Maximum likelihood inference implies a high, not a low, ancestral haploid chromosome number in Araceae, with a critique of the bias introduced by 'x'

Background and Aims

For 84 years, botanists have relied on calculating the highest common factor for series of haploid chromosome numbers to arrive at a so-called basic number, x. This was done without consistent (reproducible) reference to species relationships and frequencies of different numbers in a clade. Likelihood models that treat polyploidy, chromosome fusion and fission as events with particular probabilities now allow reconstruction of ancestral chromosome numbers in an explicit framework. We have used a modelling approach to reconstruct chromosome number change in the large monocot family Araceae and to test earlier hypotheses about basic numbers in the family.

Methods

Using a maximum likelihood approach and chromosome counts for 26 % of the 3300 species of Araceae and representative numbers for each of the other 13 families of Alismatales, polyploidization events and single chromosome changes were inferred on a genus-level phylogenetic tree for 113 of the 117 genera of Araceae.

Key Results

The previously inferred basic numbers x = 14 and x = 7 are rejected. Instead, maximum likelihood optimization revealed an ancestral haploid chromosome number of n = 16, Bayesian inference of n = 18. Chromosome fusion (loss) is the predominant inferred event, whereas polyploidization events occurred less frequently and mainly towards the tips of the tree.

Conclusions

The bias towards low basic numbers (x) introduced by the algebraic approach to inferring chromosome number changes, prevalent among botanists, may have contributed to an unrealistic picture of ancestral chromosome numbers in many plant clades. The availability of robust quantitative methods for reconstructing ancestral chromosome numbers on molecular phylogenetic trees (with or without branch length information), with confidence statistics, makes the calculation of x an obsolete approach, at least when applied to large clades.     

Chromosome numbers in African and Madagascan Loranthaceae and Viscaceae

Eighty-five chromosome numbers representing about 63 species and 23 genera are reported for African and Madagascan Loranthaceae. The base chromosome number of all genera studied is χ= 9. This chromosome number also typifies all other Old World genera of the tribe Lorantheae, and thus supports the previously suggested thesis that the Asian and African Loranthaceae are relatively closely related. Seventy-six chromosome numbers, representing three genera ( Arceuthobium, Korthalsella, Viscum ) and 40 species are reported for African and Madagascan Viscaceae. Most of the data are for Viscum. The number χ= 14 is suggested as the base chromosome number for both Viscum and the family as a whole. The Madagascan Visca , however, have a modal chromosome number of χ= 13. Aneuploid chromosome numbers of χ= 12, 11 and 10 occur in Viscum at the north-south distributional extremes in southern Africa and Europe and, with the addition χ= 13, in all the species with eastward distributions in Madagascar, Asia and Australia. Polyploidy is rare in African and Madagascan members of both families, as is generally also true for other parts of the world where they have been studied. Translocation heterozygosity is reported for a number of species in Viscum.     

Interstitial telomere‐like repeats in the monocot family Araceae

Combining molecular cytogenetics and phylogenetic modelling of chromosome number change can shed light on the types of evolutionary changes that may explain the haploid numbers observed today. Applied to the monocot family Araceae, with chromosome numbers of 2n = 8 to 2n = 160, this type of approach has suggested that descending dysploidy has played a larger role than polyploidy in the evolution of the current chromosome numbers. To test this, we carried out molecular cytogenetic analyses in 14 species from 11 genera, using probes for telomere repeats, 5S rDNA and 45S rDNA and a plastid phylogenetic tree covering the 118 genera of the family, many with multiple species. We obtained new chromosome counts for six species, modelled chromosome number evolution using all available counts for the family and carried out fluorescence in situ hybridization with three probes (5S rDNA, 45S rDNA and Arabidopsis‐like telomeres) on 14 species with 2n = 14 to 2n = 60. The ancestral state reconstruction provides support for a large role of descending dysploidy in Araceae, and interstitial telomere repeats (ITRs) were detected in Anthurium leuconerum, A. wendlingeri and Spathyphyllum tenerum, all with 2n = 30. The number of ITR signals in Anthurium (up to 12) is the highest so far reported in angiosperms, and the large repeats located in the pericentromeric regions of A. wendlingeri are of a type previously reported only from the gymnosperms Cycas and Pinus. © 2014 The Linnean Society of London, Botanical Journal of the Linnean Society, 2015, 177 , 15–26.     

