Updating the list of chromosome numbers for Philodendron ( Araceae )

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 fi rst time, thus increasing to 84 (ca. 17 % of the genus) the total number of species with available chromosome counts. Th e diploid numbers 2n = 32 and 2n = 34 were the most common, with 10 and 11 species, respectively, whereas only two species presented diff erent chromosome numbers (P. giganteum with 2n = 30 and P. adamantinum with 2n = 36). Th e 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.

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 2n = 28 to 40, most of the species present either 2n = 32
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 DAPIglycerol 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.
Considering the two diverging counts previously published for P. giganteum, 2n = 30 (Simmonds 1954) and 2n = 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 (2n = 36) and P. erubescens (2n = 32) and were divergent from the chromosome numbers observed here, which were 2n = 32 (Fig. 1E) and 2n = 34 (Fig. 1I), respectively (Tab.S1 in supplementary material).On the other hand, for P. cordatum, the number 2n = 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 (2n = 36), as well as P. corcovadense, P. saxicola and P. undulatum, for instance (Correia-da-Silva et al. 2014), while P. mello-barretoanum showed 2n = 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 (2n = 32), P. rugosum is currently the only species of the mentioned group with a double-checked diploid number of 2n = 36 (Bogner & Bunting 1983;Petersen 1989).In addition, the number 2n = 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 2n = 32 or 2n =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 2n = 32 or 2n = 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 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.