Chromosome studies of some Vernonia species (Asteraceae)

Chromosome studies of some Vernonia species (Asteraceae)^*

Massimiliano Dematteis

Instituto de Botánica del Nordeste (UNNE-CONICET), Casilla de Correo 209, (3400) Corrientes, Argentina.

ABSTRACT

Chromosomes of 12 Vernonia species (Asteraceae) from South America were studied. First counts were reported for: V. alpestris Gardn. (n = 17), V. chalybaea Mart. ex DC. (2n = 32), V. cotoneaster Less. (2n = 30), V. fruticulosa Mart. ex DC. (n = 16) and V. lithospermifolia Hieron. (n = 10, 2n = 20). Furthermore, in V. aurea Mart. ex DC. (2n = 32), V. mollissima D. Don (n = 32) and V. saltensis Hieron. (2n = 64) numbers different from those reported in the literature were found. Karyotypes were described for some species. V. aurea contains 24m + 8sm chromosomes, V. chalybaea contains 22m + 10sm chromosomes, V. cincta Griseb. 36m + 16sm + 8st and V. cotoneaster 18m + 12sm. The basic chromosome numbers x = 10, 14, 15, 16 and 17 were confirmed for South American Vernonia species. The results are discussed in relation to previous chromosome studies for the genus.

INTRODUCTION

The genus Vernonia Schreb. consists of more than 1,000 species distributed in the tropical and subtropical regions of Asia, Africa and America. There are two major centers of diversification, one in southern Brazil and the other in tropical Africa (Jones, 1977).

This genus is not well-characterized cytologically, with only 16% of the species examined for chromosome number. Most of these counts are from North American and African taxa, while reports from South American species are largely absent. The chromosomal data available to date show a wide diversity of basic chromosome numbers, which varies between x = 7 and x = 19 (Turner, 1981; Keeley and Turner, 1990; Ruas et al., 1991).

In Old World species, polyploidy is uncommon and the most frequent chromosome numbers are 2n = 18 and 2n = 20. Polyploidy is very frequent in the New World, and the most common chromosome number is 2n = 34 (Jones, 1979).

Karyological studies have been carried out in several species of the genus (Gill, 1978; Mathew and Mathew, 1982). However, only 10 taxa from South America have been analyzed in detail (Ruas et al., 1991; Dematteis, 1996).

The present paper reports chromosome number and some karyotypical data for 12 Vernonia species from Argentina, Brazil and Paraguay. The significance of the results is discussed in relation to previous studies of the genus.

MATERIAL AND METHODS

The origin of the materials studied is presented in Table I. Voucher specimens were deposited in the herbaria of the Instituto de Botánica del Nordeste (CTES), Centro de Pesquisas do Cacau (CEPEC), Prefeitura Municipal de Curitiba (MBM), Universidad Nacional de Misiones (MNES), Instituto de Botánica Darwinion (SI) and Smithsonian Institution (US).

Meiotic study was made in young inflorescences fixed in lactic acid - ethanol (1:5) and stored under refrigeration until examined. Squashes of pollen mother cells were made using the acetocarmine technique. Mitotic preparations were obtained from root-tips proceeding of germinating seeds. After a pretreatment of about 4 h in 8-oxyquinoline solution, the material was fixed in acetic acid-ethanol (1:3) and stained following Feulgen's technique.

The nomenclature used for karyotype description was proposed by Levan et al. (1964). Chromosome morphology was determined using the centromeric index (short arm x 100/total chromosome length). Accordingly, chromosomes were classified as metacentric (m) = 50-37.5, sub-metacentric (sm) = 37.5-25 and sub-telocentric (st) = 25-12.5. Ten metaphase plates for each species were measured in order to estimate the mean centromeric index (CI), total chromosome length of the complement (TLC) and mean length of the chromosomes (ML).

RESULTS

The analyzed species and their chromosome numbers are listed in Table I. Chromosome counts are reported for the first time for five species. Counts are also reported for seven taxa, three of which are new numbers.

Figure 1 - Somatic chromosomes of Vernonia. A: V. aurea, 2n = 32. B: V. chalybaea, 2n = 32. C: V. cincta, 2n = 60. D: V. cotoneaster, 2n = 30. E: V. platensis, 2n = 80. F: V. saltensis, 2n = 64. Scale = 5 µm.