A Synopsis of Chromosome Number Variation in the Cyperaceae

The Cyperaceae are well known for having a large amount of variation in chromosome numbers both within and among genera. Most of this variation has been previously attributed to agmatoploid or qualitative aneuploid chromosome number change. To date there have been 4,231 reported chromosome number counts in the family. Despite the large number of counts made, they only represent approximately 16% of the species currently recognized. These counts are here presented in an indexed list with standardized nomenclature following a draft copy of the World Checklist of Cyperaceae. Additionally, I explore variation within genera where a significant number of counts have been made. Given the distributions of counts within genera there is evidence for both agmatoploid and polyploid chromosome number changes.     

Nuclear DNA variation, chromosome numbers and polyploidy in the endemic and indigenous grass flora of New Zealand

BACKGROUND AND AIMS: Little information is available on DNA C-values for the New Zealand flora. Nearly 85 % of the named species of the native vascular flora are endemic, including 157 species of Poaceae, the second most species-rich plant family in New Zealand. Few C-values have been published for New Zealand native grasses, and chromosome numbers have previously been reported for fewer than half of the species. The aim of this research was to determine C-values and chromosome numbers for most of the endemic and indigenous Poaceae from New Zealand. SCOPE: To analyse DNA C-values from 155 species and chromosome numbers from 55 species of the endemic and indigenous grass flora of New Zealand. KEY RESULTS: The new C-values increase significantly the number of such measurements for Poaceae worldwide. New chromosome numbers were determined from 55 species. Variation in C-value and percentage polyploidy were analysed in relation to plant distribution. No clear relationship could be demonstrated between these variables. CONCLUSIONS: A wide range of C-values was found in the New Zealand endemic and indigenous grasses. This variation can be related to the phylogenetic position of the genera, plants in the BOP (Bambusoideae, Oryzoideae, Pooideae) clade in general having higher C-values than those in the PACC (Panicoideae, Arundinoideae, Chloridoideae + Centothecoideae) clade. Within genera, polyploids typically have smaller genome sizes (C-value divided by ploidy level) than diploids and there is commonly a progressive decrease with increasing ploidy level. The high frequency of polyploidy in the New Zealand grasses was confirmed by our additional counts, with only approximately 10 % being diploid. No clear relationship between C-value, polyploidy and rarity was evident.     

Chromosomal evolution in the South American Riodinidae (Lepidoptera: Papilionoidea)

We give the haploid chromosome numbers of 173 species or subspecies of Riodinidae as well as of 17 species or subspecies of neotropical Lycaenidae for comparison. The chromosome numbers of riodinids have thus far been very poorly known. We find that their range of variation extends from n =?9 to n =?110 but numbers above n =?31 are rare. While lepidopterans in general have stable chromosome numbers, or variation is limited at most a subfamily or genus, the entire family Riodinidae shows variation within genera, tribes and subfamilies with no single modal number. In particular, a stepwise pattern with chromosome numbers that are about even multiples is seen in several unrelated genera. We propose that this variation is attributable to the small population sizes, fragmented populations with little migration, and the behavior of these butterflies. Small and isolated riodinid populations would allow for inbreeding to take place. Newly arisen chromosomal variants could become fixed and contribute to reproductive isolation and speciation. In contrast to the riodinids, the neotropical Lycaenidae (Theclinae and Polyommatinae) conform to the modal n =?24 that characterizes the family.     

Chromosome Observations of Eight Species Endemic to China

A report of chromosome numbers for eight species endemic to China is made in the paper, including first counts for 4 genera and 4 species and first karyotypic analyses of two species. Sinojohnstonia chekiangensis (Migo) W. T. Wang (Boraginaceae) 2n=24*; Coptis chinenis Franch (Ranunculaceae) 2n=18**; Dichocarpum dalzielii (Drumm. et Hutch.) W. T. Wang et Hsiao (Ranunculaceae) 2n=24*; Eomecon chionantha Hance (Papaveraceae) 2n=18; Camptotheca acuminata Dcne. (Nyssaceae) 2n=44; Calycanthus chinensis Cheng et S. Y. Chang (Calycanthaceae) 2n=22**; Eucommia ulmoides Oliv. (Eucommiaceae) n=17; Pinellia pedatisecta Schott (Araceae) 2n=26; The previous reports of chromosome numbers of the same groups are compared with our own (See Table 1). The vouchers for the present study are preserved in the Herbarium of Futan University.     

