Karyotype studies in Brazilian species of Lobelia L., subgenus Tupa (Campanulaceae)

Estudo cariotípico em espécies brasileiras de Lobelia L., subgênero Tupa, Campanulaceae

Lobelia L., subgenus

Tupa (Campanulaceae)

PAULO M. RUAS¹, ANDRÉ L.L. VANZELA¹,³, ANA ODETE S. VIEIRA², CÉLIO BERNINI¹ and CLAUDETE F. RUAS¹

(received: April 15, 2000; accepted: April 25, 2001)

ABSTRACT - (Karyotype studies in Brazilian species of Lobelia L., subgenus Tupa, Campanulaceae). Karyotypes of 14 populations including eight species of the genus Lobelia were studied using root tip mitotic metaphases. All populations were tetraploid with 2n = 28 chromosomes. The chromosome base number x = 7 was confirmed for the genus. Karyotype analysis showed that chromosome size varied from 1.05 µm to 2.02 µm with predominance of M and SM chromosome types. The karyotypes were similar among themselves with small intra- and interspecific variations on the size of haploid sets, symmetry indexes and centromere position of some chromosome pairs. These results showed that karyotypes of Brazilian lobelias of the subgenus Tupa were probably due to polyploidy associated with chromosomal rearrangements probably in small chromatin segments.

RESUMO - (Estudo cariotípico em espécies brasileiras de Lobelia L., subgênero Tupa, Campanulaceae). Cariótipos de 14 populações, incluindo oito espécies do gênero Lobelia foram estudados usando metáfases mitóticas de raízes. Todas as populações mostraram-se tetraplóides com 2n = 28 cromossomos. O número cromossômico básico x = 7 foi confirmado para o gênero. A análise dos cariótipos mostrou que os tamanhos cromosômicos variaram de 1,05 µm a 2,02 µm com predominâcia de tipos cromossômicos M e SM. Os cariótipos foram similares entre si com pequenas variações intra e interespecíficas no tamanho do complemento haplóide, índice de simetria e posição do centrômero em alguns pares cromossômicos. Esses resultados mostraram que os cariótipos das lobélias brasileiras do subgênero Tupa podem ter sido originados por poliploidia associada a rearranjos cromossômicos, possivelmente em pequenos segmentos de cromatina.

Key words - Lobelia, Campanulaceae, chromosomes, Feulgen, karyotype evolution

Introduction

The genus Lobelia comprises 365 species distributed in temperate and tropical regions of America, Africa, Asia, Australia and Europe (Wimmer 1953). The African continent and Mexico seem to be the most probable genetic diversity centers (Lammers 1993). Species of Lobelia present different habit types, which vary from small delicate herbs to woody plants with rosette leaves (giant lobelias). The woody members are grouped in the subgenus Tupa in which almost all species are tetraploids with n = 14 chromosomes (Vilmorin & Simonet 1927, Nevling 1966, Lammers & Hensold 1992, Lammers 1993).

Chromosome numbers have been established for 70% of the family, but only for 13% of Lobelioïdeae subfamily (Lammers & Hensold 1992, Lammers 1993). Approximately 75% of species present chromosome numbers that are multiples of 7, which is considered the chromosome base number of the genus (Thulin 1983, Lammers & Hensold 1992). According to Vieira (1988), Vieira & Shepherd (1996) and Lammers (1993), most of the herbaceous members of Lobelioïdeae show a polyploid series with n = 7, 14 and 28 chromosomes.

In Brazil the genus Lobelia is represented by 16 species, ten belonging to the subgenus Tupa. These species are distributed mainly in swampy areas of southeastern Brazil (Vieira 1988, Vieira & Shepherd 1998). Chromosome counts for five Brazilian species of the subgenus Tupa revealed a constant number of n = 14 (Gadella et al. 1969, Vieira & Shepherd 1996). According to Lammers (1988, 1992, 1993) and Lammers & Hensold (1992) karyomorphological studies could be useful to elucidated taxonomy and evolutive mechanisms of this group, however, as up to date no karyotype data is avaliable for the subgenus Tupa or any member of the subfamily Lobelioïdeae. This work presents a comparative study applied to 14 populations of eight species of Lobelia subgenus Tupa native from Brazil. Results are discussed in relation to mechanisms of karyotype evolution.

Material and methods

Samples of eight species of Lobelia were collected in 12 Brazilian localities (table 1, figure 1), and cultivated in a greenhouse. Voucher specimens are preserved in the Herbarium FUEL, Londrina, PR.

For cytogenetic analyses, root tips of five plants of each population were collected and pretreated in 2 mM 8hydroxyquinoline at 9 °C for 4 hours, fixed in 3:1 Carnoy (ethanolacetic acid, v/v) overnight at room temperature, transferred to 70% alcohol and stored at 4 °C. Chromosome preparations were done by conventional Feulgen method.

Somatic chromosome numbers were determined in at least ten metaphases of each taxa. The chromosome and karyotype features such as haploid set lengths, centromere positions and symmetry indexes (Huziwara 1962) were obtained by measuring five well spread metaphases. Chromosome types were classified as metacentric, submetacentric, acrocentric and telocentric. Photomicrographs were taken with Agfa Pan film and Kodak Kodabromide F-3 paper.

