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Chromosome Numbers of Angiosperms From the Juan Fernndez Islands, the Tristan DA Cunha Archipelago, and From Mainland Chile1

Posted on: Tuesday, 5 July 2005, 12:00 CDT

Abstract: Chromosome counts for eight native species in six genera from Juan Fernndez Islands, five native species in three genera from Tristan da Cunha, and three species in two genera from mainland Chile are presented and discussed. They include the only chromosome number reports for angiosperms from Tristan da Cunha and first counts for the endemics Robinsonia thurifera and Wahlenbergia larrainii (Juan Fernndez), Agrostis carmichaelii, Acaena sarmentosa, A. stangii, and Nertera holmboei (Tristan da Cunha), and for Galium araucanum and Ourisia coccinea from Chile. Counts for Eryngium bupleuroides and Galium hypocarpium differ from earlier published reports.

ISOLATED OCEANIC ISLANDS are "natural laboratories" for studying patterns and processes of evolution. In this context, knowledge about the cytological situation in endemic taxa is of special interest, because chromosome numbers often reflect speciation and radiation events on islands. However, available karyological data for plants of oceanic islands are unequally distributed, ranging from no records at all for several remote archipelagos, to comparatively high percentages (e.g., ca. 40% for species native to Hawai'i [Can- 1998, Kiehn 2005] and ca. 53% for plants from the Canary Islands and from the Bonin Islands [Stuessy and Crawford 1998]). Reported numbers in most cases are obtained from single or very few individuals and populations.

This paper aims to increase the knowledge of chromosome numbers and ploidy levels of angiosperms occurring on islands by providing first reports of chromosome numbers for angiosperms from an archipelago (Tristan da Cunha), first results for taxa not investigated before, and counts for new collections of taxa already studied. In addition, counts for three species from mainland Chile are presented.

MATERIALS AND METHODS

Plant material was collected by T. F. Stuessy (Juan Fernndez Islands), G.J. (Tristan da Cunha), and P. Menzel and D. Supthut (southern Chile). Field fixations of actively growing apices, young flower buds, or youngflowers were made in a freshly mixed 3:1 solution of ethanol (96%) : glacial acetic acid (G.J., Menzel, Supthut) or in modified Carnoy's fixative (4:3:1 solution of chloroform : 96% ethanol : glacial acetic acid) (Stuessy). Fixations of root tips from plants cultivated from seeds collected in the field were made at the Botanical Garden of the University of Vienna, Austria (HBV). Somatic chromosome numbers (2n) were ultimately obtained from any of several tissue sources, including young flower buds, young ovaries, anthers, and root tips. Gametic number determinations (n) were derived from meiotic divisions of either megasporocytes or microsporocytes. In the case of root tips a pretreatment with a solution of 0.002 m 8-hydroxyquinoline (ca. 2 hr at 19-21C followed by ca. 2 hrs at 4C) was used before fixation in a freshly mixed 3 : 1 solution of ethanol (96%) : glacial acetic acid. The fixed material was transferred to 70% ethanol and kept at ca. 8C before staining with Giemsa (after hydrolysis in 5 N HCl for 50 min at 20C) or with Feulgen (see Kiehn 1995 for additional details on staining procedures). Hot aceto-carmine (2% solution in 45% acetic acid) was also used in some cases to intensify staining.

Exact counts could not be achieved in some instances because of limited material and/or clumping of chromosomes; chromosome numbers for these taxa are given with the range obtained. Data for each investigated collection are presented in Table 1. Voucher specimens for the chromosome number determinations have been deposited in the herbaria of the Ohio State University (OS) or at Vienna University (WU) as stated in Table 1. Permanent microscope slides from which chromosome numbers were determined are in the collection of M.K.

RESULTS AND DISCUSSION

Chromosome counts for 16 angiosperm species (28 collections) of 10 families are presented in Table 1. They comprise data for eight species (13 collections) from the Juan Fernandez Islands, five species, one with two investigated forms (12 collections in total) from Tristan da Cunha, and three species (three collections) native to southern Chile.

Counts from the Juan Fernndez Islands

The Juan Fernndez Islands are an isolated archipelago located ca. 600 km west of the coast of Chile at a latitude of 33 S. They have never been connected to the South American mainland and harbor a unique flora with a high degree of endemism, ca. 60% of the native species and 18% of the genera (Skottsberg 1956, Stuessy et al. 1992, Marticorena et al. 1998). Three papers have reported chromosome numbers of species occurring in this archipelago (Sanders et al. 1983, Spooner et al. 1987, Sun et al. 1990). With the results presented here, cytological data for ca. 21% of the native flora of the Juan Fernndez Islands are available.

APIACEAE: Eryngium bupleuroides is one of three endemic species of this genus on the Juan Fernndez Islands (Jakubowsky and Stuessy 1999) and so far the only one investigated cytologically. Sanders et al. (1983) reported n = 16 for E. bupleuroides, a number also present in Latin American mainland species of Eryngium (Sanders et al. 1983, Pimenov et al. 2003). According to these data, x = 8 seems to be the prevalent basic number for the genus. There are several reports of polyploids, including a few counts of n = 24, in agreement with the report here for E. bupleuroides (Table 1).