Karyotypes of four species were analyzed in detail. The idiograms are shown in Figure 2. Karyotypical data of these species are given below.

Figure 2
- Idiograms of Vernonia. A: V. aurea, 24m + 8sm. B: V. chalybaea, 22m + 10sm. C: V. cincta, 36m + 16sm + 8st. D: V. cotoneaster, 18m + 12sm. Scale = 2.5 µm.

V. aurea

Karyotype analysis showed 24m + 8sm chromosomes (Figure 2A). Chromosome size varied from 1.49 to 2.67 µm, with a ML of 2.09 µm. CI was 42.29 ± 0.20, and TLC was 66.94 ± 1.70 µm. A satellite was located in the short arm of the pair 15sm.

V. chalybaea

The karyotype was composed of 22m + 10sm chromosomes (Figure 2B). Chromosomes ranged between 1.32 and 2.35 µm, with a ML of 1.80 µm. CI of this species was 41.75 ± 0.53, and TLC was 57.85 ± 3.25 µm. It was not possible to identify satellites in this taxon.

V. cincta

The karyotype consisted of 36m + 16sm + 8st chromosomes (Figure 2C). Chromosome size varied from 1.02 to 2.80 µm, with a ML of 1.80 µm. CI was 39.65 ± 1.09, and TLC was 108.08 ± 0.12 µm. In this species no satellites were observed.

V. cotoneaster

The karyotype had 18m + 12sm chromosomes (Figure 2D). Chromosome size varied from 1.11 to 1.83 µm, with a ML of 1.37 µm. CI was 41.08 ± 0.72, and TLC was 41.39 ± 0.43 µm. The pair 11sm had a microsatellite in the short arm.

DISCUSSION

V. alpestris and V. incana are diploids with x = 17, a basic chromosome number well established in South American species of the genus. The count of 2n = 34 for V. incana agrees with the only report of this widespread taxon (Dematteis, 1997a).

In contrast, V. aurea, V. chalybaea, V. fruticulosa, V. mollissima and V. saltensis all had a base number of x = 16. V. aurea, V. chalybaea and V. fruticulosa were diploids, while the remaining species were tetraploids with the same basic number.

Chromosome number observed in V. aurea (2n = 32) differ from an earlier report of n = 17 for specimens from Goiás, Brazil (Jones, 1982). There are some possible explanations for this disagreement: chromosome number may be variable within the species, or Jones may have miscounted V. aurea. In any case, more chromosome counts should be made in order to answer this question.

The count of 2n = 32 for V. mollissima disagrees with the determination of n = ca. 54 made by Bernardello (1986) for a population from Cordoba, Argentina. According to Cabrera's (1944) systematic treatment, these specimens would belong to the typical variety of V. mollissima, while the variety analyzed here belongs to var. microcephala Hieron.

A population of V. saltensis from Cordoba (Argentina) was reported as diploid with n = 16 (Bernardello, 1986). The present count of 2n = 64 is tetraploid with x = 16 and suggests the existence of two different cytotypes for V. saltensis. Chromosome counts for other populations would help determine the geographical distribution of the cytotypes and their relationship with the morphology of the specimens.

The count of 2n = 30 for V. cotoneaster is the first for the species, which is diploid with x = 15.

The systematic position of V. cincta has been in dispute for a long time. Cabrera (1944) included it as a variety of V. scorpioides (Lam.) Pers. based on morphological features. On the other hand, Robinson (1987) recognized V. cincta and V. scorpioides as separate species based on pollen type and geographical distribution. Chromosomal data support the criteria of Robinson (1987), since both entities are cytologically different. In fact, V. cincta has 2n = 60, whereas V. scorpioides presents 2n = 66 according to the recount of Galiano and Hunziker (1987). The present count of 2n = 60 for V. cincta confirms a previous report by Wulff et al. (1996), suggesting that this species is tetraploid with x = 15.

