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.     

Coreopsis nuecensis (Compositae) and a related new species from southern Texas

An aneuploid chromosome series ofn = 6, 7, 8, 9, and 10 inCoreopsis nuecensis Heller was analyzed for morphological, distributional, and genetic correlations with the chromosome number classes. The results show that two sets of basic chromosome numbers occur within what has been treated asC. nuecensis: n = 6, 7 andn = 9, 10. Then = 7 class frequently carries a pair of B chromosomes, forming then = 8 class. The base chromosome numbers are correlated with some minor but consistent morphological differences, with distributional differences, and with strong sterility barriers in the F₁ hybrids. It is proposed that then = 9, 10 segment be recognized as a new species,Coreopsis nuecensoides.     

Chromosome numbers of Panamanian Lecythidaceae and their use in subfamilial classification

Nine species of Lecythidaceae subfamily Lecythidoideae in four genera whose chromosome numbers were previously unknown, have 17 as their basic chromosome number:Eschweilera pittieri, three other unidentified species ofEschweilera, Grias cauliflora, Gustavia dubia, G. superba, Lecythis minor, andL. tuyrana. All are diploid exceptGustavia superba, which is tetraploid.Couroupita guianensis, which was previously—and probably incorrectly—reported to have a gametic chromosome number of 18, also hasn = 17. The known chromosome numbers support recognizing at least three of Niedenzu’s subfamilies: Planchonioideae withx = 13, Napoleonaeoideae withx = 16, and Lecythidoideae withx = 17. His fourth subfamily, Foetidioideae, with one genus of five species, has not been counted. Cytological data have been and probably will be useful in indicating to what subfamily problematic genera belong and in showing interesting phytogeographic patterns within the family. On the other hand, cytological data provide no recognizable clues relating the Lecythidaceae to other families.     

A classification of the New World species of Aster (Asteraceae)

In order effectively to list and comment on the results of cytological investigations inAster in a companion paper, a scheme of infrageneric classification is presented which utilizes the basic chromosome number as a pivotal diagnostic character. Reasons are stated as to why, with the exception ofUnamia Greene which is transferred toSolidago, and the commonly recognized generaLeucelene Greene,Machaeranthera Nees andXylorhiza Nutt., none of the segregate genera previously proposed or recorded in the literature is upheld. Instead, these taxa are being given subgeneric or sectional rank. Two additional subgenera are established to accommodate the species groups traditionally placed in “Aster proper,” which are characterized by having basic chromosome numbers ofx = 5 andx = 8, respectively. Altogether ten subgenera of the genusAster, five of them subdivided further into a total of 24 sections, are recognized as having representative species in the New World. All basionyms and type species are listed, and a number of new combinations and status changes are validated in accordance with the International Rules of Botanical Nomenclature. Where known (from literature and personal research), chromosome numbers are recorded for the species.     

Cytotaxonomy of the monopodial orchids of the African and Malagasy regions

The three subtribes which are recognized within the tribe Vandeae are represented in the tropical African and Malagasy regions. All taxa of the Vandeae have a monopodial growth habit. The first subtribe, Sarcanthinae, is mainly Asian-Australasian, but a few of its species occur in Madagascar and in Africa. The other two subtribes. Angraecinae and Aerangidinae, are both represented in the latter two regions. The Angraccinae is characterized by the presence of a short rostellum, but this is elongated in the Aerangidinae. According to earlier authors there is a correlation between presence of the short rostellum and a basic chromosome number of x=19 in the Angraecinae, and between the presence of an elongated rostellum and a basic number of x=25 in the Aerangidinae. The results presented in this paper are placed in perspective with the chromosome numbers recorded by other authors. From the resulting chromosome number survey it appears that only part of the Angraecinae (Aeranthes, several species of Angraecum, Cryptopus and Jumellea) have a basic number of x=19; some members of the Angraecinae (other species of Angraecum) were found to have a basic number of x=21, 24 and 25. The Aerangidinae is not characterized by a single basic number of x=25 but by a series ranging from x=23 to x=27, of which x=23, 24 and 25 are the most frequent. The genus Calyptrochilum, although having a distinetly elongated rostellum, is characterized by a basic number of x=19 and its position within the Aerangidinae may therefore be questioned. The present evidence suggests that too much weight has been attributed to a single character, i.e. the shape of the rostellum, in distinguishing the two subtribes. As there appear to be two groups according to basic number in Angraecum, it appears questionable whether Angraecum, as presently conceived, is a ‘natural’ genus. Further chromosomal and other taxonomic evidence is needed to substantiate the challenge to the present views in respect of Angraecum. Taxa with x=19 appear to be frequent in Madagascar and the other islands in the Indian Ocean, whereas taxa with x=21 to 27 mainly occur on the African continent. The predominance of x=19 in the Malagasy flora suggests a link with the monopodial taxa of the Asian and Australasian floras, which are exclusively based on x=19.     

