Genome sizes and reported chromosome counts of two species are suggestive of polyploidy events in section
Xialophyllium. In section
Tetraspermium, with the smallest genome sizes reported to date, genome sizes reflect reported intraspecies variations in chromosome counts in
A. scandens (
Sheffer and Kamemoto 1976;
Sheffer and Croat 1983), including polyploidy, and suggest additional, subsequent genome changes occurred. The age of genome changes such as polyploidy influences the confidence with which they can be identified since subsequent mutations tend to obscure the original event (
Doyle and Egan 2009). Although the ages of polyploidy events discussed here are not known, they are maximally the ages of the relatively young crown groups,
Xialophyllium and
Tetraspermium, to which they belong, which are estimated to have arisen 2.2 and 3.43 mya, respectively. In sections other than
Xialophyllium and
Tetraspermium, our data display incongruity between interspecific and intraspecific genome size and chromosome counts reported by others for
Anthurium and other genera.
In sections
Calomystrium, Cardiolonchium, Porphyrochitoniumand
Pachyneurium, we report interspecies genome size variation without any apparent relationship between genome size and base chromosome number, suggesting that the size difference may be unrelated to a polyploidization event. Therefore, interspecies genome size variation in these sections, particularly in the youngest crown group,
Calomystrium, suggests a mechanism of genome size change capable of producing large differences in a short span of time. Transposable elements are capable of producing such changes. Up to 80 % of the current
Z. mays genome is composed of retroelements, most inserted in the last 1–3 million years (
Rabinowicz and Bennetzen 2006). Transposable elements may be deleted after initial amplification (
Shirasu et al. 2000), or may persist and play a part in local adaptation, as exemplified by the intraspecific expansion of BARE-1 retroelements in barley in response to elevation and aridity (
Kalendar et al. 2000). In
Arabidopsis and
Oryza, genome size variations are associated with changes in repetitive DNA content occurring in the last 3 million years (
Bennetzen et al. 2005). Considering
Calomystrium is estimated to have arisen ~1.5 mya, genome changes due to rapid invasion and evolution of repetitive elements may play a role in genome size differences. However, the timeframe for comparable changes to occur in
Anthurium, a long-lived tropical perennial, may be different than that of the annuals
Zea spp
.,
Arabidopsis spp. and
Oryza spp.
Some genome size changes in these sections may be attributable to DNA changes associated with chromosomal reorganization (
Kalendar et al. 2000). Chromosome reorganization in
Anthurium was reported by
Marutani et al. (1993), who detected differences in the karyotypes of
A. nymphaefolium (
Calomystrium) and
A. ochranthum (
Cardiolonchium), which she proposed to have resulted from chromosomal rearrangement. She also observed very similar karyotypes among closely related species in section
Calomystrium, noting that the
A. roseospadix karyotype resembled those of
A. kamemotoanum and
A. formosum (
Marutani et al. 1993). This is particularly interesting given that the genome size estimate for
A. roseospadix is more than twice that of each of the other two, suggesting that this may be an example where similar karyotypes among related species may be composed of chromosomes of different structure or DNA mass. However, the cytotype of the
A. roseospadix accession we sampled would have to be determined before further inferences can be made.
In
Polyphyllium, the oldest crown group in
Anthurium, we have a case of extreme differences between reported chromosome count and expected potential genome size, with reported ploidy difference between two species with measured genome sizes that were essentially the same. As ploidy differences within other
Anthurium species do exist, it is possible that we may have sampled a previously unreported cytotype, as is suspected for
A. ravenii in section
Cardiolonchium. However, if verified, this apparent incongruity between chromosome count and measured genome size allows us to consider a loss or gain of bulk nuclear DNA and permits inferences based on evidence from mechanisms of genome evolution elucidated by studies in other species. For example, in rice,
Wang et al. (2005) estimate that 35–60 % of duplicated genes were lost shortly after genome size expansion as recently as 5 mya. In
Arabidopsis,
Lagercrantz (1998) estimated ~90 chromosomal rearrangements since
Arabidopsis and
Brassica diverged ~14–24 mya, and
Yogeeswaran et al. (2005) estimated ~10 chromosomal rearrangements occurred in the divergence of
Arabidopsis thaliana and
Arabidopsis lyrata ~5 mya.
