) 2n = 12, 24, 36; (2) 2n = 24; (3) 2n = 18, 72; and (4) 2n = 20. The base chromosome number proposed for the genus is x = 6, with x = 9, x = 10 and x = 12 being considered secondary base numbers. In the present study, variability of 5S and 45S rDNA sites was investigated in 20 species of these four groups to check the reliability of this hypothesis. In the group with x = 6, five diploid species (2n = 12) exhibit two 5S rDNA sites and two (P. capsularis, P. morifolia and P. rubra) or four (P. misera 2x and P. tricuspis) 45S rDNA sites. The hexaploid cytotype of P. misera had 12 45S rDNA sites and six 5S rDNA. A tetraploid species, P. suberosa, had ten 45S rDNA sites and four 5S rDNA sites, both in the same chromosomes as the 45S rDNA sites. In the group with x = 9, P. actinia, P. amethystina, P. edmundoi, P. elegans, P. galbana, P. glandulosa and P. mucronata displayed six 45S rDNA sites, whereas P. alata, P. cincinnata, P. edulis f. flavicarpa, P. edulis var. roxo and P. laurifolia had four sites. In this group, all species were diploid (2n = 18) and had only two 5S rDNA sites. Passiflora foetida, the only species with 2n = 20, had six 45S rDNA sites and four 5S rDNA sites. The species with x = 12 (2n = 24), P. haematostigma and P. pentagona, showed four 45S rDNA sites and two 5S rDNA. In general, the number and location of 5S and 45S rDNA sites were consistent with the hypothesis of x = 6 as the probable ancestral genome for the genus, while the groups of species with x = 9, x = 10 and x = 12 were considered to be of tetraploid origin with descending dysploidy and gene silencing of some redundant gene sites, mainly those of 5S rDNA.

Fig. 1. Fluorescent in situ hybridization with 5S (green) and 45S (red or yellow) rDNA probes, in Passiflora species with x = 6, x = 10 or x = 12. A–C, P. capsularis. Note chromosomes stained with DAPI in A, 45S rDNA sites in B and a pair of 5S rDNA sites in C (arrows). D, P. rubra. Arrows indicate 5S rDNA. E, P. morifolia. Arrows indicate 5S rDNA. F, P. tricuspis. Note a longer pair of chromosomes with terminal regions characteristically less condensed, two pairs with proximal blocks of 45S rDNA, with one of the chromosomes being distended (arrowhead), and a pair with sub‐terminal 5S rDNA (arrows). G–I, P. misera 2x. Note the terminal DAPI⁺ bands in almost all chromosomes in G, 45S rDNA sites apparently sub‐terminal in one pair of chromosomes and proximal in the other one in H, and 5S rDNA sites in I (arrows). J, P. misera 6x. Broken arrows and arrowhead indicate small 45S rDNA sites. K and L, P. suberosa. Note that three of the five pairs of 45S rDNA sites are located in the smallest chromosomes, two of which also have 5S rDNA (arrows). M, P. haematostigma. Arrows indicate terminal 5S rDNA sites. N, P. pentagona. Arrows indicate 5S rDNA sites. O–Q, P. foetida. Note the secondary proximal constrictions in three chromosome pairs in O, three pairs of 45S rDNA proximal sites in P, and two pairs of 5S rDNA sub‐terminal sites in Q (arrows). All chromosomes were counterstained with DAPI (blue). Bar in Q represents 5 µm.

Fig. 2. Fluorescent in situ hybridization with 5S (green) and 45S (red or yellow) rDNA probes, in Passiflora species with x = 9. P. edmundoi (A and B); P. elegans (C and D); P. mucronata (E and F); P. galbana (G and H); P. actinia (I and J); P. glandulosa (K and L); P. amethystina (M); P. alata (N, O); P. edulis (P); P. laurifolia (Q); and P. cincinnata (R). Arrows indicate 5S rDNA sites while broken arrows indicate weak signals of 45S rDNA. All chromosomes were counterstained with DAPI (blue). Bar in R represents 5 µm.