Chromosome number determinations in Aster section Conyzopsis (Asteraceae)

Chromosome numbers forAster brachyactis, A. frondosus, andA. laurentianus were determined to be 2n=14. The latter two are the first documented records for these taxa. The basic chromosome number for sectionConyzopsis is confirmed asx=7.     

A new species of Pinguicula from Mexico (Lentibulariaceae)

A new species ofPinguicula from Mexico,P. sharpii, is described. It belongs in SubgenusIsoloba, SectionIsoloba, as defined in Casper’s monograph of the genusPinguicula (1966a). The somatic chromosome number ofPinguicula sharpii is2n = 16, which is diploid. The basic chromosome numberx = 8 is found in many members of SubgenusIsoloba ofPinguicula, while that ofx = 11 is also found in some members of the subgenus.     

Revision of the genus philactis (Compositae)

Philactis (Compositae-Heliantheae) is a genus of three species, all of limited distribution. A revision is presented, based upon herbarium specimens and field work. Cytological data are from bud material fixed in the field, and from root tips of seedlings. The basic chromosome number appears to be 14.P. nelsonii andP. liebmannii are tetraploid (2n = 56), andP. zinnioides is hexaploid (2n = 84).     

Chromosome numbers in Hypericum and related genera

New or confirmatory chromosome counts for 16 taxa ofHypericum in Britain, one of northwest Africa, and 24 of North and Central America are reported. First records for any member of the genusVisma (n = 10) are also included. The counts are discussed with respect to those previously reported forHypericum and related genera. Some of this chromosomal information is incorporated in a diagram showing suggested evolutionary trends withinHypericum. There appears to be a descending series of basic numbers (x = 12, 10, 9, 8, 7), of which all but the last have been recorded in polyploid, as well as diploid, form. Observations of chromosome morphology suggest thatHypericum is cytologically relatively unspecialized. Studies of chromosome morphology, therefore, are not likely to yield much information about the evolutionary history of the genus. It is suggested that the basic number forAllanblackia andPentadesma is 7 or 14, whereas inCalophyllum andMesua it is probably 8 or 16. Known gametic chromosome numbers inMammea (n = 16, 18) andGarcinia (n = 24, ca. 27, ca. 29, ca. 38, ca. 40, and 48) do not indicate an obvious basic number for these genera, although x = 8, 9, and 16 might be involved. Ring-formation of theOenothera-type, possibly indicative of structural hybridity, is reported for the first time inHypericum mitchellianum Rydb., a close relative ofHypericum punctatum Lam., which was previously shown to possess this anomalous chromo some condition.     

Chromosomal analysis of Leptinotarsa and Labidomera species (Coleoptera: Chrysomelidae)

Chromosomes were examined from 13 Leptinotarsa species: L. decemlineata, L. texana, L. defecta, L. juncta, L. rubiginosa, L. haldemani, L. tumamoca, L. peninsularis, L. behrensi, L. heydeni, L. lineolata, L. typographica, and L. undecimlineata. With the exception of L. undecimlineata, which has 2n=32+XO, all other species have the basic chromosome number 2n=34+XO. Of two Labidomera species also examined, L. clivicollis has 2n=32+XO and L. suturella has 2n=30+XO. Idiograms showed that the majority of the autosomes are submetacentric. All species have a large submetacentric X chromosome. Meiotic pairings were regular with both closed and open bivalents. Chiasma frequencies varied considerably among species and even between populations. No distinct correlation was evident when chromosome numbers and chiasma frequencies were compared with host plant association and geographic distribution patterns of Leptinotarsa species.     

Oogenesis and the Chromosomes of the Cystoid Nematode,Meloidodera floridensis

Three Meloidodera floridensis populations of different geographic or host origin all reproduced by mitotic parthenogenesis. One of them from pine had a somatic chromosome number of 26, whereas, another population from pine and one from azalea had 2n = 27 chromosomes. All are considered to be triploid forms derived from an amphimictic ancestor with n = 9 chromosomes, the basic number in the closely related genus Heterodera. Evidence is presented which suggests that during division the chromosomes of the germ-line cells of the developing embryo behave differently than the chromosomes of all other blastomeres.     

