Comparative cytogenetic maps of Passiflora alata and P. watsoniana (Passifloraceae) using BAC-FISH

Abstract

The genus Passiflora shows a large variation in chromosome numbers (2n = 12, 14, 18, 20, 22, 24, 36, 72) and in the distribution of ribosomal DNA (rDNA) sites. Most species of the subgenus Passiflora, however, have n = 9 and two pairs of 35S rDNA sites, such as species of economic importance Passiflora edulis and Passiflora alata. Species of another clade from the same subgenus, which includes Passiflora watsoniana, have three pairs of 35S rDNA sites. This karyotypic variability suggests a possible break in synteny within the subgenus Passiflora, but with maintenance of n = 9 in most species. To investigate synteny in three species of this subgenus, comparative cytogenetic mapping was performed in P. alata and P. watsoniana using BAC markers previously developed for P. edulis. In addition, the distribution of 35S and 5S rDNA sites and LTR retrotransposons Ty1-Copia and Ty3-Gypsy were investigated. The used single-copy BAC clones resulted in distinct dot-like signals exclusively in terminal or subterminal regions of the chromosomes. No synteny break with regard to the chromosomal distribution of these BACs or the rDNA sites could be detected, except for an additional 35S rDNA site on chromosome 3 of P. watsoniana. LTR elements showed proximal or uniformly dispersed patterns. The results revealed a similar distribution of BAC markers between species, preferably in the terminal regions, which is compatible with an increased accumulation of repetitive sequences preferentially in proximal regions of the chromosomes associated with an increase in genome size.