Carlsen (2011) estimated the crown group
Polyphyllium arose ~11 mya, well within the time required to accomplish the scope of chromosomal changes as observed in the genus
Arabidopsis.
The polyploid origin of
A. clidemioides is unknown. However, polyploids arising by interspecies hybridization (allopolyploidy) are subject to mismatch repair during recombination of homeologous chromosomes which may generate large-scale deletions contributing to chromosome loss and reorganization (
Leitch and Bennett 2004). The more broadly applicable mechanisms of ongoing unequal recombination and illegitimate recombination of homologous chromosomes also contribute to genome size reduction (
Shirasu et al. 2000;
Devos et al. 2002), in part by double-strand break repair, an essential but error-prone housekeeping function causing increases, decreases and chromosomal reorganizations, which can lead to chromosome loss (
Gorbunova and Levy 1999;
Kirik et al. 2000). Although the suppositions are intriguing, the disparity between measured genome size and reported chromosome count of
A. flexile compared with
A. clidemioides warrants further investigation. It was not possible to evaluate cytotypes for the accessions included in this study. Most samples were contributed by botanical gardens, and thus we did not have the plants available locally for fresh root tip sampling and cytotype determination.
The genus
Anthurium displays considerable flexibility of nuclear DNA quantity and organization, even within species. We report here different genome sizes for different accessions of
A. ravenii,
A. watermaliense and
A. gracile varying >20 % from the mean for the species. Conceptually, intraspecies variation can be viewed as incipient interspecies genome size variation (
Greilhuber 1998). Once, genome size seemed to offer promise for delineating species (
Ohri 1998;
Ghosh et al. 2001), so reports of intraspecies genome size variations have been scrutinized to identify and eliminate systematic sources of variation, leading to standardization of methods, attention to detail in sample handling and careful selection of internal standards (
Greilhuber 1998,
2005). Still, intraspecies variations persist: approximately 10 % of
Curcuma species sampled displayed intraspecies variation in genome size estimates (2007), while both genome sizes and ploidy levels varied widely in a survey of 244
Dianthus broteri individuals collected from 25 populations (
Balao et al. 2009), similar to results reported here for
A. scandens. While variant cytotypes may explain the largest differences observed, a lesser amount of intraspecific variation in bulk nuclear DNA content may be attributed to the presence of extrachromosomal material, which can only be convincingly excluded from genome size estimates by determining the cytotype of each accession sampled (
Teoh and Rees 1976). In particular, the origin and evolution of B chromosomes seems to be associated with amplification of tandem repeats on A chromosomes, and can be generated spontaneously following allopolyploidization (
Jones and Houben 2003). It may be that activity of extra-chromosomal DNA in sections
Calomystrium,
Cardiolonchium, Porphyrochitonium and
Pachyneurium has contributed to the range of genome sizes among accessions of the same species in those sections.
Furthermore,
Anthurium cultural practices, including
in vitro cultivation, clonal propagation and selection for sports, impose extreme selective pressures, capable of activating transposable elements causing intraspecies genome size variations without imposing a reproductive barrier (
Peschke and Phillips 1991;
Hirochika et al. 1996). Indeed, individual cultivars may be selected for phenotypes associated with transposable element activity which has affected genome size, but has more noticeably affected phenotype. For example, variegated cultivars of maize (
McClintock 1965–1966),
Antirrhinum (snapdragon) (
Coen and Carpenter 1986),
Convolvulus (morning glory) (
Hoshino et al. 1995),
Dahlia (
Ohno et al. 2011) and
Sorghum carry transposable elements associated with variegation (
Chopra et al. 1999), and it may be that the mottled ‘Shibori’ cultivar (Figs I and ), with a slightly larger genome than that of the other standard cultivars, is accomplishing its variegation by similar means.
