Updating the list of chromosome numbers for Philodendron (Araceae)

ABSTRACT

Aiming for a better understanding of karyotype evolution within Philodendron, we report chromosome counts for 23 species of the genus, of which 19 are being reported for the first time, thus increasing to 84 ( ca. 17 % of the genus) the total number of species with available chromosome counts. The diploid numbers 2 n = 32 and 2 n = 34 were the most common, with 10 and 11 species, respectively, whereas only two species presented different chromosome numbers ( P. giganteum with 2 n = 30 and P. adamantinum with 2 n = 36). The results are discussed in the context of previous analyses of karyotypes of Philodendron spp., taking into account bidirectional dysploidy as the main mechanism of chromosome number evolution within the genus.

Keywords
aroids; diploid number; dysploidy; karyotype; Philodendron

Philodendron (Araceae) is one of the most prominent monocot groups in the humid Neotropical forests, being composed mostly of lianescent species ( Grayum 1996; Croat 1997). The genus is the second largest of the aroid family, comprising almost 500 species ( Boyce & Croat 2016), which have been traditionally subdivided into three major groups: P. subgenus Meconostigma (21 spp.), P. subgenus Pteromischum (82 spp.) and P. subgenus Philodendron ( ca. 380 spp. subdivided into 10 sections) ( Sakuragui et al. 2005; Barbosa & Sakuragui 2014; Calazans et al. 2014). There is a considerable ecological variation within Philodendron, mainly observed among the species of P. subgenus Philodendron, which also presents the widest geographic distribution, ranging from Mexico to Uruguay ( Croat 1997; Mayo et al. 1997).

Considering the proportion of 19 % of angiosperms with known chromosome numbers ( Rice et al. 2015), the members of Araceae have been relatively well sampled in cytogenetic studies, with a coverage of 26 % of the approximately 3400 species ( Bogner & Petersen 2007; Cusimano et al. 2012; Boyce & Croat 2016). Recently, Correia-da-Silva et al. (2014) reviewed the list of chromosome numbers previously published for Philodendron species, besides reporting new chromosome counts for the group. According to these authors, the coverage of the genus is considerably lower than the observed in other genera of Araceae, with only 13 % of the species with available chromosome counts (66 out of ca. 500). Although there is a certain degree of variation of chromosome numbers within the genus, ranging from 2 n = 28 to 40, most of the species present either 2 n = 32 (45.4 %; 30 spp.) or 2 n = 34 (27.3 %; 18 spp.) ( Correia-da-Silva et al. 2014). Therefore, in order to increase the list of chromosome counts for Philodendron, as well as aiming for a better understanding of the karyotype evolution within the genus, we bring diploid numbers for 23 species, 19 of which are being reported for the first time.

All plant materials were obtained from the Araceae living collection held at the Royal Botanic Gardens, Kew, except for the accession of P. mello-barretoanum Burle-Marx ex G.M.Barroso, which is cultivated in the Philodendron living collection of the Department of Genetics of the Federal University of Pernambuco (Universidade Federal de Pernambuco - Recife, Brazil) ( Tab. S1 in supplementary material).

Chromosome counts followed the procedures used by Correia-da-Silva et al. (2014), with some modifications. Root tips were collected, pre-treated with 2 mM 8-hydroxyquinoline at room temperature ( ca. 25 °C) for 1 h and, and at 10 °C for 23 h, fixated in Carnoy (3:1 ethanol:acetic acid, v/v) at room temperature for 4-6 h and stored at −20 °C. Subsequently, root tips were digested for 24 h at 37 °C in an enzymatic solution containing 2 % (w/v) cellulase from Aspergillus niger Tiegh. (Sigma-Aldrich) and 20 % (v/v) pectinase from A. niger (Sigma-Aldrich) and squashed in a drop of 45 % acetic acid. Chromosome preparations were stained and mounted with a DAPI-glycerol solution (2 μg/mL 4’,6-diamidino-2-phenylindole and glycerol; 1:1, v/v) for 10-15 min. Cell images were acquired with a Leica DMLB epifluorescence microscope and a Leica DFC 340FX camera with the Leica CW4000 software.

Among the analyzed karyotypes, the diploid numbers 2 n = 32 and 2 n = 34 were the most frequent, being observed for 10 ( P. annulatum, P. ernestii ( Fig. 1A), P. glanduliferum ( Fig. 1B), P. glaziovii, P. inconcinnum ( Fig. 1C), P. jacquinii ( Fig. 1D), P. lacerum ( Fig. 1E), P. longilaminatum ( Fig. 1F), P. schmidtiae and P. smithii ( Fig. 1G)) and 11 ( P. angustilobum, P. burle-marxii ( Fig. 1H), P. cordatum, P. erubescens ( Fig. 1I), P. krugii, P. maximum ( Fig. 1J), P. mello-barretoanum ( Fig. 1K), P. renauxii, P. tenue, P. tripartitum and P. uleanum ( Fig. 1L)) species, respectively ( Tab. S1 in supplementary material). Only P. giganteum (2 n = 30; Fig. 1M) and P. adamantinum (2 n = 36) had different chromosome numbers. Therefore, we further confirm the numbers 2 n = 32 and 2 n = 34, particularly the first one, as the most important in the genus, although without any clear pattern of distribution among the different sections of P. subgenus Philodendron ( Tab. S1 in supplementary material).