According to Stuessy and Crawford (1998), Eryngium is one of only two potential cases of hybridogeneous speciation on the Juan Fernndez Islands. Jakubowsky and Stuessy (1999) confirmed a hybridogeneous origin for E. fernandezianum from E. bupleuroides and E. inaccessum, a situation that had already been suspected by Skottsberg (1922).

The data presented here for E. bupleuroides reflect studies of all developmental stages from microsporocytes to pollen grains of nine different flowers. Most stages of meiosis showed marked irregularities; cytoplasmic bridges between microsporocytes, anaphase bridges, univalents, and restitution nuclei were abundant. In some of the irregular anaphase II divisions it was possible to count 11-25 chromosomes or chromosome fragments in different parts of the microsporocytes. Usually 15-24 chromosomes were moving together, but lagging fragments were always present. Very few normal tetrads and only a few well-formed pollen grains were observed; cells with three to six unequally sized nuclei were present instead. In all of the very few regular-looking divisions that could be counted n = (22)-24 chromosomes were observed. Additional cytological investigations of the genus Eryngium on the Juan Fernndez Islands are needed to appreciate more fully the results reported here.

ASTERACEAE: The endemic genus Robinsonia comprises seven species (Skottsberg 1922, Marticorena et al. 1998). Chromosome numbers for two of them (R. gayana and R. gracilis) were reported and discussed by Sanders et al. (1983). Both species show a haploid number of n = 20. The new count of n = 20 for R. thurifera was obtained from megasporocytes. The genus Robinsonia is considered to be related to Senecio, where n = 20 is common (Sanders et al. 1983). The two species of Robinsonia investigated earlier represent two sections of the genus. Thus Sanders et al. (1983) considered speciation in the genus on the Islands to have taken place without change in chromosome number. The new result for R. thurifera is in accordance with this hypothesis.

CAMPANULACEAE: According to Lammers (1996) and Marticorena et al. (1998), the genus Wahlenbergia has five endemic species in the Juan Fernndez Islands. Chromosome counts are available for two of them: W. fernandeziana from Masatierra and W. masafuerae from Masafuera. For both species n = 11 was counted in individuals from several populations (Sanders et al. 1983, Spooner et al. 1987, Sun et al. 1990). Our new reports revealed n = x = 11 (or 2n = 22) for W. fernandeziana (four populations) and W. larraini. The latter, treated as distinct species by Ricci and Eaton (1994), has a very narrow distribution on Masatierra and is now probably extinct. However, the taxonomic status of this taxon is doubtful, because neither Lammers (1996) nor Crawford et al. (1990) nor Anderson et al. (2000) have found characters that distinguish it from W. fernandeziana. Because n = x = 11 is a basic number not known otherwise in the genus Wahknbergia, chromosome numbers give no hint of geographic origin or taxonomic relationships of the species from the Juan Fernndez Islands.

HALORAGACEAE: The count presented here for Haloragis masatierrana (2n =14) is in accord with the earlier report of n = 7 by Sanders et al. (1983). Because the predominantly reported basic chromosome number in the family Haloragaceae is x = 7, our count has little bearing on questions of speciation and radiation in Haloragis.

LAMIACEAE: Cuminia is an endemic genus of the Juan Fernndez Islands consisting of two clearly distinguishable species, C. eriantha and C. fernandezia (Ruiz et al. 2001). Sanders et al. (1983) reported n = 22 for two populations of C. eriantha and one population of C. fernandezia. The results presented here of n = 22- 24 for C. eriantha and of 2n = ca. 44 for C. fernandezia are in accordance with the earlier reports and emphasizethe tetraploid status of these species based on x = 11 (Sanders et al. 1983).

SAXIFRAGACEAE (ESCALLONIACEAE): Investigations of individuals from three different populations of the Juan Fernndez Islands endemic Escallonia cattcottiae revealed 2n = 24 or 2n = (22-)24, in accordance with the earlier report of n = 12 for this species by Sanders et al. (1983).

Counts from the Tristan da Cunha, Archipelago

The Tristan da Cunha archipelago is situated in the South Atlantic Ocean, ca. 2,800 km west of South Africa and ca. 3,200 km east of South America at a latitude of 37 15'-37 28' S and comprises three islands (Inaccessible, Nightingale, and Tristan da Cunha). In addition to ca. 100 species of alien flowering plants, 78 indigenous vascular plant species (37 ferns and 41 angiosperms) have been recorded for the Islands so far. Twenty-three of the angiosperm species (56%) are endemic (G.J. and M.K., unpubl. data).

The only counts of angiosperm taxa from the archipelago are those reported here, including four endemic and one nonendemic indigenous species. Together they comprise ca. 12% of the indigenous angiosperm flora. The addition of the new counts to previous reports for 20 species of ferns from the archipelago (Manton and Vida 1968) brings the proportion of indigenous vascular plant species of Tristan da Cunha that has been investigated cytologically to 32%.