V. lithospermifolia is diploid with x = 10, which is a primitive basic number infrequent in the New World and restricted to species of the V. flexuosa group (Dematteis, 1996). This group was first recognized formally by Cabrera (1944) under the title subsection Flexuosae and later by Jones (1981) as series Flexuosae. Within the Flexuosae group, V. lithospermifolia is probably related to V. flexuosa, from which it can be distinguished by its shorter heads and narrower leaves. Cytologically, the two species are different. V. lithospermifolia is diploid, and V. flexuosa is tetraploid with x = 10 (Ruas et al., 1991).

Chromosome numbers of 2n = 20, 40, 60 and 80 have been reported previously for V. platensis (Núñez in Cabrera, 1944; Hunter, 1964; Galiano and Hunziker, 1987; Dematteis, 1996, 1997b). These records indicate the occurrence of a large euploid series with base number x = 10. The material analyzed here was octoploid and constituted apparently the second population with this ploidy level reported for V. platensis. The octoploid plants appeared more robust and had larger leaves than the diploids, but it was impossible to distinguish them from the tetraploids or hexaploids.

The count of 2n = 2x = 28 for V. remotiflora agrees with the single preceeding report for the species (Dematteis, 1996). The basic number x = 14 has been reported for only two species of Vernonia, including V. remotiflora. The other species with x = 14 is V. racemosa Delp. (Torres and Liogier, 1970), a closely related entity to V. remotiflora endemic to the Dominican Republic.

Jones (1979) suggested x = 17 as the only basic number in the New World. However, recent reports show Vernonia as multibasic (Keeley and Turner, 1990; Ruas et al., 1991; Dematteis, 1996). The data presented in this paper support five different basic numbers x = 10, 14, 15, 16 and 17. Previous reports for other American species also suggest the base numbers x = 18 and x = 19 (Turner, 1981; Ruas et al., 1991). The numbers cited above indicate that a complex combination of polyploidy and aneuploidy changes has occurred in the evolution of the genus Vernonia.

Although chromosomes are relatively small, differences in their size and morphology were detected. Most species analyzed to date have symmetric karyotypes, composed mainly of metacentric pairs with several sub-metacentrics. The presence of sub-telocentric chromosomes such as in V. cincta would seem to be exceptional in the genus. This fact strongly suggests that diversification of genus Vernonia was accompanied by few changes in the karyotype constitution of the species.

Further cytological work is needed to obtain chromosome counts and karyotypes in Vernonia primarily from South American species. Additional information should provide data important to the understanding of the systematics of the genus.

ACKNOWLEDGMENTS

I would like to particularly thank Ing. Aveliano Fernández for his guidance and continuous support. This work was supported in part by a grant from the Secretaría General de Ciencia y Técnica of the Universidad Nacional del Nordeste.

RESUMO

Os cromossomos de doze espécies sul-americanas de Vernonia (Asteraceae) foram estudados, sendo que as contagens foram feitas pela primeira vez em V. alpestris Gardn. (n = 17), V. chalybaea Mart. ex DC. (2n = 32), V. cotoneaster Less. (2n = 30), V. fruticulosa Mart. ex DC. (n = 16) e V. lithospermifolia Hieron. (n = 10, 2n = 20). Além disso, encontram-se em V. aurea Mart. ex DC. (2n = 32), V. mollissima D. Don (n = 32) e V. saltensis Hieron. (2n = 64) números diferentes dos relatados na literatura. Os cariótipos foram descritos para algumas espécies: V. aurea contém 24m + 8sm, V. chalybaea contém 22m + 10sm, V. cincta Griseb. 36m + 16sm + 8st e V. cotoneaster 18m + 12sm. Os números cromossômicos básicos x = 10, 14, 15, 16 e 17 foram confirmados para essas espécies sul-americanas de Vernonia. Os resultados são discutidos em relação a prévios estudos cromossômicos para o gênero Vernonia.

(Received July 23, 1997)

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*

Part of a thesis that will be presented by M.D. to the Facultad de Ciencias Exactas, Físicas y Naturales of the Universidad Nacional de Cordoba, Argentina, in partial fulfillment of the requirements for the Doctoral degree.

Publication Dates

• Publication in this collection
  23 Feb 1999
• Date of issue
  Sept 1998

History

• Received
  23 July 1997