Brassicaceae: Chromosome number index and database on CD-Rom

A summary of chromosome numbers has been prepared for the Brassicaceae (Cruciferae) family. Over 9000 chromosome counts have been compiled from the literature. Chromosome numbers are known from 232 of the 338 (68.6%) genera and 1558 of the 3709 (42.0%) of the species in the family. Counts in the excel database have been arranged according to currently accepted taxonomy, and includes for each count, country of origin and reference citation. Electronic supplementary material to this article is available at and is accessible for authorized users.     

Cytotaxonomy of Commelinaceae: chromosome numbers of some African and Asiatic species

Chromosome counts are reported for 32 taxa (31 species and 1 subspecies) belonging to 10 genera of Commelinaceae from seven African and Asiatic countries. Counts for 13 species and 1 subspecies are recorded for the first time. Published chromosome numbers for Anhicopsis and Polyspatha are confirmed. It is suggested that Pdisota, Pollia and Stanfieldidla each has a single basic number (x = 20, 16 and 11, respectively). The known cytological diversity in Floscopa is extended. The third continental African species of Coleolrype is found to have the same chromosome number (2n = 36) as the other two. The preponderance of the basic number x = 15 in Commelina is supported. The uncommon basic number x = 13 is reported in four taxa of Cyanotis together with karyotypic differences. The basic number x = 6 is found in a second species of Murdannia . Karyotypic data in addition to chromosome numbers are presented for 24 of the 32 taxa investigated. Karyotypes are found to be useful in assessing relationships in the family, and evolutionary trends in the karyotype are noted.     

Chemicals in aroids: a survey, including new results for polyhydroxy alkaloids and alkylresorcinols

The Araceae is a large, mainly tropical family, many members of which contain bioactive substances which are often either toxic or irritating. Inflorescences frequently emit strong fragrances. The active compounds have not been adequately determined nor is the chemotaxonomy of the family very well known, except for one major study of flavonoids. Two groups of compounds of known bioactivity–alkylresorcinols and nitrogenous sugar analogues–have been sought in representative species from most of the genera in the family. Although percentage sampling was necessarily small, a few examples of species containing these substances were found and some taxonomic suggestions are made and compared with results in the literature. This survey is a basis for further work on the family.     

CHROMOSOME NUMBERS IN COMPOSITAE VIII: HELIANTHEAE

One hundred and ninety-three new counts are reported for the tribe Heliantheae of Compositae, mostly based on determinations of meiotic material, including first counts for the genera Adenothamnus, Chrysogonum, Enceliopsis, Guardiola, Isocarpha, Lipochaeta, Otopappus, and Oyedaea, as well as first counts for 66 species. The original counts are discussed in relation to those previously reported for the tribe, by genera and subtribe. Two-thirds of the approximately 150 genera and more than a third of the roughly 1500 species have now been examined. The incomplete knowledge of generic relationships in the tribe often make the interpretation of these chromosome numbers difficult. Three observations are documented and discussed: (1) genera with low chromosome numbers are few; (2) genera with aneuploid series are abundant; and (3) the original basic chromosome number in the tribe is probably in the range of x = 8 to x = 12.     

Chemicals in aroids: a survey, including new results for polyhydroxy alkaloids and alkylresorcinols

The Araceae is a large, mainly tropical family, many members of which contain bioactive substances which are often either toxic or irritating. Inflorescences frequently emit strong fragrances. The active compounds have not been adequately determined nor is the chemotaxonomy of the family very well known, except for one major study of flavonoids. Two groups of compounds of known bioactivity—alkylresorcinols and nitrogenous sugar analogues—have been sought in representative species from most of the genera in the family. Although percentage sampling was necessarily small, a few examples of species containing these substances were found and some taxonomic suggestions are made and compared with results in the literature. This survey is a basis for further work on the family.     

中国滇西天南星科一新种——保山犁头尖

描述了天南星科Araceae南星族Areae犁头尖属Tiphonium Schoa一新种——保山犁头尖Typhonium baoshanense Z.L.Dao ＆ H.Li,它的叶形和佛焰苞形态与犁头尖T.blumei Nicolson ＆ Sivadasan相似,但附属器下部,雄花序之上,有1-2轮不育雄花（退化雄蕊）,雌花序圆锥形,附属器基部截形和佛焰焰苞檐部狭窄而易于区别。同域分布的犁头尖染色体数目为2n=52,本种染色体数目为2n=10,为天南星科的最低染色体基数。     