Figure 1
Mitotic chromosomes of Philodendron species stained with DAPI (4',6-diamidino-2-phenylindole). Philodendron ernestii (2 n = 32; A); P. glanduliferum (2 n = 32; B); P. inconcinnum (2 n = 32; C); P. jacquinii (2 n = 32; D); P. lacerum (2 n = 32; E); P. longilaminatum (2 n = 32; F); P. smithii (2 n = 32; G); P. burle-marxii (2 n = 34; H); P. erubescens (2 n = 34; I); P. maximum (2 n = 34; J); P. mello-barretoanum (2 n = 34; K); P. uleanum (2 n = 34; L); ; and P. giganteum (2 n = 30; M).

Considering the two diverging counts previously published for P. giganteum, 2 n = 30 ( Simmonds 1954) and 2 n = 34 ( Jones 1957), we confirmed the data from the first analysis ( Tab. S1 in supplementary material). As previously pointed out by Correia-da-Silva et al. (2014) for several other Philodendron species, this seems to be the case of a miscount by Jones (1957), instead of the existence of a chromosome number polymorphism within the species. Such likely miscounts may be linked to the use paraffin sections by the author to obtain the chromosome preparations for materials with small chromosomes such as Philodendron spp. ( Jones 1957), instead of the most usual flattening of macerated meristems by squashing between slide and coverslip. Similarly, the data previously published by Sharma & Mukhopadhyay (1965) for P. lacerum (2 n = 36) and P. erubescens (2 n = 32) and were divergent from the chromosome numbers observed here, which were 2 n = 32 ( Fig. 1E) and 2 n = 34 ( Fig. 1I), respectively ( Tab. S1 in supplementary material). On the other hand, for P. cordatum, the number 2 n = 34, which Jones (1957) had previously observed, was corroborated here ( Tab. S1 in supplementary material).

Regarding the analyzed species of P. subgenus Meconostigma, P. adamantinum, showed a commonly observed number for eastern Brazilian species of the subgenus (2 n = 36), as well as P. corcovadense, P. saxicola and P. undulatum, for instance ( Correia-da-Silva et al. 2014), while P. mello-barretoanum showed 2 n = 34, which is being reported for the first time for the subgenus. As our new results for P. lacerum associates the species with a quite common diploid number for P. subgenus Philodendron (2 n = 32), P. rugosum is currently the only species of the mentioned group with a double-checked diploid number of 2 n = 36 ( Bogner & Bunting 1983; Petersen 1989). In addition, the number 2 n = 36 has been observed almost only in the heliophytes of P. subgenus Meconostigma, which has been indicated as a derivative habit within the group ( Calazans et al. 2014; Loss-Oliveira et al. 2016). Thus, x = 18 may not be the primitive basic number in Philodendron, not representing the whole genus, as previously discussed by Correia-da-Silva et al. (2014). Instead, we suggest either 2 n = 32 or 2 n =34 as the ancestral diploid number for the group, although further confirmation is necessary through an analysis of ancestral chromosome number reconstruction. Therefore, bidirectional dysploidy (starting from 2 n = 32 or 2 n = 34) could be regarded as the main cause of chromosome number variation among Philodendron species, as largely observed among Araceae genera, probably being the most significant events during the karyotype evolution within the family (see Sousa & Renner 2015), besides being frequently reported for other angiosperm groups, such as Brachypodium (Poaceae) ( Idziak et al. 2014) and Melampodium (Asteraceae) ( McCann et al. 2016), for instance.

Including the results presented here, chromosome counts are now available for 84 species of Philodendron (ca. 17 % of the ca. 500 species), excluding the findings for the cultivated hybrids presented by Catalano et al. (1964), Catalano & Landi (1966) and Jones (1957) ( Tab. S1 in supplementary material). As previously mentioned, assumptions regarding the association between taxa and diploid numbers cannot be easily defined, due to the still low coverage of some groups, such as P. subgenus Pteromischum, for which there are chromosome counts for only two species ( Tab. S1 in supplementary material; see Correia-da-Silva et al. 2014). On the other hand, only by increasing the knowledge on the cytogenetic features of the Philodendron species, one may understand the evolutionary pathways of the karyotypes within the genus, as reported for Typhonium ( Sousa et al. 2014), another aroid genus from the subfamily Aroideae.

Acknowledgements

This study was supported by CNPq (Conselho Nacional de Desenvolvimento Científico e Tecnológico) and CAPES (Coordenação de Aperfeiçoamento de Pessoal de Nível Superior). The authors are grateful to the Royal Botanic Gardens, Kew, especially to Dr. Ilia J. Leitch, Dr. Ralf Kynast, Dr. Simon J. Mayo and Marcelo Sellaro, for providing access to the sampled materials.

