Karyotype heterogeneity in Philodendron s.l. (Araceae) revealed by chromosome mapping of rDNA loci

Abstract

Philodendron s.l. (Araceae) has been recently focus of taxonomic and phylogenetic studies, but karyotypic data are limited to chromosome numbers and a few published genome sizes. In this work, karyotypes of 34 species of Philodendron s.l. (29 species of Philodendron and five of Thaumatophyllum), ranging from 2n = 28 to 36 chromosomes, were analyzed by fluorescence in situ hybridization (FISH) with rDNA and telomeric probes, aiming to understand the evolution of the karyotype diversity of the group. Philodendron presented a high number variation of 35S rDNA, ranging from two to 16 sites, which were mostly in the terminal region of the short arms, with nine species presenting heteromorphisms. In the case of Thaumatophyllum species, we observed a considerably lower variation, which ranged from two to four terminal sites. The distribution of the 5S rDNA clusters was more conserved, with two sites for most species, being preferably located interstitially in the long chromosome arms. For the telomeric probe, while exclusively terminal sites were observed for P. giganteum (2n = 30) chromosomes, P. callosum (2n = 28) presented an interstitial distribution associated with satellite DNA. rDNA sites of the analyzed species of Philodendron s.l. species were randomly distributed considering the phylogenetic context, probably due to rapid evolution and great diversity of these genomes. The observed heteromorphisms suggest the accumulation of repetitive DNA in the genomes of some species and the occurrence of chromosomal rearrangements along the karyotype evolution of the group.

Fig 1. Fluorescent in situ hybridization of 35S (green) and 5S rDNA (red) on mitotic chromosomes of Philodendron species, counterstained with DAPI and pseudocolored in gray.

(a) Philodendron billietiae; (b) P. quinquenervium; (c) P. maximum; (d) P. uleanum; (e) P. giganteum; (f) P. distantilobum; (g) P. krugii; (h) P. nadruzianum; (i) P. fragrantissimum. Bar in i represents 5 μm.

Fig 2. Distribution of repetitive sequences in species of Thaumatophyllum and Philodendron species, counterstained with DAPI and pseudocolored in gray (35S rDNA in green, 5S rDNA in red, and telomeric probe in pink).

(a) T. lundii; (b) T. mello-barretoanum; (c) P. angustilobum; (d) P. bipennifolium; (e) P. glaziovii; (f) P. giganteum; (g) P. callosum. Bar in g represents 5 μm.

Fig 3. Chromosomal patterns for rDNA in 22 Philodendron and five Thaumatophyllum species plotted in a modified phylogeny based on Vasconcelos et al. [11].

Underlined chromosomes represent single chromosomes with heteromorphism, for which it was not possible to identify their respective homologs. Each non-underlined chromosome represents a pair of homologs.

Fig 4. Chromosomes mapped with 35S (green) and 5S (red) rDNA probes in six species of Philodendron subgenus Philodendron not included on Vasconcelos et al. [11] phylogeny.

Underlined chromosomes represent single chromosomes with heteromorphism, for which it was not possible to identify their respective homologs. Non-underlined chromosomes represent the pair of homologues.

Fig 5. Features of the satellite DNA PcSat1 from Philodendron callosum.

(a) Reconstructed monomer using WebLogo [63]. A cluster of conserved and degenerated plant telomeric motifs is present at the repeat unit and represents more than half of its full size (box). (b) Layout of cluster 228 obtained from Repeat Explorer output (Vasconcelos et al. unpublished data) and (c) Dotplot graph showing the internal organization of the sequence.