CHROMOSOME NUMBERS IN COMPOSITAE. I. ASTEREAE

Raven , Peter H. (U. California, Los Angeles.), Otto T. Solbrig , Donald W. Kyhos , and Richard Snow . Chromosome numbers in Compositae. I. Astereae. Amer. Jour. Bot. 47(2) : 124—132. Illus. 1960.–Ninety-two new counts are reported for the tribe Astereae of Compositae, mostly based on determinations of meiotic material. These include the first counts reported for the genera Acamptopappus, Amphipappus, Benitoa, Chrysothamnus, Corethrogyne, Lessingia, Monoptilon, and Xanthocephalum, as well as for many species. The original counts are discussed in relation to those previously reported for the tribe; together these constitute a total of 39 genera examined cytologically out of the approximately 100 known. Because of its widespread occurrence in diverse phylogenetic lines within the tribe and the family, and because of its high degree of correlation with the woody habit, which is thought to be primitive, x=9 is regarded as the original basic number for Astereae. Within the Haplopappus alliance there is a strong secondary mode of chromosome numbers centering around x=5. The hiatus between these two modes in number is explained on the basis of ancient phylogenetic reduction in chromosome number followed by the extinction of less successful intermediate types, and is compared with similar trends that have been reported for Cichorieae. It is suggested that the family is not of polyploid origin but may have had an original diploid basic number.     

Characters of Leaf Epidermis in Genus Sauromatum (Araceae) and It’s Relative Genera

Leaf epidermis of 2 species in genus Sauromatum Schott were examined under light microscope and scanning electron microscope ( SEM) and compared with that of 13 species in genera Typhonium, Pinellia and Arisaema in the same tribe Area of Araceae . The result reveals that the characters of leaf epidermis between the several genera generally are very similar . However , some of the characters in the species of the same genus are variable and can be used to identify its species . The characters of leaf epidermis support to merge Typhonium kunmingense into Typhonium calcicolum . The stomata apparatus of 15 species are all brachyparacytic types with 2 subsidiary cells . This type of stomata apparatus is considered more evolutional than that with more than 2 subsidiary cells which existed in majority tribes of Araceae . This also verify some taxonomists’viewpoint that tribe Area is more evolutional in family Araceae .     

斑龙芋属(天南星科) 及近缘属植物的叶表皮形态

光镜、电镜下观察了天南星科天南星族下斑龙芋属2 个种及近缘属13 个种植物的叶表皮形态特征。实验结果显示, 4 属植物的叶表皮组成及其形态特征较相似, 属间不存在明显差异, 但某些特征在种间存在差异, 可作为种的鉴别特征。叶表皮特征支持将单籽犁头尖和昆明犁头尖两个种合并为一个种。15 个种的气孔器均具有2 个副卫细胞, Stebbins and Khush 认为这是气孔器类型中较具2 个以上副卫细胞更进化的一种类型, 而天南星科大多数族的气孔器都具有2 个以上的副卫细胞, 这也证明了天南星族是天南星科较进化的族。     

POLYPLOIDY,DYSPLOIDY, AND CHROMOSOME PAIRING IN ECHEVERIA (CRASSULACEAE) AND ITS HYBRIDS

The 140+ species of Echeveria have more than 50 gametic chromosome numbers, including every number from 12 through 34 and polyploids to n = ca. 260. With related genera, they comprise an immense comparium of 200+ species that have been interconnected in cultivation by hybrids. Some species with as many as 34 gametic chromosomes include none that can pair with each other, indicating that they are effectively diploid, but other species with fewer chromosomes test as tetraploids. Most diploid hybrids form multivalents, indicating that many translocations have rearranged segments of the chromosomes. Small, nonessential chromosomal remnants can be lost, lowering the number and suggesting that higher diploid numbers (n = 30–34) in the long dysploid series are older. These same numbers are basic to most other genera in the comparium (Pachyphytum, Graptopetalum, Sedum section Pachysedum), and many diploid intergeneric hybrids show very substantial chromosome pairing. Most polyploid hybrids here are fertile, even where the parents belong to different genera and have very different chromosome numbers. This seems possible only if corresponding chromosomes from a polyploid parent pair with each other preferentially, strong evidence for autopolyploidy. High diploid numbers here may represent old polyploids that have become diploidized by loss, mutation, or suppression of duplicate genes, but other evidence for this is lacking. Most species occur as small populations in unstable habitats in an area with a history of many rapid climatic and geological changes, presenting a model for rapid evolution.     