Chromosome numbers in some Polish critical or rare Liverworts (Hepaticae)

Chromosome numbers of several hepatic species collected in Poland are published. Polymorphism in chromosome numbers was discovered in Jungermannia leiantha Grolle: most Polish populations showed n = 9 and only two n = 18; chromosome number n = 9 in Jungermannia subulata was confirmed. For Lophozia hyperborea Schust. & Dams. (n = 9) and for Lophozia kunzeana (Hüb.) Evans (n = 1*) the chromosome numbers are published for the first time.     

Cytological variation and evolution withinPelargonium sectionHoarea (Geraniaceae)

Chromosome numbers of 65 species of sect.Hoarea have been determined. These show three basic chromosome numbers, x = 11, 10 and 9. Only a few species are tetraploid. In five species both diploid and tetraploid cytotypes are reported. Several cases of deviations in chromosome numbers and cytological abnormalities were found, most of these being related to the presence of B chromosomes that occur in eight species. Evidence is presented to suggest that the basic chromosome numbers of x = 10 and x = 9 are derived from x = 11 by centric fusion. Although variation in basic chromosome number withinPelargonium has been the subject of detailed study, this is the first time that evidence has been found for a mechanism of change in basic number, that of centric fusion by Robertsonian translocation. For the species of sect.Hoarea with x = 9, where the evidence for Robertsonian translocation is greatest, this process has probably taken place quite recently. In contrast to results from other sections of the genusPelargonium, the three different basic numbers of sect.Hoarea do not contradict its delimitation as a natural taxon.     

华南地区3种兔儿风属植物(菊科-帚菊木族)的细胞学研究

首次报道了华南地区兔儿风属(Ainsliaea DC.)(菊科-帚菊木族Asteraceae-Pertyeae)3种植物共4个居群的染色体数目和核型。其中长穗兔儿风(A.henryi Diels)的染色体数目为2n=24,核型公式为2n=16m+8sm;三脉兔儿风(A.trinervis Y.C.Tseng)的染色体数目为2n=26,核型公式为2n=16m+10sm;莲沱兔儿风(A.ramosa Hemsl.)2个居群的染色体数目均为2n=26,核型公式为2n=26=22m+4sm。所有居群的染色体由大到小逐渐变化,核型没有明显的二型性。这些结果表明兔儿风属植物确有x=12和x=13两个基数,其中x=13可能是该属的原始基数。     

First report of chromosome numbers and karyotypes of two monotypic genera endemic to eastern Asia: Brachystemma (Caryophyllaceae) and Craspedolobium (Fabaceae)

The chromosome numbers and karyotypes of Brachystemma and Craspedolobium, two monotypic genera endemic to eastern Asia, are reported here for the first time. The somatic chromosome numbers are 2n=40 for Brachystemma calycinum and 2n=22 for Craspedolobium unijugum. A karyotype of 2n=2x=40=28m+12sm was found in B. calycinum and that of 2n=2x=22=12m+10sm in C. unijugum, both of them have a moderately symmetrical karyotype type 2B and small‐sized chromosomes. Brachystemma has a unique basic chromosome number in Alsinoideae, which may support its isolated taxonomic position. As do some morphological characters, the basic chromosome number x=11 suggests that Craspedolobium belongs in the Millettioid clade.     

Chromosome numbers in Heterotheca,including Chrysopsis (Compositae: Astereae), with Phylogenetic interpretations

A summation of both previously reported and original data on chromosome numbers is presented for species ofHeterotheca sens. lat. Chromosome numbers are listed for 24 taxa, including 20 different species. Chromosome numbers for the genus aren = 4, 5, 9, 12, and 18. The most common and possibly basic number for the genus isn = 9, with then = 18 taxa considered to be tetraploids, and those withn = 4 and 5 as probable aneuploid derivatives ; alternativelyn = 4 and 5 could be considered the basic numbers withn = 9 the result of polyploidy. Several nomenclatural changes have been made including the following new combinations:Heterotheca bolanderi (Gray) Harms,H. fastigiata (Greene) Harms, andH. villosa var.hispida (Hook.) Harms.     