POACEAE: There are five endemic species of Agrostis known from Tristan da Cunha. Molecular data (G.J., unpubl. data) indicate monophyly for these endemics, which all seem to be derived from A. magellanica subsp. magellanica, a species widespread in the Southern Hemisphere. Beuzenberg and Hair (1983) reported n = 42 for A. magellanica from Campbell Island, whereas Moore (1960) counted 2n = 72 in plants from Macquarie Island (both islands south of New Zealand). Edgar and Forde (1991) mentioned x = 7 as the basic chromosome number for Agrostis and expressed doubts about the result of Moore (1960). The count presented here for Agrostis carmichaelii (2n = 56) is in accord with a basic number of x = 7 and represents octoploidy, which otherwise is unknown in the A. magellanica species alliance.

TABLE 1

Chromosome Numbers of Angiosperms from Tristan da Cunha, Juan Fernndez Islands, and Chile

TABLE 1

Chromosome Numbers of Angiosperms from Tristan da Cunha, Juan Fernndez Islands, and Chile

ROSACEAE: The new chromosome data (2n = 42; 2n = 40-42) for Acaena sarmentosa and A. stangii, the two Acaena species endemic to Tristan da Cunha, are in accordance with earlier reports for the genus that show a basic number of x = 21 for all Acaena species from New Zealand (see survey in Dawson 2000). The cytological data are consistent with earlier hypotheses (Bitter 1911) and DNA sequencing data (Hibbs and G.J., unpubl. data) that link the Tristan da Cunha species to those of New Zealand.

RUBIACEAE: The proposed inclusion of Nertera into Coprosma (Heads 1996) is not supported by molecular studies (Anderson et al. 2001, Markey et al. 2004) and is, therefore, not followed in this paper. The new chromosome data for Nertera from Tristan da Cunha (Table 1) are in accord with x = 11 and tetraploidy obtained in most earlier counts for the genus (see Kiehn 1996). The only exceptions are the results for N. setulosa (= Leptostigma setulosa: n = 20 [Hair 1963]; 2n = 40 [Dawson and Beuzenberg 2000]) and N. yamashitae (2n = 22 [Kokubugata et al. 1998]). Based on morphological characters and molecular data (Markey et al. 2004, unpubl. data), and the chromosome numbers for N. setulosa and N. yamashitae, placement of these two species in Nertera seems doubtful and deserves further study.

Counts from Mainland Chile

RUBIACEAE: All Galium species considered here share a base number of x = 11 and are members of the group formerly treated as the genus Relbunium (Dempster 1990). Diploidy (Huynh 1965) and tetraploidy (Diers 1961) has been found in G. hirsutum Ruiz & Pavon from Peru. The widespread G. hypocarpium L. has been reported as a hexaploid (Peru [Huynh 1965]), a tetraploid (Costa Rica [M.K., unpubl. data]), and is here documented from Chile at the diploid level for the first time. The first report for another Chilean species, G. araucanum Philippi, also establishes it as a diploid. Additional cytological work on the Relbunium group of Galium would likely be highly rewarding.

SCROPHULARIACEAE: The genus Ourisia occurs in the mountains of New Zealand, in alpine regions of Tasmania, and throughout the South American Andes (from Venezuela to Tierra del Fuego). It comprises about 35 species (H. Meudt, unpubl. data). All 15 New Zealand species have been investigated cytologically, and all have 2n = 48 chromosomes (Hair and Arroyo 1984). Ourisia integrifolia from Tasmania was reported as 2n = 32 (Hair and Arroyo 1984). The count presented here of n = 8 for 0. coccinea is the first published report for a species from the Andes and the lowest number published so far for the genus. However, H. Meudt (pers. comm.) has obtained the same number for at least one additional species of the genus from the Andes. This indicates that x = 8 may be the basic number for Ourisia and that the Andean species are diploids, whereas the Tasmanian species is tetraploid and all New Zealand species are hexaploids.

ACKNOWLEDGMENTS

The authors thank Tod F. Stuessy (Institute of Botany, University of Vienna) for the fixations and identifications of plants from the Juan Fernndez Islands and for useful comments. Hania Weiss- Schneeweiss (Institute of Botany, University of Vienna) carried out some of the counts and was helpful in critically discussing the manuscript. Heidi M. Meudt (University of Texas at Austin) is thanked for the identification of Ourisia cocdnea and for sharing unpublished data on Ourisia, including chromosome counts.

1 The work of M.K. and G.J. was supported by the Austrian Science Foundation (FWF) research project P-14108-Bio.

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Michael Kiehn, Marion Jodl, and Gerhard Jakubowsky2

2 Institute of Botany, University of Vienna, Rennweg 14, A-1030 Vienna (Austria) (e-mail: michael.kiehn@univic.ac.at).

Copyright University Press of Hawaii Jul 2005

Source: Pacific Science

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