斑龙芋属(天南星科)及近缘属植物的叶表皮形态

光镜、电镜下观察了天南星科天南星族下斑龙芋属2个种及近缘属13个种植物的叶表皮形态特征。实验结果显示,4属植物的叶表皮组成及其形态特征较相似,属间不存在明显差异,但某些特征在种间存在差异,可作为种的鉴别特征。叶表皮特征支持将单籽犁头尖和昆明犁头尖两个种合并为一个种。15个种的气孔器均具有2个副卫细胞,Stebbins andKhush认为这是气孔器类型中较具2个以上副卫细胞更进化的一种类型,而天南星科大多数族的气孔器都具有2个以上的副卫细胞,这也证明了天南星族是天南星科较进化的族。     

Quantitative karyology of some species ofLuzula

The dimensions of metaphase chromosomes and nuclear DNA contents were measured in eight species ofLuzula. The 2 C DNA contents ranged from 8.51 pg inL. purpurea to 0.55 pg inL. pilosa. Total chromosome volume shows a linear relationship with DNA content; however, the total chromosome length of the complement of the different species is approximately constant. Nucleolar volume and the number of chromocentres in the different species also show a relationship with DNA content. Taken together, these data suggest that while chromosome fragmentation could have generated the present-day range of chromosome numbers in the genus, there have also been changes in the total quantity of DNA with the result that species with similar chromosome numbers have different DNA contents. The relationships of DNA content with chromosome volume inLuzula and other genera are compared and the differences discussed.     

Chromosome observations of three ranunculaceous genera in relation to their systematic positions

1. The present paper describes the observations of chromosome numbers and karyomorphology of 2 species of 2 endemic genera and I endemic species of Chinese Ranunculaceae: Asteropyrum peltatum (Franch.) Drumm et Hutch. 2n=16, x=8; Kingdonia unifolia Balf. f. et W. W. Sm. 2n=18, x=9 and Calathodes oxycarpa Sprague 2n=16, x=8. The chromosome counts of three ranunculaceous genera are reported for the first time. 2. The morphylogical, palynological and cytological date in relation to the systematic postition of Asteropyrum, Kingdonia and Calathodes within the family Ranunculaceae are diseussed and resulted in following conclusions: (1). On the basis of the basic number x=8 in Asteropyrum, it is further confirmed that this genus is distinct from the r elated genera such as Isopyrum, Dichocarpum and other allied taxa. The comparison of Asteropyrum with Coptis shows that they are identical in short chromosomes, with magnoflorina and benzylisaquinodine type of alkaloides, but different from coptis in the chromosome numbers (T-type), pantocolpate pollens, united carpels and the dorsi-ventral type of petioles. In view of these fundamental morphological and cytological differences, Asterop yrum is better raised to the level of Tribe. However Asteropyrum and Coptis may represent two divaricate evolutional lines of Thalictroideae. (2). The systematic position of the genus Kingdonia has been much disputed in the past. We support the view of Sinnote (1914), namely, the trilacunar in leaf traces “the ancient type”, appeared in the angiosperm line very early, while the unilacunar of Kingdonia may be derived from the trilacunar. On the basis of the chromosome numbers and morphylogical observation, the present writer accept Tamura’s and Wang’s treatment by keeping Kingdonia in Ranunculaceae instead of raising it to a family rank as has been been done by Forster (1961). Kingdonia and Coptis are similar in having short chromosome with x=9, but with one-seeded fruits; therefore it is suggested that placed into Thalictroideae as an independent tribe, indicating its close relationship with Coptideae. (3). Comparing with its allies, Calathodes being with out petals, seems to be more primitive than Trollius. But Calathodes differs from Trollius with R-type chromosomes in having T-type chromosome with x=8 and subterminal centromere. Those characteristics show that it is very similar to the related genera of Thalictroideae. But as Kurita already pointed out that most speci es of Ranunculus have usually large long chromosomes but some species have compar ativelly short chromosomes, therefore we regard T-type and R-type chromosomes appear independently in different subfamilies of Ranunculaceae. According to Tamura, G alathodes seems to be closely related to Megaleranthis, because of the resemblance in follicles. But due to lack of cytological data of the latter genus, the relationship between the two genera still is not clear pending further studies. From the fact that the morphology and chromosomes of the Calathodes differs from that of all other genera of the Helleboroideae, we consider Calathodes may form an independent tribe of its own with a closer relationship withTrollieae.9841