New chromosome numbers for plant taxa endemic to the Balearic Islands

Mitotic chromosome numbers are reported from 25 vascular plant taxa, endemic to the Balearic Islands that are poorly known cytogenetically. The chromosome numbers ofAnthyllis vulneraria subsp.balearica (2n=12),Cymbalaria fragilis (2n=56), andPolygonum romanum subsp.balearicum (2n=40) were determined for the first time. A new chromosome number was found in several populations ofAnthyllis hystrix (2n=70) suggesting that this species is decaploid, in contrast to an earlier work reporting a higher ploidy level (2n=12x=84). The new chromosome number 2n=32 was reported inHypericum hircinum subsp.cambessedesii. It is suggested that the previous count (2n=40) could be explained by the presence of anomalous pentaploid cells in some tissues, contrating with the presence of a regular tetraploid complement (2n=32). Cytogenetic observations suggest thatSibthorpia africana has a diploid chromosome complement of 2n=18, with 0–2 accessory chromosomes. Accessory chromosomes are also reported forPhlomis italica, being the first record of B chromosomes in this genus. Chromosomal instability was found inGalium crespianum andG. friedichii species, with three numbers 2n=44, 55 and 66. Two cytotypes differing in ploidy level were documented within single plants. It is suggested that both species share a regular complement of 2n=44 and that the past hybridization events and formation of regenerating roots from the typical rootstock ofG. crespianum andG. friedrichii could be involved in the genesis of chromosome variants through partial endopolyploidy and concomitant somatic segregation.     

Genetic relations among basil taxa (Ocimum L.) based on molecular markers, nuclear DNA content, and chromosome number

Twenty-eight basil accessions including six Ocimum species and six botanical varieties or cultivars of O. basilicum were studied using molecular markers, nuclear DNA content, and chromosome counting. This is the first study reporting the nuclear DNA content in the genus Ocimum. The results supported the existence of more infrageneric groups within the genus. The section Ocimum was further divided into two separate clades. The first clade contained the accessions belonging to different botanical varieties and cultivars of O. basilicum as well as O. minimum, indicating that the separate species rank of O. minimum was not justified. The second clade, comprising O. americanum, O. africanum, and two O. basilicum var. purpurascens accessions, could represent a set of allopolyploid species sharing some common parental genomes. O. tenuiflorum was the most divergent species according to genetic distance; it had the smallest genome size, organized in small chromosomes, and the lowest chromosome number. Chromosome data obtained in our research could indicate that the basic chromosome number for species belonging to section Ocimum is x = 12. This suggestion implies that species belonging to O. basilicum clade are tetraploids, while species belonging to O. americanum clade are hexaploids. It seems that the basic chromosome number for O. gratissimum could be x = 10 and for O. tenuiflorum x = 9. The differences in genome size and chromosome number among Ocimum species indicate that evolution of their genomes was accompanied by both sequence deletion/amplification and chromosome rearrangements and polyploidization.     

Systematic studies on mexican coreopsis (compositae). Coreopsis mutica: Flavonoid chemistry,chromosome numbers,morphology, and hybridization

Coreopsis mutica is a highly variable species occurring in the highlands from Central Mexico southeastward barely into El Salvador and Honduras. It is not continuous over this range, however, but is found in three geographic population centers: one in Guatemala and Chiapas, a second in Oaxaca, and the third in Central Mexico. Populations in Guatemala and Chiapas are uniform in chromosome number (2n = 56), leaf flavonoid chemistry, and morphology. Var.microcephala is proposed to accommodate these assemblages. Plants comprising populations centered around Cd. Oaxaca have a chromosome number of 2n = ca. 112. This large complex consists of two distinct varieties and their putative hybrids. Those plants to the northwest of Cd. Oaxaca (var.subvillosa) are constant in leaf flavonoid chemistry (producing only flavones) and possess a combination of distinctive morphological traits. To the southeast of Cd. Oaxaca plants invariably contain flavonols and anthochlors in their leaves in addition to flavones. Moreover, these plants (the newly proposed var.carnosijolia) are readily separable from var.subvillosa by a number of morphological features. Evidence is presented that the two taxa hybridize in the vicinity of Cd. Oaxaca. On the southeastern edge of the var.subvittosavar. carnosifolia complex a population was encountered which has a chromosome number of 2n = 56 and a very distinctive morphology and flavonoid chemistry. These plants have been accorded taxonomic status as var.multiligulata. Two morphologically similar, yet distinguishable, varieties occur in Central Mexico. It has been determined that the two differ also in chromosome number and leaf flavonoid chemistry. One taxon (var.leptomera) has a chromosome complement of 2n = 56 and produces only flavones in its leaves, whereas var.mutica has a chromosome number of 2n = ca. 112 and produces flavones, flavonols, and anthochlors.