References

• Barbosa JF, Sakuragui CM. 2014. Taxonomy and conservation of the Brazilian extra-Amazonian species of Philodendron subg. Pteromischum (Araceae). Phytotaxa 191: 45-65.
• Bogner J, Bunting GS. 1983. A new Philodendron species (Araceae) from Ecuador. Willdenowia 13: 183-185.
• Bogner J, Petersen G. 2007. The chromosome numbers of the aroid genera. Aroideana 30: 82-90.
• Boyce PC, Croat TB. 2016. The Überlist of Araceae, totals for published and estimated number of species in aroid genera. http://aroid.org/genera/140601uberlist.pdf 3 Nov. 2016.
  » http://aroid.org/genera/140601uberlist.pdf
• Calazans LSB, Sakuragui CM, Mayo SJ. 2014. From open areas to forests? The evolutionary history of Philodendron subgenus Meconostigma (Araceae) using morphological data. Flora 209: 117-121.
• Catalano M, Landi A. 1966. Dati citotassonomici su Philodendron squamiferum Poepp. et Endl., Philodendron eximium Schott e sul loro ibrido Philodendron × pausilypum Landi. Delpinoa 6/7: 139-147.
• Catalano M, Landi A, Virzo A. 1964. Osservazioni cariologiche su Philodendron eximium Schott, Ph. erubescens C. Koch. et Augustin e sul loro ibrido Ph. × parthenopaeum Landi. Delpinoa 5: 129-137.
• Correia-da-Silva M, Vasconcelos S, Soares MLC, Mayo SJ, Benko-Iseppon AM. 2014. Chromosomal diversity in Philodendron (Araceae): taxonomic significance and a critical review. Plant Systematics and Evolution 300: 1111-1122.
• Croat TB. 1997. A revision of Philodendron subgenus Philodendron (Araceae) for Mexico and Central America. Annals of the Missouri Botanical Garden 84: 311-704.
• Cusimano N, Sousa A, Renner SS. 2012. Maximum likelihood inference implies a high, not a low, ancestral haploid chromosome number in Araceae, with a critique of the bias introduced by ‘x’. Annals of Botany 109: 681-692.
• Grayum MH. 1996. Revision of Philodendron subgenus Pteromischum (Araceae) for Pacific and Caribbean tropical America. Systematic Botany Monographs 47: 1-233.
• Idziak D, Hazuka I, Poliwczak B, Wiszynska A, Wolny E, Hasterok R. 2014. Insight into the karyotype evolution of Brachypodium species using comparative chromosome barcoding. PLoS One 9: e93503. http://dx.doi.org/10.1371/journal.pone.0093503
  » http://dx.doi.org/10.1371/journal.pone.0093503
• Jones GE. 1957. Chromosome numbers and phylogenetic relationships in the Araceae. PhD Thesis, University of Virginia, Charlottesville.
• Loss-Oliveira L, Sakuragui C, Soares ML, Schrago CG. 2016. Evolution of Philodendron (Araceae) species in Neotropical biomes. PeerJ 4: e1744. http://dx.doi.org/10.7717/peerj.1744
  » http://dx.doi.org/10.7717/peerj.1744
• Mayo SJ, Bogner J, Boyce PC. 1997. The genera of Araceae. Richmond, Kew Publishing.
• McCann J, Schneeweiss GM, Stuessy TF, Villaseñor JL, Weiss-Schneeweiss H. 2016. The impact of reconstruction methods, phylogenetic uncertainty and branch lengths on inference of chromosome number evolution in American daisies ( Melampodium, Asteraceae). PLoS One 11: e0162299. http://dx.doi.org/10.1371/journal.pone.0162299
  » http://dx.doi.org/10.1371/journal.pone.0162299
• Petersen G. 1989. Cytology and systematics of Araceae. Nordic Journal of Botany 9: 119-165.
• Rice A, Glick L, Abadi S, et al 2015. The chromosome counts database (CCDB) - a community resource of plant chromosome numbers. New Phytologist 206: 19-26.
• Sakuragui CM, Mayo SJ, Zappi DC. 2005. Taxonomic revision of Brazilian species of Philodendron section Macrobelium Kew Bulletin 60: 465-513.
• Sharma AK, Mukhopadhyay S. 1965. Cytological study on two genera of Araceae and correct assessment of their taxonomic status. Genetica Agraria 18: 603-616.
• Simmonds NW. 1954. Chromosome behaviour in some tropical plants. Heredity 8: 139-146.
• Sousa A, Cusimano N, Renner SS. 2014. Combining FISH and model-based predictions to understand chromosome evolution in Typhonium (Araceae). Annals of Botany 113: 669-680.
• Sousa A, Renner SS. 2015. Interstitial telomere-like repeats in the monocot family Araceae. Botanical Journal of the Linnean Society 177: 15-26.

Publication Dates

• Publication in this collection
  02 May 2017
• Date of issue
  Apr-Jun 2017

History

• Received
  06 Dec 2016
• Accepted
  02 Mar 2017