Results and Discussion

All populations (14) belonging to the eight species of Lobelia exhibited 2n = 28 chromosomes (table 1, figure 2). Previous chromosome counts showed n = 14 for Hawaiian Islands lobelias (Lammers 1988), the giant African species (Mabberley 1974, Knox & Kowal 1993), and also for the Brazilian species of the subgenus Tupa (Vieira & Shepherd 1996). The chromosome number n = 7 was reported for two other species of subgenus Tupa, the Peruvian L. decurrens Cav (Diers 1961) and the West Indian L. portoricensis (Vatke) Urban (Nevling 1966), while n = 21 was found in Chilean lobelias (Stace & James 1996). Chromosome numbers n = 14 and 21 should be considered as polyploid (2n = 4x = 28 and 2n = 6x = 42) derivatives from ancestral diploid species with n = 7 (Nevling 1966, Lammers & Hensold 1992, Lammers 1993). This hypothesis was followed by Raven (1975) and Lammers (1988) who suggested that low diploid numbers are a primitive characteristic in this subfamily. This hypothesis also follows the general tendency of most of the angiosperms in which 80% of the species are polyploid (see Leitch & Bennett 1997). Moreover, Mabberley (1974) and Stace & James (1996) proposed that high chromosome numbers n = 21 and n = 14 should represent the most primitive number for the genus. This statement is supported by the absence of multivalents in meiosis, as observed in four Chilean species (Lammers & Hensold 1992), and by a cpDNA phylogenetic tree (Knox et al. 1993). These authors showed that certain herbaceous taxa of the subgenus Lobelia section Lobelia derived from ancestral stocks including the old woody giants of subgenus Tupa and other woody lobeliads. Carlquist (1969, 1992) also pointed out that woodiness is an apomorphic character in the subfamily Lobelioïdeae. Therefore, if n = 21 is the diploid primitive number, the woodiness with n = 7 and n = 14 could be apomorphic derived by diploid redution (n = 21 ® n = 14 ® n = 7). In the present study, all karyotypes were very similar with metacentric and submetacentric chromosomes. Satellites were observed in chromosome 8 for all taxa. These results are not consistent with the mechanism of diploid chromosome reduction discussed before.

In general, only few inter- and intraspecific variations were found in the karyotypes (see populations of L. thapsoidea from Nova Friburgo and Vale da Revolta, table 1). Variations in chromosome morphology, as observed in some chromosome pairs, were probably due to the occurrence of chromosome rearrangements involving the centromere (figure 2: pair 2 of B and D, pair 4 of I and K, pair 6 of J and K and pair 14 of B and D). The range of the chromosome pairs varied from 1.7 µm (L. hassleri Zahlb.) to 2.3 µm (L. exaltata Pohl) and from 0.9 µm (L. brasiliensis A.O.S. Vieira & G.J. Shepherd, L. hassleri, and L. exaltata) to 1.3 µm (L. fistulosa Vell.). The total size of the haploid complement varied from 19.7 µm in L. thapsoidea Schott to 24.1 µm in L. exaltata while the karyotype symmetry index ranged from 36.8% in L fistulosa to 43.5% in L. thapsoidea (table 1, figure 2). All these variations could be related to the accumulation of minor chromosome rearrangements, as small amplifications, deletions or inversions. The occurrence of chromosomes rearrangements was reported in recent study using fluorescence in situ hybridization (FISH) by Vanzela et al. (1999). These authors showed differences in the distribution and number of 5S rDNA sites between L. brasiliensis and L. imperialis var. kanitzii E. Wimm., which result from amplification and interruption of continuous arrays by insertion of unrelated DNA sequences in 5S rDNA repeated units.

The maintenance of karyotype variability of Brazilian Lobelia species can be also explained by some ecological features. These species are distributed in isolated populations (figure 1), most of them adapted to swamps and small river banks at relatively high altitudes (Vieira 1988, Vieira & Shepherd 1998). The isolation of these populations and the occurrence of self-pollinated seeds, as observed in Japanese Lobelia species (Mariko & Kachi 1995), could also explain the accumulation of karyotype differences.

Acknowledgments - The authors are thankful to Dulce M.S. Rocha (Universidade de Brasília), Kykio Yamamoto, and Renato Belinello (Universidade Estadual de Campinas) for their help in collecting the specimens. We are also grateful to Maria Augusta S. Vieira who drew figure 1.

2. Universidade Estadual de Londrina, Centro de Ciências Biológicas, Departamento de Biologia Animal e Vegetal, Caixa Postal 6001, 86051-990 Londrina, PR, Brazil.

3. Corresponding author: [e-mail]

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1

. Universidade Estadual de Londrina, Centro de Ciências Biológicas, Departamento de Biologia Geral, Caixa Postal 6001, 86051-990 Londrina, PR, Brazil.

Publication Dates

• Publication in this collection
  10 Dec 2001
• Date of issue
  Sept 2001

History

• Accepted
  25 Apr 2001
• Received
  15 Apr 2000